The R-C-R differential low-pass filter at the input of the PGA helps reduce EMI/RFI high frequency extrinsic

noise. This filter can be customized per the bandwidth and application requirements.

Two first-order filters are implemented with the PGA855 circuit. The first filter is provided by CFB in parallel with

the PGA 5

‑kΩ feedback resistors. The PGA resistors are ±15% absolute tolerance, such as, consider the effect

of the tolerance on the filter cutoff frequency, the filter frequency changes to 126 kHz. At this tolerance, the filter

maintains –0.1 dB flatness to 24 kHz.

There is flexibility of modifying the CFB capacitor value to adjust bandwidth, with the trade-off on the broadband

noise of the circuit.

The second filter placed directly at the ADS8900B 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 that otherwise can degrade because of incomplete amplifier settling. The RC filter combination

(RFIL, CDIFF) is optimized for the SAR ADC sample and hold settling. This combination reduces nonlinear charge

kickback of the SAR ADC and is optimized for best THD performance. This combination allows for the best

trade-off between harmonic distortion while maintaining stability of the PGA output stage.

High-grade C0G (NPO) are used everywhere in the signal path (CIN_DIFF, CIN_CM, CFB, CDIFF, CCM) for the low

distortion properties.

The results are shown in Table 9-5, which includes the typical signal-to-noise ratio (SNR) and total harmonic

distortion (THD) of the PGA855 driving the ADS8900B SAR ADC. For the SNR and THD measurements, a

1-kHz differential signal is applied. The signal amplitude is adjusted to produce a PGA855 output at –0.5 dBFS

of the ADC full-scale range. Table 9-5 shows the equivalent input voltage amplitude signal for different PGA855

gain configurations. At gain = 1 V/V, the design achieves a –121.4

‑dB THD and 101.2‑dB SNR.

Table 9-5. PGA855 and ADS8900B FFT Data Summary: fSAMPLE = 1 MSPS, fIN = 1 kHz

PGA GAIN (V/V)

INPUT AMPLITUDE

(VPP)

ADC SIGNAL POWER

(dBFS)

SNR (dB)

THD (dB)

ENOB (Bits)

0.125

40.10

–6.0

95.9

–118.2

15.6

0.25

36.48

–0.8

101.0

–118.6

16.5

0.5

18.24

–0.8

101.2

–121.0

16.5

1

9.12

–0.8

101.2

–121.7

16.5

2

4.56

–0.8

100.5

–121.6

16.4

4

2.28

–0.8

99.5

–121.3

16.2

8

1.14

–0.8

97.4

–119.4

15.9

16

0.58

–0.8

93.6

–117.3

15.2

9.2.2.3 Application Curves

Frequency (Hz)

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

100

1000

10000

100000

Gain = 1 V/V, fSAMPLE = 1 MSPS, fIN = 1 kHz

Figure 9-10. Performance FFT Plot With ADS8900B

Frequency (Hz)

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

100

1000

10000

100000

Gain = 4 V/V, fSAMPLE = 1 MSPS, fIN = 1 kHz

Figure 9-11. Performance FFT Plot With ADS8900B

PGA855

SBOSAE0B – APRIL 2023 – REVISED SEPTEMBER 2023

www.ti.com

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