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).