ADS1292 datasheet(21/69 Pages) TI

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PgaP

150k

60k

(forGain=6)

150k

FromMuxP

PgaN

FromMuxN

R =2k

4.7nF

FILTER

PGA1P

PGA1N

−105

−100

−95

−90

−85

CFILTER (nF)

G025

ADS1291

ADS1292

ADS1292R

www.ti.com

SBAS502A – DECEMBER 2011 – REVISED MARCH 2012

PGA SETTINGS AND INPUT RANGE

The PGA is a differential input or differential output amplifier, as shown in Figure 23. It has seven gain settings

(1, 2, 3, 4, 6, 8, and 12) that can be set by writing to the CHnSET register (see the CH1SET and CH2SET

Registers in the Register Map section for details). The ADS1291, ADS1292, and ADS1292R have CMOS inputs

and hence have negligible current noise.

Figure 23. PGA Implementation

The PGA resistor string that implements the gain has 360 k

Ω of resistance for a gain of 6. This resistance

provides a current path across the outputs of the PGA in the presence of a differential input signal. This current

is in addition to the quiescent current specified for the device in the presence of a differential signal at the input.

The PGA output is filtered by an RC filter before it goes to the ADC. The filter is formed by an internal resistor RS

= 2 k

Ω and an external capacitor CFILTER (4.7 nF, typical). This filter acts as an anti-aliasing filter with the –3-dB

bandwidth of 8.4 kHz. The internal RS resistor is accurate to 15% so actual bandwidth will vary. This RC filter

also suppresses the glitch at the PGA output caused by ADC sampling. The minimum value of CEXT that can be

used is 4 nF. A larger value CFILTER capacitor can be used for increased attenuation at higher frequencies for

anti-aliasing purposes. If channel 1 of the ADS1292R is used for respiration measurement, then a 4.7-nF

external capacitor is recommended. The tradeoff is that a larger capacitor value gives degraded THD

performance. See Figure 24 for a diagram explaining the THD versus CFILTER value for a 10-Hz input signal.

Figure 24. THD versus CFILTER Value

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