The right leg drive (RLD) circuitry is used as a means to counter the common-mode interference in a ECG system as a result of power lines and other sources, including fluorescent lights. The RLD circuit senses the common-mode of a selected set of electrodes and creates a negative feedback loop by driving the body with an inverted common-mode signal. The negative feedback loop restricts the common-mode movement to a narrow range, depending on the loop gain. Stabilizing the entire loop is specific to the individual user system based on the various poles in the loop. The ADS1291, ADS1292, and ADS1292R integrates the muxes to select the channel and an operational amplifier. All the amplifier terminals are available at the pins, allowing the user to choose the components for the feedback loop. The circuit shown in Figure 40 shows the overall functional connectivity for the RLD bias circuit.
The reference voltage for the right leg drive can be chosen to be internally generated (AVDD + AVSS) / 2 or it can be provided externally with a resistive divider. The selection of an internal versus external reference voltage for the RLD loop is defined by writing the appropriate value to the RLDREF_INT bit in the RESP2 register.
If the RLD function is not used, the amplifier can be powered down using the PDB_RLD bit. This bit is also used in daisy-chain mode to power-down all but one of the RLD amplifiers.
The functionality of the RLDIN pin is explained in the Input Multiplexer section.