In a mixed analog and digital integrated circuit for physiological signal instrumentation such as electrocardiographs, electromyographs, and electroencephalographs, an AD converter (346) adds an analog signal input (416) to an analog offset input to produce a digital signal output (724), and an offset means (736,740) generates the analog offset input if the digital output exceeds a threshold (1006,1008) for a predetermined number of A/D conversion. The integrated circuit has five signal channels, each with analog amplification and analog to digital conversion. The channels can be configured for various combinations of input signal amplification, input signal summation, analog output driving, and AC impedance measurement. The integrated circuit has two digital serial input lines and two digital serial output lines, all designed for direct connection to optical couplers. Channel configuration, gain, and other parameters are externally controllable by a serial digital input signal. Up to six compatible devices can be serially connected in a chain.