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LTC6258 Low Noise Reference Use LT6656 for a Low Current Starting Reference

Figure 5a. Low Noise Reterence Use LT6656 for a Low current Starting Reterence

The LT6656 is a 1µA precision series voltage reference. Yet with low power comes low drive current capability and higher noise. The LTC6259 can be used as a buffer that follows a filter to enhance the utilization of the LT6656 in low power applications. Figure 5a shows such a configuration. First a very low cutoff frequency follows the LT6656 output (RIN1 and CIN1, lower than 5Hz cutoff). Choice of filter resistor RIN1 is such that the bias current in the LTC6259, multiplied by the resistance value, is lower than the nominal offset voltage of the op amp. CIN1 can be larger or smaller, with more or less filtering accordingly. The voltage withstanding requirement of CIN1 is low, resulting in large capacitance in a small volume.

This circuit takes advantage of the ability of the LTC6259 to drive large capacitive loads. Use of a large output capacitor attached to the LT6659 enables significant bypassing of follow-on circuits that use the reference voltage. In total, the combination of LT6656 and LTC6259, in this configuration, develops a reference voltage, with low noise, at low power, and with appreciably large available bypass capacitance.

Voltage spectral noise densities are shown in Figure 5b. Larger noise from the reference below 10kHz noticeably drops down once a filter (RIN1 and CIN1) follow the reference. The op amp, configured in unity gain, with a large 44µF load, remains stable and contributes only a small amount of low frequency noise. Figure 5c shows the transient response of the combination of RIN1-CIN1 filter and op amp circuit, with and without the 44µF output capacitor.

Figure 5b. Noise Density, Reference Buffer

Figure 5c shows the time domain response of the reference buffer.

Figure 5c. Reference Butter Transient Response

The total measured supply current consumption is 21µA.

The LTC®6258/LTC6259/LTC6260 are single/dual/quad operational amplifiers with low noise, low power, low supply voltage, and rail-to-rail inputs and outputs. They are unity gain stable with or without capacitive loads. They feature 1.3MHz gain-bandwidth product, 0.24V/μs slew rate while consuming only 20μA of supply current per amplifier operating on supply voltages ranging from 1.8V to 5.25V. The combination of low supply current, low supply voltage, high gain bandwidth product and low noise makes the LTC6258 family unique among rail-to-rail input/output op amps with similar supply current. These operational amplifiers are ideal for power efficient applications.




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