LTC2656 - Octal 16-/12-Bit Rail-to-Rail DACs with 10ppm/°C Max Reference

Features

  • Precision 10ppm/°C Max Reference
  • Maximum INL Error: ±4LSB at 16 Bits
  • Guaranteed Monotonic over Temperature
  • Selectable Internal or External Reference
  • 2.7V to 5.5V Supply Range (LTC2656-L)
  • Integrated Reference Buffers
  • Ultralow Crosstalk Between DACs(<1nV•s)
  • Power-On-Reset to Zero-Scale/Mid-scale
  • Asynchronous LDAC Update Pin
  • Tiny 20-Lead 4mm × 5mm QFN and 20-Lead Thermally Enhanced TSSOP Packages

Typical Application

LTC2656 Typical Application
LTC2656 Typical Application

Description

The LTC2656 is a family of octal 16-/12-bit rail-to-rail DACs with a precision integrated reference. The DACs have built-in high performance, rail-to-rail, output buffers and are guaranteed monotonic.The LTC2656-L has a full-scale output of 2.5V with the integrated 10ppm/°C reference and operates from a single 2.7V to 5.5V supply. The LTC2656-H has a full-scale output of 4.096V with the integrated reference and operates from a 4.5V to 5.5V supply. Each DAC can also operate with an external reference, which sets the DAC full-scale output to two times the external reference voltage.

These DACs communicate via a SPI/MICROWIRE™ compatible 4-wire serial interface which operates at clock rates up to 50MHz. The LTC2656 incorporates a power-on reset circuit that is controlled by the PORSEL pin. If PORSEL is tied to GND the DACs reset to zero-scale. If PORSEL is tied to VCC, the DACs reset to mid-scale.

Packaging

CAD Symbols and Footprints: The downloadable Zip file below contains the schematic symbol and PCB footprints.

For complete and up to date package information and drawings, please refer to our packaging page

Part Number Package Code Temp Package
Drawing
RoHS
LTC2656BCFE-H16#PBF TSSOP-20 FE C 05-08-1663 (CB) Yes
LTC2656BCFE-H16#TRPBF TSSOP-20 FE C 05-08-1663 (CB) Yes
LTC2656BCFE-L16#PBF TSSOP-20 FE C 05-08-1663 (CB) Yes
LTC2656BCFE-L16#TRPBF TSSOP-20 FE C 05-08-1663 (CB) Yes
LTC2656BCUFD-H16#PBF 4x5 QFN-20 UFD C 05-08-1711 Yes
LTC2656BCUFD-H16#TRPBF 4x5 QFN-20 UFD C 05-08-1711 Yes
LTC2656BCUFD-L16#PBF 4x5 QFN-20 UFD C 05-08-1711 Yes
LTC2656BCUFD-L16#TRPBF 4x5 QFN-20 UFD C 05-08-1711 Yes
LTC2656BIFE-H16#PBF TSSOP-20 FE I 05-08-1663 (CB) Yes
LTC2656BIFE-H16#TRPBF TSSOP-20 FE I 05-08-1663 (CB) Yes
LTC2656BIFE-L16#PBF TSSOP-20 FE I 05-08-1663 (CB) Yes
LTC2656BIFE-L16#TRPBF TSSOP-20 FE I 05-08-1663 (CB) Yes
LTC2656BIUFD-H16#PBF 4x5 QFN-20 UFD I 05-08-1711 Yes
LTC2656BIUFD-H16#TRPBF 4x5 QFN-20 UFD I 05-08-1711 Yes
LTC2656BIUFD-L16#PBF 4x5 QFN-20 UFD I 05-08-1711 Yes
LTC2656BIUFD-L16#TRPBF 4x5 QFN-20 UFD I 05-08-1711 Yes
LTC2656CCFE-L16#PBF TSSOP-20 FE C 05-08-1663 (CB) Yes
LTC2656CCFE-L16#TRPBF TSSOP-20 FE C 05-08-1663 (CB) Yes
LTC2656CCUFD-L16#PBF 4x5 QFN-20 UFD C 05-08-1711 Yes
LTC2656CCUFD-L16#TRPBF 4x5 QFN-20 UFD C 05-08-1711 Yes
LTC2656CFE-H12#PBF TSSOP-20 FE C 05-08-1663 (CB) Yes
LTC2656CFE-H12#TRPBF TSSOP-20 FE C 05-08-1663 (CB) Yes
LTC2656CFE-L12#PBF TSSOP-20 FE C 05-08-1663 (CB) Yes
LTC2656CFE-L12#TRPBF TSSOP-20 FE C 05-08-1663 (CB) Yes
LTC2656CIFE-L16#PBF TSSOP-20 FE I 05-08-1663 (CB) Yes
LTC2656CIFE-L16#TRPBF TSSOP-20 FE I 05-08-1663 (CB) Yes
LTC2656CIUFD-L16#PBF 4x5 QFN-20 UFD I 05-08-1711 Yes
LTC2656CIUFD-L16#TRPBF 4x5 QFN-20 UFD I 05-08-1711 Yes
LTC2656CUFD-H12#PBF 4x5 QFN-20 UFD C 05-08-1711 Yes
LTC2656CUFD-H12#TRPBF 4x5 QFN-20 UFD C 05-08-1711 Yes
LTC2656CUFD-L12#PBF 4x5 QFN-20 UFD C 05-08-1711 Yes
LTC2656CUFD-L12#TRPBF 4x5 QFN-20 UFD C 05-08-1711 Yes
LTC2656IFE-H12#PBF TSSOP-20 FE I 05-08-1663 (CB) Yes
LTC2656IFE-H12#TRPBF TSSOP-20 FE I 05-08-1663 (CB) Yes
LTC2656IFE-L12#PBF TSSOP-20 FE I 05-08-1663 (CB) Yes
LTC2656IFE-L12#TRPBF TSSOP-20 FE I 05-08-1663 (CB) Yes
LTC2656IUFD-H12#PBF 4x5 QFN-20 UFD I 05-08-1711 Yes
LTC2656IUFD-H12#TRPBF 4x5 QFN-20 UFD I 05-08-1711 Yes
LTC2656IUFD-L12#PBF 4x5 QFN-20 UFD I 05-08-1711 Yes
LTC2656IUFD-L12#TRPBF 4x5 QFN-20 UFD I 05-08-1711 Yes


LTC2656 Package Drawing
LTC2656 Package Drawing

Order Info

  • Part numbers ending in PBF are lead free. Solder plated terminal finish (SnPb) versions are non-standard and special terms and conditions and pricing applies if available. Please contact LTC marketing for information.
  • Part numbers containing TR or TRM are shipped in tape and reel or 500 unit mini tape and reel, respectively
  • Please refer to our general ordering information or the product datasheet for more details

