Sampling variation caused by A/D cards due to external trigger.

In electrophysiological recording, a microcomputer-based analog-to-digital (A/D) card is an indispensable instrument for signal acquisition and analysis. In our studies, evoked responses sampled by our A/D card showed variation among different cycles. If several cycles had been averaged, the resultant waveform would have a smaller peak amplitude and a longer duration. To explain this phenomenon, a simulated sampling model of compound action potential was proposed. Our experimental data agreed very well with the prediction of the simulated model. The long and varied delay time between the external trigger and the first sampling in each cycle by our A/D card might be the main cause of such variation. This problem could not be solved by any post-sampled programming. Hence, for those electrophysiological laboratories which sampled evoked responses, to buy a new A/D card might be the most straightforward solution to the problem.