MULTIPLE-CHANNEL DIGITAL LOCK-IN AMPLIFIER WITH PPM RESOLUTION

A digital lock-in amplifier built with a couple of input/output boards in a personal computer is described. The use of direct memory access allows the generation of a reference sinewave and the sampling of up t o eight channels synchronously with it, leaving enough time for the pr ocessor to calculate the in phase and in quadrature responses at and o nly at selected harmonics (h1,h2,...) of the reference frequency. This digital lock-in is drift free and has the gain stability of the analo g-to-digital converter, that is within +/- 1 ppm for a few minutes, in creasing to +/- 20 ppm for 24 h. Even a simple Mac IIci can monitor in real time h1 at 24 ksamples/s for two channels, corresponding to an u pper frequency of 3 kHz, with no limitations on the low-frequency side . Higher sampling rates and processing power are available with more r ecent hardware.