AN017: The Integrating A/D Converter

Integrating A/D converters have two characteristics in common. First, as the name implies, their output represents the integral or average of an input voltage over a fixed period of time. Compared with techniques which require that the input is “frozen” with a sample-and-hold, the integrating converter will give repeatable results in the presence of high frequency noise (relative to the measurement period). A second and equally important characteristic is that they use time to quantize the answer, resulting in extremely small nonlinearity errors and no possibility of missing output codes. Furthermore, the integrating converter has very good rejection of frequencies whose periods are an integral multiple of the measurement period. This feature can be used to advantage in reducing line frequency noise, for example in laboratory instruments (Figure 1).