  We present a method for precise monitoring of the loop gain of transition
edge sensors (TES) under electrothermal feedback. The measurement is
implemented on the ICE DfMux electronics and operates simultaneously with
Digital Active Nulling (DAN). It uses one additional bias sinusoid per TES and
does not require any additional readout channels. The loop gain monitor is
being implemented on the Simons Array and is an integral part of the baseline
calibration strategy for the upcoming LiteBIRD satellite.
