[FAQ] ADS129x: How do I calculate the DC gain of the RLD or BIAS amplifier?

In the ADS129x family of bio-potential ADCs, each electrode input can be individually selected to derive the common-mode cancellation signal at the RLD/BIAS amplifier output. In the ADS1292, ADS1298, and ADS1299, these signals are connected to the amplifier's inverting input through a series resistor to form a summing junction. Below is an example from the ADS1298 datasheet:

FIGURE 1: ADS1298 RLD Block Diagram

The DC gain of the RLD/BIAS amplifier is then calculated by dividing the feedback resistor by the parallel combination of resistors connected to the summing junction. If 3 electrode inputs are selected (i.e. RA, LA, and LL), the gain would be calculated as:

In the ADS1293, the summing junction is buffered by the CM Detect circuit before connecting to the RLD amplifier via an external resistor. This allows you set a fixed DC gain for the RLD amplifier regardless of the number of electrodes selected for common-mode derivation. In the figure below, the RLD amplifier gain at DC is set by R1 and R2  (suggested values are 100 kΩ and 1 MΩ, respectively).

FIGURE 2: ADS1293 RLD Block Diagram

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This post applies to ADS1191, ADS1192, ADS1291, ADS1292, ADS1292R, ADS1293, ADS1294, ADS1294R, ADS1296, ADS1296R, ADS1298,

 ADS1298R, ADS1299-4, ADS1299-6, and ADS1299.