DualComparatorFormsTemperature-CompensatedProximityDetector

Dual Comparator Forms Temperature-Compensated Proximity Detector

Abstract: Simple comparator and op amp circuit forms a temperature-compensated proximity detector.

In the proximity detector shown in Figure 1, a 4-inch-square piece of copper-plated PC board serves as an antenna that forms src="/data/attachment/portal/201007/ET34468201007230651241.gif">
Figure 1. This proximity detector lights the LED when a person approaches the antenna plate within a threshold set by the potentiometer.

The method for transforming this proximity distance into a proportional voltage is illustrated by a simplified circuit that lacks temperature compensation (Figure 2). Transitions of the input square wave apply directly to the lower input of the exclusive-OR (XOR) gate, but are delayed 0.693(R1)(C1) seconds before being reconstructed by the comparator and applied to the upper input. R4 and C2 filter the resulting XOR output to produce a voltage proportional to distance.


Figure 2. This circuit, uncompensated for temperature, illustrates the principle of capacitance-to-voltage conversion.

The XOR output's duty cycle is proportional to the sum of R1 + C1 delay plus comparator propagation delay, so a small variation in comparator delay can mask small changes in antenna capacitance. The Figure 1 circuit overcomes this limitation with a dual comparator (IC1). Passing the XOR inputs through nearly identical comparators largely nullifies the effect of offset voltage, drift, and propagation delay through the comparators.

Figure 1's delay capacitance consists of a 33pF capacitor (C1) in parallel with 15pF (6 inches of coaxial cable at 30pF per foot) and the 4-inch-square antenna plate. It charges to 5V via R5 during each positive half cycle of the input square wave. When no body is near the detector, this capacitance equals 48pF and produces a delay of 16.5ns at the upper XOR input. With a hand placed 6 inches from the detector, the capacitance rises to 50pF and produces a delay of 17.3ns, yielding a time difference of src="/data/attachment/portal/201007/ET34468201007230651243.gif">
Figure 3. The dual-comparator technique of Figure 1 offers much better temperature stability than that of the uncompensated circuit in Figure 2.

A similar idea appeared in the 2/16/98 issue of EDN.

MAX912/MAX913 pdf datasheet (TTL比较器)