Tracking Window Comparator

by H Gulitz The use of comparator circuits in many different appearances and practical realizations is common in a wide variety of electronic control and measure- ment systems. Usually, the volt- age from a sensor device is fed to a comparator which, as its name implies, compares the measured level, Um, with a fixed reference, Uref, and pro- duces a negative output (0) or positive output (1) when UinUref, respect- ively. A window comparator can be made by connecting two comparators with different reference levels, which define the upper and lower limit of the switching range. In practice, these references are usually adjusted with presets to dimension the win- dow as required. This arrange- ment makes it impossible, however, to automatically shift the window up or down in ac- cordance with, say, ambient light conditions to be measured with a light dependent resistor. This circuit has no fixed threshold levels, but derives its reference from the measured signal, so tha...