Package Variations and Pricing

Part Number Package Temp Price
(1-99)
Price
(1k)*
RoHS
LTC2656BCFE-H16#PBF TSSOP-20 C $25.64 $17.95 Yes
LTC2656BCFE-H16#TRPBF TSSOP-20 C $18.01 Yes
LTC2656BCFE-L16#PBF TSSOP-20 C $25.64 $17.95 Yes
LTC2656BCFE-L16#TRPBF TSSOP-20 C $18.01 Yes
LTC2656BCUFD-H16#PBF 4x5 QFN-20 C $25.64 $17.95 Yes
LTC2656BCUFD-H16#TRPBF 4x5 QFN-20 C $18.01 Yes
LTC2656BCUFD-L16#PBF 4x5 QFN-20 C $25.64 $17.95 Yes
LTC2656BCUFD-L16#TRPBF 4x5 QFN-20 C $18.01 Yes
LTC2656BIFE-H16#PBF TSSOP-20 I $29.50 $20.65 Yes
LTC2656BIFE-H16#TRPBF TSSOP-20 I $20.71 Yes
LTC2656BIFE-L16#PBF TSSOP-20 I $29.50 $20.65 Yes
LTC2656BIFE-L16#TRPBF TSSOP-20 I $20.71 Yes
LTC2656BIUFD-H16#PBF 4x5 QFN-20 I $29.50 $20.65 Yes
LTC2656BIUFD-H16#TRPBF 4x5 QFN-20 I $20.71 Yes
LTC2656BIUFD-L16#PBF 4x5 QFN-20 I $29.50 $20.65 Yes
LTC2656BIUFD-L16#TRPBF 4x5 QFN-20 I $20.71 Yes
LTC2656CCFE-L16#PBF TSSOP-20 C $20.36 $14.25 Yes
LTC2656CCFE-L16#TRPBF TSSOP-20 C $14.31 Yes
LTC2656CCUFD-L16#PBF 4x5 QFN-20 C $20.36 $14.25 Yes
LTC2656CCUFD-L16#TRPBF 4x5 QFN-20 C $14.31 Yes
LTC2656CFE-H12#PBF TSSOP-20 C $12.50 $8.75 Yes
LTC2656CFE-H12#TRPBF TSSOP-20 C $8.81 Yes
LTC2656CFE-L12#PBF TSSOP-20 C $12.50 $8.75 Yes
LTC2656CFE-L12#TRPBF TSSOP-20 C $8.81 Yes
LTC2656CIFE-L16#PBF TSSOP-20 I $23.41 $16.39 Yes
LTC2656CIFE-L16#TRPBF TSSOP-20 I $16.45 Yes
LTC2656CIUFD-L16#PBF 4x5 QFN-20 I $23.41 $16.39 Yes
LTC2656CIUFD-L16#TRPBF 4x5 QFN-20 I $16.45 Yes
LTC2656CUFD-H12#PBF 4x5 QFN-20 C $12.50 $8.75 Yes
LTC2656CUFD-H12#TRPBF 4x5 QFN-20 C $8.81 Yes
LTC2656CUFD-L12#PBF 4x5 QFN-20 C $12.50 $8.75 Yes
LTC2656CUFD-L12#TRPBF 4x5 QFN-20 C $8.81 Yes
LTC2656IFE-H12#PBF TSSOP-20 I $14.37 $10.06 Yes
LTC2656IFE-H12#TRPBF TSSOP-20 I $10.12 Yes
LTC2656IFE-L12#PBF TSSOP-20 I $14.37 $10.06 Yes
LTC2656IFE-L12#TRPBF TSSOP-20 I $10.12 Yes
LTC2656IUFD-H12#PBF 4x5 QFN-20 I $14.37 $10.06 Yes
LTC2656IUFD-H12#TRPBF 4x5 QFN-20 I $10.12 Yes
LTC2656IUFD-L12#PBF 4x5 QFN-20 I $14.37 $10.06 Yes
LTC2656IUFD-L12#TRPBF 4x5 QFN-20 I $10.12 Yes
Buy NowRequest Samples
* The USA list pricing shown is for BUDGETARY USE ONLY, shown in United States dollars (FOB USA per unit for the stated volume), and is subject to change. International prices may differ due to local duties, taxes, fees and exchange rates. For volume-specific price or delivery quotes, please contact your local Linear Technology sales office or authorized distributor.

Demo Boards

Linear Technology offers many demo boards free of charge to qualified customers. Contact your local sales office or distributor to inquire about a demo board. Certain demo boards are also available for sale via credit card on this website. Demo boards are for evaluation purposes only. It remains the customer’s responsibility to verify proper and reliable operation in the actual end application.

Part Number Description Price Documentation
DC1397B-A LTC2656-L16 Demo Board | Octal SPI 16-bit Voltage Output DAC with 1.25V Reference, req DC2026 $100.00
DC1397B-B LTC2656-H16 Demo | Octal SPI 16-bit Voltage Output DAC with 2.048V Reference, (req DC2026) $100.00
Buy Now

Companion Boards

Part Number Description Price Documentation
DC2026C Linduino One Isolated USB Demo Board: An Arduino- and QuikEval-Compatible Code Development Platform $75.00
DC590B Isolated USB Serial Controller for Linear Technology QuikEval-Compatible Demo Boards $50.00
Buy Now
Click here to view our complete list of demo boards

Applications

  • Mobile Communications
  • Process Control and Industrial Automation
  • Instrumentation
  • Automatic Test Equipment
  • Automotive
  • Product Notifications

    Please login to your MyLinear account for notifications of datasheet updates, new document releases and LTspice model announcements for your favorite products. If you do not have a MyLinear account you may Sign Up Now.

    Forgot your password? Click here.
    Need help? Email mylinear@linear.com with questions and comments.

    Design Tools

    Linduino

    Linduino is an Arduino compatible platform for developing and distributing firmware libraries and code for SPI and I²C-compatible integrated circuits. The Linduino One board interfaces to more than 300 QuikEval demonstration cards, supporting a variety of product types including analog-to-digital converters (ADCs)digital-to-analog converters (DACs)power monitors, and more. Firmware libraries for individual devices are written in C and designed to be portable to a wide variety of processors and microcontrollers. Each library has a demonstration program that can be uploaded to the Linduino One platform to allow the circuit and software to be quickly and easily verified.

    Click here for more information on Linduino

    Code

    Linduino is Linear Technology's Arduino compatible system for developing and distributing firmware libraries and example code for Linear Technology’s integrated circuits. The code below can be downloaded or copied and pasted into your project. Please visit the Linduino Home Page for demo board, manual and setup information.

    This part is Code Supported: There is example code available for this part. The code below may rely on other drivers available in the full library.

    Download LTC2656 - DC1397A Linduino .INO File

    /*!
    Linear Technology DC1397A Demonstration Board.
    LTC2656: Quad SPI 16-/12-Bit Rail-to-Rail DACs with 10ppm/C Max Reference.
    
    @verbatim
    NOTES
     Setup:
       Set the terminal baud rate to 115200 and select the newline terminator.
       The program displays calculated voltages which are based on the voltage
       of the reference used, be it internal or external. A precision voltmeter
       is needed to verify the actual measured voltages against the calculated
       voltage displayed. If an external reference is used, a precision voltage
       source is required to apply the external reference voltage. A
       precision voltmeter is also required to measure the external reference
       voltage. No external power supply is required. Any assembly option may be
       used: DC1397A-A, DC1397A-B
    
    
     Explanation of Commands:
       1- Select DAC: Select one of four DACs to test : A, B, C, D, E, F, G, H.
    
