HA4905

Window Comparator With Independent Adjustments (HA2841, HA4905) Application Note April 1998 AN9802 Introduction Window comparators, or limit comparators as they are sometimes called, determine when the input voltage is within a pre-selected range. The output voltage usually goes to a TTL high when the input voltage is within the pre-selected range. A typical window comparator circuit normally consists of two comparators with two reference adjustments: one for the low trip level, and one for the high trip level. If the reference voltages are derived from the same resistor divider string, hystersis can not be added without having the trip levels interact. If the reference voltages are independently derived they can drift in different directions causing the window to widen or possibly disappear. The window comparator shown in Figure 1 circumvents these problems with independent trip and window adjustments. The HA2841 op amp (selected because it has high speed and low input currents) adds the input signal to the trip voltage developed by RT. The window width voltage RW is summed into the output of the op amp as an offset voltage. The op amp has two feedback loops each of which contains a steering diode, D1 or D2. The high open loop gain of the op amp insures that one diode will always be forward biased unless RW supplies the feedback current, thus when the sum of the input voltages (closed loop gain is one) and IRW passes through zero the current flow switches from one diode to t...