Window comparators, sometimes called limit comparators, determine when the input voltage is within a preselected range. The output voltage usually goes to a TTL high when the input voltage is within this preselected range. A typical windowcomparator circuit consists of two comparators with two reference adjustments ¾one for the low trip level and one for the high trip level. If the reference voltages are derived from the same resistor-divider string, hysteresis can’t be added without having the trip levels interact. If the reference voltages are independently derived, they can drift in different directions. This causes the window to widen or possibly disappear.
The window comparator shown circumvents these problems with independent trip and window adjustments (Fig. 1). 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 contain a steering diode, D1 or D2. The high open-loop gain of the op amp ensures 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 the other. The output comparator senses the polarity switch across the diodes and the comparator output voltage changes.
The HA4905 comparator has inputs that can work from 15-V supplies. As a result, the input voltage swing is matched to the op amp’s output voltage swing. Even though the input section of the HA4905 is connected to 15 V, the output section can be connected from +5 V to ground, thus offering a TTL-compatible output voltage.
RT sets the trip point at any point between the power supplies, and RW sets the window width from zero, when the center tap voltage is zero, to about 90% of the input waveform. The trip point should be set first, then the window can be set to the desired width. Because the adjustments don’t interact, the trip point stays constant as the window width changes.
Resist the temptation to change the comparator to an op amp. This is poor practice for any circuit that requires high-speed performance because the opamp output would saturate, causing uncontrolled time delays. The comparator is subject to multiple switching caused by high-frequency noise riding on slow-rising input signals. This multiple switching is eliminated by the capacitor (C), which is placed across its input leads. Effectively, C couples the high-frequency noise to both comparator leads, enabling it to be rejected by the comparator’s commonmode rejection capability. Notice that no multiple switching effects are occurring in the waveform (Fig. 2).
This circuit yields a window comparator with independent adjustments, temperature stability, wide voltage range compatibility, and TTL compatibility.