The circuit uses the LTC1043 in a variable gain amplifier which features continuously variable gain, gain stability of 20 ppm/°C, and single-ended or differential inputs. The circuit uses two separate LTC1043s. LTC1043B is continuously clocked by a 1-kHz source, which could also be processor supplied. Both LTC1043s function as the sampled data equivalent of a resistor within the bandwidth set by Al`s 0.01-I`F value and the switched-capacitor equivalent feedback resistor.
Wide-range-digitally-controlled-variable-gain-amplifier - schematic

The time-averaged current delivered to the summing point by LTC1043A is a function of the 0.01-I`F capacitor`s input-derived voltage and the commutation frequency at pin 16. Low-commutation frequencies result in small time-averaged current values, and require a large input resistor. Higher frequencies require an equivalent small input resistor.