Datasheet

−105
−100
−95
−90
−85
5 10 15 20 25
C
FILTER
(nF)
THD (dB)
G025
PgaP
R
150kW
2
R
60k
(forGain=6)
W
1
R
150kW
2
FromMuxP
PgaN
FromMuxN
R =2kW
S
R =2kW
S
C
4.7nF
FILTER
PGA1P
PGA1N
C
P1
C
P2
ADS1291
ADS1292
ADS1292R
www.ti.com SBAS502B –DECEMBER 2011–REVISED SEPTEMBER 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 kΩ 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 R
S
= 2 kΩ and an external capacitor C
FILTER
(4.7 nF, typical). This filter acts as an anti-aliasing filter with the –3-dB
bandwidth of 8.4 kHz. The internal R
S
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 C
EXT
that can be
used is 4 nF. A larger value C
FILTER
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 C
FILTER
value for a 10-Hz input signal.
Figure 24. THD versus C
FILTER
Value
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Product Folder Links: ADS1291 ADS1292 ADS1292R