       2- Write to DAC input register: Value is stored in the DAC for updating
          later, allowing multiple channels to be updated at once, either
          through a software "Update All" command or by asserting the LDAC# pin.
          User will be prompted to enter either a code in hex or decimal, or a
          voltage. If a voltage is entered, a code will be calculated based on
          the active scaling and reference parameters - ideal values if no
          calibration was ever stored.
    
       3- Write and Update: Similar to item 1, but DAC is updated immediately.
    
       4- Update DAC: Copies the value from the input register into the DAC
          Register. Note that a "write and update" command writes the code to
          BOTH the input register and DAC register, so subsequent "update"
          commands will simply re-copy the same data (no change in output.)
    
       5- Power Down DAC: Disable DAC output. Power supply current is reduced.
          DAC code present in DAC registers at time of shutdown are preserved.
    
       6- Set reference mode, either internal or external: Selecting external
          mode prompts for the external reference voltage, which is used directly
          if no individual DAC calibration is stored. The selection and entered
          voltage are stored to EEPROM so it is persistent across reset / power cycles.
    
       7- Calibrate DAC: Use a precision voltmeter to obtain and enter VOUT
          readings taken with different DAC codes. Set reference mode FIRST,
          as values are stored separately for internal and external reference
          mode. Entries are used to calculate the closest code to send to the
          DAC to achieve an entered voltage.
    
       8- Enable / Disable calibration: Switch between stored calibration
          values and defaults. Calibration parameters are stored separately for
          internal and external reference modes. Ideal calibration will be used
          if the calibration parameter valid key is not set.
    
    USER INPUT DATA FORMAT:
     decimal : 1024
     hex     : 0x400
     octal   : 02000  (leading 0 "zero")
     binary  : B10000000000
     float   : 1024.0
    
    @endverbatim
    
    http://www.linear.com/product/LTC2656
    
    http://www.linear.com/product/LTC2656#demoboards
    
    REVISION HISTORY
    $Revision: 4906 $
    $Date: 2016-04-07 15:56:58 -0700 (Thu, 07 Apr 2016) $
    
    Copyright (c) 2013, Linear Technology Corp.(LTC)
    All rights reserved.
    
    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions are met:
    
    1. Redistributions of source code must retain the above copyright notice, this
       list of conditions and the following disclaimer.
    2. Redistributions in binary form must reproduce the above copyright notice,
       this list of conditions and the following disclaimer in the documentation
       and/or other materials provided with the distribution.
    
    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
    ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
    WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
    DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
    ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
    (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
    ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
    (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
    SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
    
    The views and conclusions contained in the software and documentation are those
    of the authors and should not be interpreted as representing official policies,
    either expressed or implied, of Linear Technology Corp.
    
    The Linear Technology Linduino is not affiliated with the official Arduino team.
    However, the Linduino is only possible because of the Arduino team's commitment
    to the open-source community.  Please, visit http://www.arduino.cc and
    http://store.arduino.cc , and consider a purchase that will help fund their
    ongoing work.
     */
    
    /*! @file
        @ingroup LTC2656
    */
    
    #include <Arduino.h>
    #include <stdint.h>
    #include "Linduino.h"
    #include "LT_SPI.h"
    #include "UserInterface.h"
    #include "LT_I2C.h"
    #include "QuikEval_EEPROM.h"
    #include "LTC2656.h"
    #include <SPI.h>
    #include <Wire.h>
    
    #define EEPROM_CAL_KEY_INT  0x5678          //!< Calibration associated with internal reference
    #define EEPROM_CAL_KEY_EXT  0x9ABC          //!< Calibration associated with external reference
    
    // DAC Reference State
    // Could have been zero or 1, this allows you to use the
    // variable "reference_mode" as the command argument to a write
    #define REF_INTERNAL LTC2656_CMD_INTERNAL_REFERENCE   //!< Stored reference state is Internal
    #define REF_EXTERNAL LTC2656_CMD_EXTERNAL_REFERENCE   //!< Stored reference state is External
    
    // EEPROM memory locations
    #define STORED_REF_STATE_BASE     EEPROM_CAL_STATUS_ADDRESS     //!< Base address of the stored reference state
    #define INT_CAL_VALID_BASE        STORED_REF_STATE_BASE + 2     //!< Base address of the "internal ref calibration valid" flag
    #define INT_CAL_PARAMS_BASE       INT_CAL_VALID_BASE + 2        //!< Base address of the internal ref calibration parameters
    #define EXT_CAL_VALID_BASE        INT_CAL_PARAMS_BASE + 32      //!< Base address of the "external ref calibration valid" flag
    #define EXT_CAL_PARAMS_BASE       EXT_CAL_VALID_BASE + 2        //!< Base address of the external ref calibration parameters
    #define EXT_REF_V_BASE            EXT_CAL_PARAMS_BASE + 32      //!< Base address of the stored external reference voltage
    
    // Function Declaration
    int8_t restore_calibration();               // Read the DAC calibration from EEPROM, Return 1 if successful, 0 if not
    void store_calibration();                   // Store the DAC calibration to the EEPROM
    void print_title();                         // Print the title block
    void print_prompt(int16_t selected_dac);    // Prompt the user for an input command
    int16_t prompt_voltage_or_code();
    uint16_t get_voltage(float LTC2656_lsb, int16_t LTC2656_offset);
    uint16_t get_code();
    void calibrate_dac(uint8_t index);       // Calibrate the selected DAC using a voltmeter. The routine does a linear curve fit given two data points.
    
    void menu_1_select_dac(int16_t *selected_dac);
    void menu_2_write_to_input_register(int16_t selected_dac);
    void menu_3_write_and_update_dac(int16_t selected_dac);
    void menu_4_update_power_up_dac(int16_t selected_dac);
    void menu_5_power_down_dac(int16_t selected_dac);
    void menu_6_set_reference_mode();         // int, ext, if ext, prompt for voltage
    void menu_7_calibrate_dacs();
    void menu_8_enable_calibration();
    
    // Global variables
    static uint8_t demo_board_connected;   //!< Set to 1 if the board is connected
    static uint8_t shift_count = 0;        //!< The data align shift count. For 16-bit=0, for 12-bits=4
    static uint8_t reference_mode;         //!< Tells whether to set internal or external reference
    
    // Global calibration variables
    static float reference_voltage;        //!< Reference voltage, either internal or external
    static int16_t LTC2656_offset[9];      //!< DAC offset - index 8 for "all DACs"
    static float LTC2656_lsb[9];           //!< The LTC2656 lsb - index 8 for "all DACs"
    
    // Constants
    
    //! Lookup table for DAC address. Allows the "All DACs" address to be indexed right after DAC D in loops.
    //! This technique is very useful for devices with non-monotonic channel addresses.
    const uint8_t address_map[9] = {LTC2656_DAC_A, LTC2656_DAC_B, LTC2656_DAC_C, LTC2656_DAC_D, LTC2656_DAC_E,
                                    LTC2656_DAC_F, LTC2656_DAC_G, LTC2656_DAC_H,LTC2656_DAC_ALL
                                   };  //<! Map entered option 0..2 to DAC address
    
