Sample & Hold For Analogue Signals

0 SAMPLE & HOLD FOR ANALOGUE SIGNALS Conventional analogue sample and hold circuits are notorious for their tendency to drift, a phenomenon unknown in digi- tal memories. It is, therefore, interesting to study the use of a digital memory el- ement for storing an analogue signal. The present circuit is based on in- termediate storage of digitized analogue information, and therefore re- quires an analogue-to-digital converter (ADC) at the input, and a digital-to- analogue converter (DAC) at the output. Unfortunately, DACs and ADCs are typically expensive components, and the present circuit is therefore set up with a DAC only, driven by an up/down counter-see Fig. 1. The counter is es- sentially an ADC, since the output voltage of the R-2R based DAC is con- tinuously compared to the input voltage with the aid of a window comparator. The error signal produced by the com- parator arranges for the counter to count up or down, depending on the magnitude of the difference between the in...