The impact of multiplexing on the dynamic requirements of analog-to-digital converters

In data acquisition applications where the signals being digitized are produced in a time-division multiplexed system, the required dynamic performance of the analog-to-digital converter (ADC) is no longer bound by the conditions set forth in the sampling theorem. This results from the introduction of very high-frequency information by the multiplexing process which, while not necessarily containing information of interest, must be processed by the input circuitry of the ADC. In this situation, signal bandwidths and slew rates can greatly exceed those produced in a Nyquist limited system and can surpass the capability of the ADC, thus degrading overall system performance. This paper examines two common multiplexing schemes and their impact on ADC dynamic requirements. First, the authors examine a simple voltage multiplexing scheme typically found in state-of-health or data-logging applications and develop the necessary equations to show how the ADC dynamic requirements are affected. Then, the analysis is extended to a multiplexed photodiode array readout to see how this application further challenges the dynamic performance of the ADC. Finally, the issues associated with developing dynamic test methodologies for assessing ADC performance in multiplexed systems are discussed