the 10:1 ratio for differential capacitor CIN_DIFF versus common-mode capacitors CIN_CM offers good differential

and common-mode noise rejection. This arrangement tends to be less sensitive to the tolerance variation and

mismatch of the filter capacitors. in the measurement above a narrow-band input filter is used to limit the noise

and harmonics from the source waveform generator.

The feedback capacitor, CFB, is in parallel with the PGA854 output-stage 6.67kΩ feedback resistors to help

implement additional noise filtering. The internal resistors have ±15% absolute resistance variation; take this

variation into account when implementing noise filtering. In this example, CFB is set to 680pF, providing a typical

f–3dB corner frequency of 35kHz. The estimated minimum f–3dB corner frequency for this circuit is approximately

30kHz when accounting for the feedback-resistor variation.

The filter at the ADS127Lx1 inputs works as a charge reservoir to filter the sampled input of the ADC. The

charge reservoir reduces the instantaneous charge demand of the amplifier, maintaining low distortion and low

gain error that otherwise potentially degrade because of incomplete amplifier settling. The ADC input filter values

are RFIL = 47.4Ω, CDIFF = 470pF, and CCM = 47pF. The ADC input precharge buffers significantly reduce the

sample-phase input charge that raises the ADC input impedance to decrease gain error.

High-grade COG (NPO) are used everywhere in the signal path (CIN_DIFF, CIN_CM, CFB, CDIFF, CCM) for

low distortion. Among ceramic surface-mount capacitors, COG (NPO) ceramic capacitors provide the best

capacitance accuracy. The type of dielectric used in COG (NPO) ceramic capacitors provides the most stable

electrical properties over voltage, frequency, and temperature changes.

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PGA854

SBOSAN2A – AUGUST 2025 – REVISED DECEMBER 2025

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