A sensitivity switchable detection circuit includes a high gain circuit
for outputting a first signal, a low gain circuit for outputting a second
signal, and an output switching circuit for switching between the first
signal and the second signal. When the first signal is smaller than a
lower limit, the first signal determines a detected signal. When the
second signal is larger than an upper limit, the second signal determines
the detected signal. When the first signal is larger than the lower limit
and the second signal is lower than the upper limit, a weighting function
that uses the first signal and the second signal as input variables
determines the detected signal.