    //! Used to choose between voltage and code
    enum
    {
      PROMPT_VOLTAGE = 0, /**< 0 */
      PROMPT_CODE = 1     /**< 1 */
    };
    
    //! Initialize Linduino
    void setup()
    // Setup the program
    {
      char demo_name[] = "DC1397";  // Demo Board Name stored in QuikEval EEPROM
      quikeval_SPI_init();          // Configure the spi port for 4MHz SCK
      quikeval_SPI_connect();       // Connect SPI to main data port
      quikeval_I2C_init();          // Configure the EEPROM I2C port for 100kHz
      Serial.begin(115200);         // Initialize the serial port to the PC
      print_title();
      demo_board_connected = discover_demo_board(demo_name);
      if (demo_board_connected)
      {
        restore_calibration();
        print_prompt(0);
      }
    }
    
    //! Repeats Linduino loop
    void loop()
    {
      int16_t user_command;
      static  int16_t selected_dac = 0;     // The selected DAC to be updated (0=A, 1=B ... 5=All).  Initialized to "A".
      // The main control loop
      if (demo_board_connected)             // Do nothing if the demo board is not connected
      {
        if (Serial.available())             // Check for user input
        {
          user_command = read_int();        // Read the user command
          Serial.println(user_command);
          Serial.flush();
          switch (user_command)
          {
            case 1:
              menu_1_select_dac(&selected_dac);
              break;
            case 2:
              menu_2_write_to_input_register(selected_dac);
              break;
            case 3:
              menu_3_write_and_update_dac(selected_dac);
              break;
            case 4:
              menu_4_update_power_up_dac(selected_dac);
              break;
            case 5:
              menu_5_power_down_dac(selected_dac);
              break;
            case 6:
              menu_6_set_reference_mode(); // int, ext, if ext, prompt for voltage
              restore_calibration();
              break;
            case 7:
              menu_7_calibrate_dacs();
              restore_calibration();
              break;
            case 8:
              menu_8_enable_calibration();
              restore_calibration();
              break;
            default:
              Serial.println("Incorrect Option");
              break;
          }
          Serial.println("\n*****************************************************************");
          print_prompt(selected_dac);
        }
      }
    }
    
    // Function Definitions
    
    //! Select which DAC to operate on
    void menu_1_select_dac(int16_t *selected_dac)
    {
      // Select a DAC to operate on
      Serial.print("Select DAC to operate on (0=A, 1=B, 2=C, 3=D, 4=E, 5=F, 6=G, 7=H, 8=All)");
      *selected_dac = read_int();
      if (*selected_dac == 8)
        Serial.println("All");
      else
        Serial.println(*selected_dac);
    }
    
    //! Write data to input register, but do not update DAC output
    void menu_2_write_to_input_register(int16_t selected_dac)
    {
      uint16_t dac_code;
    
      if (prompt_voltage_or_code() == PROMPT_VOLTAGE)
        dac_code = get_voltage(LTC2656_lsb[selected_dac], LTC2656_offset[selected_dac]);
      else
        dac_code = get_code();
    
      LTC2656_write(LTC2656_CS, LTC2656_CMD_WRITE, address_map[selected_dac], dac_code << shift_count);
    }
    
    //!Write data to DAC register (which updates output immediately)
    void menu_3_write_and_update_dac(int16_t selected_dac)
    {
      uint16_t dac_code;
    
      if (prompt_voltage_or_code() == PROMPT_VOLTAGE)
        dac_code = get_voltage(LTC2656_lsb[selected_dac], LTC2656_offset[selected_dac]);
      else
        dac_code = get_code();
    
      LTC2656_write(LTC2656_CS, LTC2656_CMD_WRITE_UPDATE, address_map[selected_dac], dac_code << shift_count);
    }
    
    //! Update DAC with data that is stored in input register, power up if sleeping
    void menu_4_update_power_up_dac(int16_t selected_dac)
    {
      // Update DAC
      LTC2656_write(LTC2656_CS, LTC2656_CMD_UPDATE, address_map[selected_dac], 0x0000);
    }
    
    //! Power down DAC
    void menu_5_power_down_dac(int16_t selected_dac)
    {
      // Power down DAC
      LTC2656_write(LTC2656_CS, LTC2656_CMD_POWER_DOWN, address_map[selected_dac], 0x0000);
    }
    
    //! Set reference mode and store to EEPROM
    void menu_6_set_reference_mode(void) // int, ext, if ext, prompt for voltage
    {
      int16_t user_input;
      Serial.println("Select reference mode - 0 for Internal, 1 for External");
      user_input = read_int();
      if (user_input == 1)
      {
        reference_mode = REF_EXTERNAL;
        Serial.println("External reference mode; enter external reference voltage");
        reference_voltage = read_float();
        Serial.print(reference_voltage, 5);
        Serial.println("V");
        eeprom_write_float(EEPROM_I2C_ADDRESS, reference_voltage, EXT_REF_V_BASE);
      }
      else
      {
        reference_mode = REF_INTERNAL;
        Serial.println("Internal reference mode selected");
      }
      Serial.println("Writing reference mode to EEPROM\n\n");
      eeprom_write_byte(EEPROM_I2C_ADDRESS, reference_mode, STORED_REF_STATE_BASE);
    }
    
    //! Calibrate all DACs by measuring two known outputs
    void menu_7_calibrate_dacs()
    {
      // Calibrate the DACs using a multimeter
      uint8_t i;
      for (i = 0; i < 8; i++)
      {
        calibrate_dac(i);   // Run calibration routine
      }
      store_calibration();
    }
    
    //! Enable / Disable calibration. Use with caution - behavior is undefined if you enable calibration and an actual
    //! calibration cycle has not been performed.
    void menu_8_enable_calibration()
    {
      int16_t user_input;
      Serial.println(F("\n\nSelect option -  0: Enable Internal, 1: Disable Internal, 2: Enable External, 3: Disable External"));
      user_input = read_int();
      switch (user_input)
      {
        case 0:
          Serial.println(F("Enabling Internal Cal Params"));
          eeprom_write_int16(EEPROM_I2C_ADDRESS, EEPROM_CAL_KEY, INT_CAL_VALID_BASE);
          break;
        case 1:
          Serial.println(F("Disabling Internal Cal Params"));
          eeprom_write_int16(EEPROM_I2C_ADDRESS, 0x0000, INT_CAL_VALID_BASE);
          break;
        case 2:
          Serial.println(F("Enabling External Cal Params"));
          eeprom_write_int16(EEPROM_I2C_ADDRESS, EEPROM_CAL_KEY, EXT_CAL_VALID_BASE);
          break;
        case 3:
          Serial.println(F("Disabling External Cal Params"));
          eeprom_write_int16(EEPROM_I2C_ADDRESS, 0x0000, EXT_CAL_VALID_BASE);
          break;
      }
    }
    
    //! Read stored calibration parameters from nonvolatile EEPROM on demo board
    //! @return Return 1 if successful, 0 if not
    int8_t restore_calibration()
    // Read the DAC calibration from EEPROM
    // Return 1 if successful, 0 if not
    {
      int16_t intvalid, extvalid;
      uint8_t i, eeaddr;
      float dac_count;                                  // The number of codes, 4096 for 12 bits, 65536 for 16 bits
    
