Last week we went into details of ADC and how source impedance can effect timing. Now there will be cases where you are looking to sample two independent signals, say bio-med signals, on 2 separate analog channels on an MCU with a single ADC.
With a time-multiplexed SAR ADC, the internal sampling capacitor first charges from channel A. The mux flips to channel B. If the source is not low impedance or acquisition time is short, some charge from A rides into B. You see a ghost voltage of the previous channel. One fix is to wait after each switch so it settles. That reduces ghosting, but it creates timing skew between channels. Meaning it’s non-time synchronised.
Sometimes this skew is not acceptable. In bio-med you might want ECG and PPG at the same instant. This is where sample or track-and-hold stages help. You place a T/H per channel and drive all holds with one edge. In track the output follows the input. On hold the switch opens, a small capacitor freezes the value, and a buffer drives the ADC. The ADC then converts sequentially, but the samples come from the same instant.
There are few parameters to keep in mind while selecting one, Aperture delay and Jitter set the exact instant you grab the signal. Lower jitter means cleaner SNR and less channel-to-channel timing error. Droop is how the held voltage slowly sags because of leakage. Too much droop changes readings. Hold step is the small jump when the switch opens due to charge injection. Big steps look like offsets or spikes. I usually aim for low jitter, low droop, and small hold step, so both channels look clean and truly simultaneous.
So next time when you want absolutely 2 signals at the same time look into sample and hold Amplifiers. TI and Analog devices have a few in their portfolio.
