Another method for removing V_OS is to use some form of a DC feedback loop. This can be done in many ways, but the general form of the circuit is shown in Figure 6-3. Such a loop is used to limit the V_OS of a section of a circuit, usually just before some critical input where the offset must be removed. It reduces the offset voltage to that of the error amplifier only, which can be a DAC, op amp, or some other more elaborate circuit. The DC measurement must be made when there is no input, as represented by the switches. Such offset correction takes a long time to make—usually milliseconds—compared with the speed of the system, and they are made during some noncritical time such as during start-up.

Figure 6-3 General Form of DC Feedback Loop