      Serial.println(F("\n\nReading Calibration parameters from EEPROM..."));
      float full_scale; // To avoid confusion - in internal ref mode, FS=Vref, in ext mode, FS=2*Vref
      // Read the cal keys from the EEPROM.
      eeprom_read_int16(EEPROM_I2C_ADDRESS, &intvalid, INT_CAL_VALID_BASE);
      eeprom_read_int16(EEPROM_I2C_ADDRESS, &extvalid, EXT_CAL_VALID_BASE);
      // Read the stored reference state
      eeprom_read_byte(EEPROM_I2C_ADDRESS, (char *) &reference_mode, STORED_REF_STATE_BASE);
      // Read external ref V unconditionally, overwrite with defaults if no cal found
      eeprom_read_float(EEPROM_I2C_ADDRESS, &reference_voltage, EXT_REF_V_BASE);
    
      if (reference_mode == REF_EXTERNAL)
      {
        Serial.println(F("Restored external ref. Voltage:"));
        Serial.println(reference_voltage, 5);
      }
      else  // EITHER reference is set to internal, OR not programmed in which case default to internal
      {
        reference_mode = REF_INTERNAL; // Redundant if already set
        Serial.println("Internal reference mode set");
      }
    
      // Write the reference mode to the DAC right away
      LTC2656_write(LTC2656_CS, reference_mode, 0x0F, 0x0000);
    
      // Set up default values, shift count, DAC count
      // Calibration parameters MAY be changed next, if match
      // between reference mode and stored calibration
      full_scale = reference_voltage * 2.0; // If external ref mode, this applies.
    
      // The following two IF statements are used to allow the program to run with
      // a QuikEval string that does not contain the demo board option.
      // If the demo board option is found then these values are overwritten.
      if (strcmp(demo_board.product_name, "LTC2656-L16") == 0)
      {
        // LTC2657CUF-L16, 16-bits, 2.5V full scale
        shift_count = 0;
        if (reference_mode == REF_INTERNAL) full_scale = 2.5;
        dac_count = 65536;
      }
      if (strcmp(demo_board.product_name, "LTC2656-H16") == 0)
      {
        // LTC2657CUF-H16, 16-bits, 4.096V full scale
        shift_count = 0;
        if (reference_mode == REF_INTERNAL) full_scale = 4.096;
        dac_count = 65536;
      }
    
      switch (demo_board.option)
      {
        case 'A':
          // LTC2656CUF-L16, 16-bits, 2.5V full scale
          shift_count = 0;
          if (reference_mode == REF_INTERNAL) full_scale = 2.5;
          dac_count = 65536;
          break;
        case 'B':
          // LTC2656CUF-H16, 16-bits, 4.096V full scale
          shift_count = 0;
          if (reference_mode == REF_INTERNAL) full_scale = 4.096;
          dac_count = 65536;
          break;
        case 'C':
          // LTC2656CUF-L12, 12-bits, 2.5V full scale
          shift_count = 4;
          if (reference_mode == REF_INTERNAL) full_scale = 2.5;
          dac_count = 4096;
          break;
        case 'D':
          // LTC2656CUF-H12, 12-bits, 4.096V full scale
          shift_count = 4;
          if (reference_mode == REF_INTERNAL) full_scale = 4.096;
          dac_count = 4096;
          break;
      }
    
      for (i = 0; i <= 8; i++)
      {
        LTC2656_offset[i] = 0;
        LTC2656_lsb[i] = full_scale / dac_count;
      }
    
      // Restore calibration IF reference mode matches stored calibration
      eeaddr = 0;   // Assume no calibration present or mismatch between cal and reference mode
    
      if ((intvalid == EEPROM_CAL_KEY) && (reference_mode == REF_INTERNAL))
      {
        eeaddr = INT_CAL_PARAMS_BASE;
        Serial.println(F("Found internal calibration, restoring...)"));
      }
      else if ((extvalid == EEPROM_CAL_KEY) && (reference_mode == REF_EXTERNAL))
      {
        eeaddr = EXT_CAL_PARAMS_BASE;
        Serial.println(F("Found external calibration, restoring...)"));
      }
      else Serial.println(F("Calibration not found for this\nreference mode, using ideal calibration"));
    
      if (eeaddr != 0) // If cal key was enabled and reference mode is correct, read offset and lsb
      {
        eeprom_read_int16(EEPROM_I2C_ADDRESS, &LTC2656_offset[0], eeaddr);
        eeprom_read_int16(EEPROM_I2C_ADDRESS, &LTC2656_offset[1], eeaddr + 2);
        eeprom_read_int16(EEPROM_I2C_ADDRESS, &LTC2656_offset[2], eeaddr + 4);
        eeprom_read_int16(EEPROM_I2C_ADDRESS, &LTC2656_offset[3], eeaddr + 6);
        eeprom_read_int16(EEPROM_I2C_ADDRESS, &LTC2656_offset[4], eeaddr + 8);
        eeprom_read_int16(EEPROM_I2C_ADDRESS, &LTC2656_offset[5], eeaddr + 10);
        eeprom_read_int16(EEPROM_I2C_ADDRESS, &LTC2656_offset[6], eeaddr + 12);
        eeprom_read_int16(EEPROM_I2C_ADDRESS, &LTC2656_offset[7], eeaddr + 14);
        eeprom_read_float(EEPROM_I2C_ADDRESS, &LTC2656_lsb[0], eeaddr + 16);
        eeprom_read_float(EEPROM_I2C_ADDRESS, &LTC2656_lsb[1], eeaddr + 20);
        eeprom_read_float(EEPROM_I2C_ADDRESS, &LTC2656_lsb[2], eeaddr + 24);
        eeprom_read_float(EEPROM_I2C_ADDRESS, &LTC2656_lsb[3], eeaddr + 28);
        eeprom_read_float(EEPROM_I2C_ADDRESS, &LTC2656_lsb[4], eeaddr + 32);
        eeprom_read_float(EEPROM_I2C_ADDRESS, &LTC2656_lsb[5], eeaddr + 36);
        eeprom_read_float(EEPROM_I2C_ADDRESS, &LTC2656_lsb[6], eeaddr + 40);
        eeprom_read_float(EEPROM_I2C_ADDRESS, &LTC2656_lsb[7], eeaddr + 44);
        LTC2656_offset[8] = LTC2656_offset[0]; // Copy cal value for DAC A to cal value for
        LTC2656_lsb[8] = LTC2656_lsb[0];       // DAC ALL
        Serial.println("Calibration Restored");
      }
      for (i=0; i<=8; ++i)
      {
        Serial.print("DAC ");
        Serial.print((char) ('A' + i));
        Serial.print(" offset: ");
        Serial.print(LTC2656_offset[i]);
        Serial.print(" , lsb: ");
        Serial.print(LTC2656_lsb[i]*1000, 4);
        Serial.println(" mV");
      }
      Serial.println("(DAC I applies to ALL DACs selections)");
      if (eeaddr != 0) return (1);
      return (0);
    }
    
