Time to settle your measurements

Many data-acquisition systems measure multiple channels through a single analog-to-digital converter (ADC) and a multiplexer. When you switch channels, you reduce the sample rate. A 100-ksamples/s rate may reduce to under 20 ksamples/s when you need more than one channel because the multiplexer needs time to settle after switching channels. If you scan channels too fast, you'll introduce errors because capacitance in the multiplexer will retain artifacts from the previous measurement.

Capacitance in multiplexers may cause a previous sample to influence the current smaple. In this case, a signal (red trace) may never reach its full voltage level (blue trace) before a sample occurs.

Capacitance, in combination with the system's input impedance and amplifier gain, places a low-pass filter on the signal. If the filter's R-C time constant gets too large for the switching speed, the ADC will never sample the full voltage (red trace) because the capacitance won't have sufficient time to charge.

To minimize the error, take sufficient samples on each channel until you measure the full voltage before switching to the other channel. Discard all but the last measurement in each series of samples in your application. Minimize your signal source's output impedance to minimize settling time. Use a signal-conditioning amplifier if necessary. You should also select a scan sequence that measures channels with similar amplitudes.

Application note AN045, "Is Your Data Inaccurate Because of Instrumentation Amplifier Settling Time?" National Instruments, Austin, TX. zone.ni.com (search for AN045).

"IOtech Signal Conditioning Handbook," Chapter 3: Multiplexing. IOtech, Cleveland, OH. www.iotech.com/sigcon3.html.