REF5045 datasheet(44/62 Pages) TI2

44 / 62 page

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C004

ADS8881

SBAS547D – MAY 2013 – REVISED AUGUST 2015

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The input driver circuit uses extremely low-power, dual amplifiers (such as the OPA2333) with a maximum

quiescent current of 28 µA per channel to drive the ADC inputs. The input amplifiers are configured in a modified

unity-gain buffer configuration. The filter capacitor at the ADC inputs attenuates the sampling charge-injection

noise from the ADC but effects the stability of the input amplifiers by degrading the phase margin. This

attenuation requires a series isolation resistor to maintain amplifier stability. The value of the series resistor is

directly proportional to the open-loop output impedance of the driving amplifier to maintain stability, which is high

(in the order of kΩ) in the case of low-power amplifiers such as the OPA333. Therefore, a high value of 1 kΩ is

selected for the series resistor at the ADC inputs. However, this series resistor creates an additional voltage drop

in the signal path, thereby leading to linearity and distortion issues. The dual-feedback configuration used in

Figure 72 corrects for this additional voltage drop and maintains system performance at ultralow-power

consumption.

11.2.4.3 Application Curve

Vdiff close to 0 V, 32768 data points, standard deviation = 1.7 bits,

ENOB (dc) = 16.3 bits

Figure 73. DC Input Histogram

For step-by-step design procedure, circuit schematics, bill of materials, PCB files, simulation results, and test

results, see 18-Bit, 10kSPS Data Acquisition (DAQ) Block Optimized for Ultra Low Power < 1mW (SLAU514).

REF5045 Datasheet(PDF) 44 Page - Texas Instruments