    //! Store measured calibration parameters to nonvolatile EEPROM on demo board
    void store_calibration()
    // Store the DAC calibration to the EEPROM
    {
      uint8_t eeaddr;
      if (reference_mode == REF_INTERNAL)
      {
        eeprom_write_int16(EEPROM_I2C_ADDRESS, EEPROM_CAL_KEY, INT_CAL_VALID_BASE);
        eeaddr = INT_CAL_PARAMS_BASE;
      }
      else
      {
        eeprom_write_int16(EEPROM_I2C_ADDRESS, EEPROM_CAL_KEY, EXT_CAL_VALID_BASE);
        eeaddr = EXT_CAL_PARAMS_BASE;
      }
    
      eeprom_write_int16(EEPROM_I2C_ADDRESS, LTC2656_offset[0], eeaddr);        // Offset
      eeprom_write_int16(EEPROM_I2C_ADDRESS, LTC2656_offset[1], eeaddr + 2);
      eeprom_write_int16(EEPROM_I2C_ADDRESS, LTC2656_offset[2], eeaddr + 4);
      eeprom_write_int16(EEPROM_I2C_ADDRESS, LTC2656_offset[3], eeaddr + 6);
      eeprom_write_int16(EEPROM_I2C_ADDRESS, LTC2656_offset[4], eeaddr + 8);
      eeprom_write_int16(EEPROM_I2C_ADDRESS, LTC2656_offset[5], eeaddr + 10);
      eeprom_write_int16(EEPROM_I2C_ADDRESS, LTC2656_offset[6], eeaddr + 12);
      eeprom_write_int16(EEPROM_I2C_ADDRESS, LTC2656_offset[7], eeaddr + 14);
      eeprom_write_float(EEPROM_I2C_ADDRESS, LTC2656_lsb[0], eeaddr + 16);      // lsb
      eeprom_write_float(EEPROM_I2C_ADDRESS, LTC2656_lsb[1], eeaddr + 20);
      eeprom_write_float(EEPROM_I2C_ADDRESS, LTC2656_lsb[2], eeaddr + 24);
      eeprom_write_float(EEPROM_I2C_ADDRESS, LTC2656_lsb[3], eeaddr + 28);
      eeprom_write_float(EEPROM_I2C_ADDRESS, LTC2656_lsb[4], eeaddr + 32);      // lsb
      eeprom_write_float(EEPROM_I2C_ADDRESS, LTC2656_lsb[5], eeaddr + 36);
      eeprom_write_float(EEPROM_I2C_ADDRESS, LTC2656_lsb[6], eeaddr + 40);
      eeprom_write_float(EEPROM_I2C_ADDRESS, LTC2656_lsb[7], eeaddr + 44);
      Serial.println(F("Calibration Stored to EEPROM"));
    }
    
    //! Prompt user to enter a voltage or digital code to send to DAC
    int16_t prompt_voltage_or_code()
    {
      int16_t user_input;
      Serial.print(F("Type 1 to enter voltage, 2 to enter code:"));
      Serial.flush();
      user_input = read_int();
      Serial.println(user_input);
    
      if (user_input != 2)
        return(PROMPT_VOLTAGE);
      else
        return(PROMPT_CODE);
    }
    
    //! Get voltage from user input, calculate DAC code based on lsb, offset
    uint16_t get_voltage(float LTC2656_lsb, int16_t LTC2656_offset)
    {
      float dac_voltage;
    
      Serial.print(F("Enter Desired DAC output voltage: "));
      dac_voltage = read_float();
      Serial.print(dac_voltage);
      Serial.println(" V");
      Serial.flush();
      return(LTC2656_voltage_to_code(dac_voltage, LTC2656_lsb, LTC2656_offset));
    }
    
    //! Get code to send to DAC directly, in decimal, hex, or binary
    uint16_t get_code()
    {
      uint16_t returncode;
      Serial.println("Enter Desired DAC Code");
      Serial.print("(Format 32768, 0x8000, 0100000, or B1000000000000000): ");
      returncode = (uint16_t) read_int();
      Serial.print("0x");
      Serial.println(returncode, HEX);
      Serial.flush();
      return(returncode);
    }
    
    //! Prints the title block when program first starts.
    void print_title()
    {
      Serial.println("");
      Serial.println(F("*****************************************************************"));
      Serial.println(F("* DC1397 Demonstration Program                                  *"));
      Serial.println(F("*                                                               *"));
      Serial.println(F("* This program demonstrates how to send data to the LTC2656     *"));
      Serial.println(F("* quad 16/12-bit DAC found on the DC1397 demo board.            *"));
      Serial.println(F("*                                                               *"));
      Serial.println(F("* Set the baud rate to 115200 and select the newline terminator.*"));
      Serial.println(F("*                                                               *"));
      Serial.println(F("*****************************************************************"));
    }
    
    //! Prints main menu.
    void print_prompt(int16_t selected_dac)
    {
      Serial.println(F("\nCommand Summary:"));
      Serial.println(F("  1-Select DAC"));
      Serial.println(F("  2-Write to input register (no update)"));
      Serial.println(F("  3-Write and update DAC"));
      Serial.println(F("  4-Update / power up DAC"));
      Serial.println(F("  5-Power down DAC"));
      Serial.println(F("  6-Set reference mode"));
      Serial.println(F("  7-Calibrate DAC"));
      Serial.println(F("  8-Enable / Disable calibration"));
    
      Serial.println("\nPresent Values:");
      Serial.print("  Selected DAC: ");
      if (selected_dac != 8)
        Serial.println((char) (selected_dac + 0x41));
      else
        Serial.println("All");
    
      Serial.print("  DAC Reference: ");
      if (reference_mode == REF_INTERNAL)
        Serial.println("Internal");
      else
      {
        Serial.print(F("External "));
        Serial.print(reference_voltage, 5);
        Serial.println(F("V reference, please verify"));
      }
      Serial.print(F("Enter a command:"));
      Serial.flush();
    }
    
    //! Calibrate the selected DAC using a voltmeter. The routine
    //! does a linear curve fit given two data points.
    void calibrate_dac(uint8_t index)
    {
      uint16_t code1 = 0x0200;                            //! Calibration code 1
      uint16_t code2 = 0xFFFF;                            //! Calibration code 2
      float voltage1;                                     //! Calibration voltage 1
      float voltage2;                                     //! Calibration voltage 2
      Serial.println("");
      Serial.print("Calibrating DAC ");
      Serial.println((char) (0x41 + index));
      // Left align 12-bit code1 to 16 bits & write to DAC
      LTC2656_write(LTC2656_CS, LTC2656_CMD_WRITE_UPDATE, index, code1 << shift_count);
      Serial.print("DAC code set to 0x");
      Serial.println(code1, HEX);
      Serial.print("Enter measured DAC voltage:");
      voltage1 = read_float();
      Serial.print(voltage1, 6);
      Serial.println(" V");
      // Left align 12-bit code2 to 16 bits & write to DAC
      LTC2656_write(LTC2656_CS, LTC2656_CMD_WRITE_UPDATE, index, code2 << shift_count);
      Serial.print("DAC code set to 0x");
      Serial.println(code2, HEX);
      Serial.print("Enter measured DAC voltage:");
      voltage2 = read_float();
      Serial.print(voltage2, 6);
      Serial.println(" V");
      LTC2656_calibrate(code1, code2, voltage1, voltage2, &LTC2656_lsb[index], &LTC2656_offset[index]);
    }
    

