Table 8-2 and Table 8-3 show the typical signal-to-noise (SNR) and total harmonic distortion (THD) of the PGA854 driving the ADS127Lx1 delta-sigma ADC using a sinc4 or wideband filter. A 1kHz differential signal is applied to the input. The signal amplitude is adjusted to produce a PGA854 output at –0.2dBFS of the ADC full-scale range. For a list of the equivalent input voltage amplitude signals for the different PGA854 gain configurations, see Table 8-2 and Table 8-3. At gain = 1V/V, the design achieves –94dB THD and 80.2dB SNR for Sinc4 filter.

The R-C-R differential low-pass filter at the input of the instrumentation amplifier helps reduce EMI/RFI high-frequency extrinsic noise. This filter is customizable per the bandwidth and application requirements. This design example (see Figure 8-9) suggests a filter with the capacitor ratio of C_{IN_DIFF} = 10 × C_{IN_CM}. Using the 10:1 ratio for differential capacitor C_{IN_DIFF} versus common-mode capacitors C_{IN_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, C_FB, 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, C_FB 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 R_FIL = 47.4Ω, C_DIFF = 470pF, and C_CM = 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 (C_{IN_DIFF,} C_{IN_CM,} C_FB, C_DIFF, C_CM) 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.