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 can be customized per the bandwidth and application requirements. This

design example (see Figure 9-6) suggests a filter with the capacitor ratio of CIN_DIFF = 10 × CIN_CM. Using the

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

and common-mode noise rejection, and this arrangement tends to be less sensitive to the tolerance variation

and mismatch of the filter capacitors.

The feedback capacitor, CFB, is in parallel with the PGA855 output-stage 5-kΩ feedback resistors to implement

additional noise filtering. The internal resistors have ±15 % absolute resistance variation, and this variation must

be taken in to account when implementing noise filtering. In this example, CFB is set to 25 pF, providing a typical

f–3dB corner frequency of 1 MHz. The estimated minimum f–3dB corner frequency for this circuit is approximately

938 kHz 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 can degrade because of incomplete amplifier settling. The ADC input filter values are

RFIL = 47.4 Ω, CDIFF = 560 pF, and CCM = 51 pF. 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.

9.2.1.3 Application Curves

Frequency (Hz)

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

100

1000

10000

10000

G = 1 V/V, fIN = 1 kHz, SNR = 109.6 dB, THD = –121.4 dB

Figure 9-7. Performance FFT Plots With

ADS127L11, OSR = 64, Sinc4 Filter

Frequency (Hz)

-220

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

100

1000

10000

10000

G = 1 V/V, fIN = 1 kHz, SNR = 107.6 dB, THD = –121.4 dB

Figure 9-8. Performance FFT Plots With

ADS127L11, OSR = 64, Wideband Filter

PGA855

SBOSAE0B – APRIL 2023 – REVISED SEPTEMBER 2023

www.ti.com

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