    Download LTC2656 Linduino .CPP File

    /*!
    LTC2656: Octal SPI 16-/12-Bit Rail-to-Rail DACs with 10ppm/C Max Reference
    
    @verbatim
    
    The LTC2656 is a family of octal 16-/12-bit rail-to-rail DACs with a precision
    integrated reference. The DACs have built-in high performance, rail-to-rail,
    output buffers and are guaranteed monotonic. The LTC2656-L has a full-scale
    output of 2.5V with the integrated 10ppm/C reference and operates from a single
    2.7V to 5.5V supply. The LTC2656-H has a full-scale output of 4.096V with the
    integrated reference and operates from a 4.5V to 5.5V supply. Each DAC can also
    operate with an external reference, which sets the DAC full-scale output to two
    times the external reference voltage.
    
    These DACs communicate via a SPI/MICROWIRE compatible 4-wire serial interface
    which operates at clock rates up to 50MHz. The LTC2656 incorporates a power-on
    reset circuit that is controlled by the PORSEL pin. If PORSEL is tied to GND the
    DACs reset to zero-scale. If PORSEL is tied to VCC, the DACs reset to mid-scale.
    
    @endverbatim
    
    http://www.linear.com/product/LTC2656
    
    http://www.linear.com/product/LTC2656#demoboards
    
    REVISION HISTORY
    $Revision: 3659 $
    $Date: 2015-07-01 10:19:20 -0700 (Wed, 01 Jul 2015) $
    
    Copyright (c) 2013, Linear Technology Corp.(LTC)
    All rights reserved.
    
    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions are met:
    
    1. Redistributions of source code must retain the above copyright notice, this
       list of conditions and the following disclaimer.
    2. Redistributions in binary form must reproduce the above copyright notice,
       this list of conditions and the following disclaimer in the documentation
       and/or other materials provided with the distribution.
    
    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
    ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
    WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
    DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
    ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
    (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
    ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
    (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
    SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
    
    The views and conclusions contained in the software and documentation are those
    of the authors and should not be interpreted as representing official policies,
    either expressed or implied, of Linear Technology Corp.
    
    The Linear Technology Linduino is not affiliated with the official Arduino team.
    However, the Linduino is only possible because of the Arduino team's commitment
    to the open-source community.  Please, visit http://www.arduino.cc and
    http://store.arduino.cc , and consider a purchase that will help fund their
    ongoing work.
    */
    
    //! @defgroup LTC2656 LTC2656: Octal SPI 16-/12-Bit Rail-to-Rail DACs with 10ppm/C Max Reference
    
    /*! @file
        @ingroup LTC2656
        Library for LTC2656 Octal SPI 16-/12-Bit Rail-to-Rail DACs with 10ppm/C Max Reference
    */
    
    #include <stdint.h>
    #include <math.h>
    #include <Arduino.h>
    #include "Linduino.h"
    #include "LT_SPI.h"
    #include "LTC2656.h"
    #include <SPI.h>
    
    void LTC2656_write(uint8_t cs, uint8_t dac_command, uint8_t dac_address, uint16_t dac_code)
    // Write the 16-bit dac_code to the LTC2656
    {
      uint8_t data_array[3], rx_array[3];
      LT_union_int16_2bytes data;
    
      data.LT_uint16 = dac_code;
    
      data_array[2] = dac_command | dac_address;
      data_array[1] = data.LT_byte[1];
      data_array[0] =data.LT_byte[0];
    
      spi_transfer_block(cs, data_array, rx_array, (uint8_t) 3);
    }
    
    uint16_t LTC2656_voltage_to_code(float dac_voltage, float LTC2656_lsb, int16_t LTC2656_offset)
    // Calculate a LTC2656 DAC code given the desired output voltage and DAC address (0-7)
    {
      int32_t dac_code;
      float float_code;
      float_code = dac_voltage / LTC2656_lsb;                                                             //! 1) Calculate the DAC code
      float_code = (float_code > (floor(float_code) + 0.5)) ? ceil(float_code) : floor(float_code);       //! 2) Round
      dac_code =(int32_t)float_code - LTC2656_offset;                                                     //! 3) Subtract offset
      if (dac_code < 0)                                                                                   //! 4) If DAC code < 0, Then DAC code = 0
        dac_code = 0;
      return ((uint16_t)dac_code);                                                                        //! 5) Cast DAC code as uint16_t
    }
    
    float LTC2656_code_to_voltage(uint16_t dac_code, float LTC2656_lsb, int16_t LTC2656_offset)
    // Calculate the LTC2656 DAC output voltage given the DAC code and DAC address (0-7)
    {
      float dac_voltage;
      dac_voltage = ((float)(dac_code + LTC2656_offset)* LTC2656_lsb);                                    //! 1) Calculates the dac_voltage
      return (dac_voltage);
    }
    
    void LTC2656_calibrate(uint16_t dac_code1, uint16_t dac_code2, float voltage1, float voltage2, float *LTC2656_lsb, int16_t *LTC2656_offset)
    // Calculate the LTC2656 offset and LSB voltage given two measured voltages and their corresponding codes
    {
      float temp_offset;
      *LTC2656_lsb = (voltage2 - voltage1) / ((float) (dac_code2 - dac_code1));                           //! 1) Calculate the LSB
      temp_offset = voltage1/(*LTC2656_lsb) - (float)dac_code1;                                           //! 2) Calculate the offset
      temp_offset = (temp_offset > (floor(temp_offset) + 0.5)) ? ceil(temp_offset) : floor(temp_offset);  //! 3) Round offset
      *LTC2656_offset = (int16_t)temp_offset;                                                             //! 4) Cast as int16_t
    }
    

    Download LTC2656 Linduino Header File

    /*!
    LTC2656: Octal SPI 16-/12-Bit Rail-to-Rail DACs with 10ppm/C Max Reference
    
    @verbatim
    
    The LTC2656 is a family of octal 16-/12-bit rail-to-rail DACs with a precision
    integrated reference. The DACs have built-in high performance, rail-to-rail,
    output buffers and are guaranteed monotonic. The LTC2656-L has a full-scale
    output of 2.5V with the integrated 10ppm/C reference and operates from a single
    2.7V to 5.5V supply. The LTC2656-H has a full-scale output of 4.096V with the
    integrated reference and operates from a 4.5V to 5.5V supply. Each DAC can also
    operate with an external reference, which sets the DAC full-scale output to two
    times the external reference voltage.
    
    These DACs communicate via a SPI/MICROWIRE compatible 4-wire serial interface
    which operates at clock rates up to 50MHz. The LTC2656 incorporates a power-on
    reset circuit that is controlled by the PORSEL pin. If PORSEL is tied to GND the
    DACs reset to zero-scale. If PORSEL is tied to VCC, the DACs reset to mid-scale.
    
