A Digital Feedback Integrator for Precision Photocurrent Measurements at High Data Rates

A digital feedback circuit has been developed that integrates the current from a photomultiplier tube or other current source and delivers a digital output signal. The high conversion rate permits precise integration of the photocurrent resulting from chopped or pulsed-light sources. The processing lag of approximately 300 ns makes this a real-time system for most applications in that it delivers discrete pulse-by-pulse readouts for signals spaced at this or longer intervals. The measurement of pulse area represents optimum processing in photo-limited situations. The circuitry is based upon the delta-modulation encoder with the refinement that return-to-zero (RZ) pulses are used to eliminate nonlinearities due to contiguous-pulse effects. With the component values shown, the circuit area-encodes pulses of 7-μA peak amplitude with a maximum nonlinearity of about 0.1 percent of full scale. The feedback error of the circuit produces a random noise component that varies inversely with the square-root of pulse duration and is about 0.1 percent of full scale, peak-to-peak, for pulses of 10-ms duration.