Combination of ADC and DAC to measure small variation with large standing signal

In some cases, the variation of a signal is small compared with the standing signal level. It is difficult to measure small variation with a large standing signal. One method of achieving more accurate measurement is to cancel the standing signal first, and then measure the variation alone. This paper presents a circuit combining analogue-to-digital converter (ADC) with digital-to-analogue converter (DAC), to cope with this particular problem. Two DACs are used, one providing an offset voltage for standing signal compensation and another as a programmable gain amplifier (PGA). An equation is derived for calculation of the absolute signal from the measured data. Experimental results are shown.