    SPI DATA FORMAT (MSB First):
    
    24-Bit Load Sequence:
    
                 Byte #1                   Byte #2                         Byte #3
                 Command                   MSB                             LSB
    LTC2656-16 : C3 C2 C1 C0 A3 A2 A1 A0   D15 D14 D13 D12 D11 D10 D9 D8   D7 D6 D5 D4 D3 D2 D1 D0
    LTC2656-12 : C3 C2 C1 C0 A3 A2 A1 A0   D11 D10 D9  D8  D7  D6  D5 D4   D3 D2 D1 D0 X  X  X  X
    
    32-Bit Load Sequence:
    
                 Byte #1                   Byte #2                   Byte #3                         Byte #4
                 Command                   MSB                                                       LSB
    LTC2656-16 : X  X  X  X  X  X  X  X    C3 C2 C1 C0 A3 A2 A1 A0   D15 D14 D13 D12 D11 D10 D9 D8   D7 D6 D5 D4 D3 D2 D1 D0
    LTC2656-12 : X  X  X  X  X  X  X  X    C3 C2 C1 C0 A3 A2 A1 A0   D11 D10 D9  D8  D7  D6  D5 D4   D3 D2 D1 D0 X  X  X  X
    
    Cx   : DAC Command Code
    Ax   : DAC Address (0=DACA, 1=DACB, 2=DACC, 3=DACD, 4=DACE, 5=DACF, 6=DACG, 7=DACH, 0xFF=All DACs)
    Dx   : DAC Data Bits
    X    : Don't care
    
    
    Example Code:
    
    Set DAC A to to 2V for 16-bit DAC.
    
        shift_count = 0;    // 16-bit DAC does not have to be shifted
        dac_voltage = 2.0;  // Sets dac voltage variable to 2v
    
        dac_code = LTC2656_voltage_to_code(dac_voltage, LTC2656_lsb, LTC2656_offset);       // Calculate DAC code from voltage, lsb, and offset
        LTC2656_write(LTC2656_CS, LTC2656_CMD_WRITE_UPDATE, LTC2656_DAC_A, dac_code << shift_count);    // Set DAC A with DAC code
    
    @endverbatim
    
    http://www.linear.com/product/LTC2656
    
    http://www.linear.com/product/LTC2656#demoboards
    
    REVISION HISTORY
    $Revision: 3659 $
    $Date: 2015-07-01 10:19:20 -0700 (Wed, 01 Jul 2015) $
    
    Copyright (c) 2013, Linear Technology Corp.(LTC)
    All rights reserved.
    
    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions are met:
    
    1. Redistributions of source code must retain the above copyright notice, this
       list of conditions and the following disclaimer.
    2. Redistributions in binary form must reproduce the above copyright notice,
       this list of conditions and the following disclaimer in the documentation
       and/or other materials provided with the distribution.
    
    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
    ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
    WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
    DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR
    ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
    (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
    ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
    (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
    SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
    
    The views and conclusions contained in the software and documentation are those
    of the authors and should not be interpreted as representing official policies,
    either expressed or implied, of Linear Technology Corp.
    
    The Linear Technology Linduino is not affiliated with the official Arduino team.
    However, the Linduino is only possible because of the Arduino team's commitment
    to the open-source community.  Please, visit http://www.arduino.cc and
    http://store.arduino.cc , and consider a purchase that will help fund their
    ongoing work.
    */
    
    /*! @file
        @ingroup LTC2656
        Header for LTC2656 Octal SPI 16-/12-Bit Rail-to-Rail DACs with 10ppm/C Max Reference
    */
    
    
    #ifndef LTC2656_H
    #define LTC2656_H
    
    #include <SPI.h>
    
    //! Define the SPI CS pin
    #ifndef LTC2656_CS
    #define LTC2656_CS QUIKEVAL_CS
    #endif
    
    //! @name LTC2656 Command Codes
    //! @{
    //! OR'd together with the DAC address to form the command byte
    #define  LTC2656_CMD_WRITE               0x00  //!< Write to input register n
    #define  LTC2656_CMD_UPDATE              0x10  //!< Update (power up) DAC register n
    #define  LTC2656_CMD_WRITE_UPDATE_ALL    0x20  //!< Write to input register n, update (power up) all
    #define  LTC2656_CMD_WRITE_UPDATE        0x30  //!< Write to input register n, update (power up) all
    #define  LTC2656_CMD_POWER_DOWN          0x40  //!< Power down n
    #define  LTC2656_CMD_POWER_DOWN_ALL      0x50  //!< Power down chip (all DACs and reference)
    #define  LTC2656_CMD_INTERNAL_REFERENCE  0x60  //!< Select internal reference (power up reference)
    #define  LTC2656_CMD_EXTERNAL_REFERENCE  0x70  //!< Select external reference (power down internal reference)
    #define  LTC2656_CMD_NO_OPERATION        0xF0  //!< No operation
    //! @}
    
    //! @name LTC2656 DAC Addresses
    //! @{
    //! Which DAC to operate on
    #define  LTC2656_DAC_A     0x00
    #define  LTC2656_DAC_B     0x01
    #define  LTC2656_DAC_C     0x02
    #define  LTC2656_DAC_D     0x03
    #define  LTC2656_DAC_E     0x04
    #define  LTC2656_DAC_F     0x05
    #define  LTC2656_DAC_G     0x06
    #define  LTC2656_DAC_H     0x07
    #define  LTC2656_DAC_ALL   0x0F
    //! @}
    
    //! Write the 16-bit dac_code to the LTC2656
    //! @return Void
    void LTC2656_write(uint8_t cs,                            //!< Chip Select Pin
                       uint8_t dac_command,                   //!< Command Nibble, left-justified, lower nibble set to zero
                       uint8_t dac_address,                   //!< DAC Address Nibble, right justified, upper nibble set to zero
                       uint16_t dac_code                      //!< 16-bit DAC code
                      );
    
    //! Calculate a LTC2656 DAC code given the desired output voltage and DAC address (0-7)
    //! @return @return The 16-bit code to send to the DAC
    uint16_t LTC2656_voltage_to_code(float dac_voltage,       //!< Voltage to send to DAC
                                     float LTC2656_lsb,       //!< LSB value (volts)
                                     int16_t LTC2656_offset   //!< Offset (volts)
                                    );
    
    //! Calculate the LTC2656 DAC output voltage given the DAC code, offset, and LSB value
    //! @return Calculated voltage
    float LTC2656_code_to_voltage(uint16_t dac_code,          //!< DAC code
                                  float LTC2656_lsb,          //!< LSB value (volts)
                                  int16_t LTC2656_offset        //!< Offset (volts)
                                 );
    
    //! Calculate the LTC2656 offset and LSB voltage given two measured voltages and their corresponding codes
    //! @return Void
    void LTC2656_calibrate(uint16_t dac_code1,                //!< First DAC code
                           uint16_t dac_code2,                //!< Second DAC code
                           float voltage1,                    //!< First voltage
                           float voltage2,                    //!< Second voltage
                           float *LTC2656_lsb,                //!< Returns resulting LSB (volts)
                           int16_t *LTC2656_offset            //!< Returns resulting Offset (volts)
                          );
    
    #endif  // LTC2656_H
    

    Technical Support