D.C. Detector

0 The d.c. component of a signal can only be detected by separating it from the a.c. component. This is most con- veniently done by filtering the a.c. com- ponent. In the proposed circuit this is effected with the aid of the common- mode rejection ratio (CMRR) of an opamp. (The CMRR is a measure of the ability of the opamp to produce a zero output for like inputs). The complete signal is applied to the in- verting input of opamp Ai, and only the a.c. component, via CI, to the non- inverting input. The lowest frequency that can be detected is determined by time constant (123+ R4)Ci. With values as shown, a.c. suppression amounts to about 50 dB at 20 Hz. The output of Ai is fed to a low-pass filter to further attenuate high fre- quencies. This is necessary because the CMRR of an opamp decreases at higher frequencies. The difference signal is then applied to comparator A2. Diodes Di and D2 ensure that A2 reacts only to voltages greater than ±300 mV. A negative direct voltage at the ...