The PDA-750 from Terahertz Technologies Inc. is a low-noise, high-gain transimpedance amplifier that provides direct digital readout of the current generated from a photodiode or photomultiplier. Applications include unity quantum-efficient detectors, characterization of detector dark current, a readout interface for spectrometers and spectral calibration of detectors. Full-scale input ranges from ±20 nA to ±20 mA, with a noise level of <1 pA. Features include variable bias supply, background cancellation, a bidirectional RS-232 port and rechargeable batteries. A 4.5-digit LCD provides maximum resolution of one part in 20,000, enabling detection of small changes in the signal under test.