EDN -- 03.01.96 Precision waveform converter uses sawtooth wav

-March 01, 1996

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Design Ideas:March 1, 1996

Precision waveform converter uses sawtooth wave

Richard Kowitch,
Chicago, IL

The circuit in Figure 1 can convert almost any waveform into another by coupling a sawtooth waveform with the reference inputs of a conventional bank of comparators. This addition allows the circuit to control the discharge time of CD more precisely. The circuit has numerous applications, but the specific application for this design is to fire a circuit breaker when the voltage on CD drops below a certain setpoint. In this case, VIN, from a current-sensing transformer, is the voltage proportional to high-tension-line current.

As VIN rises, the time required to discharge CD drops. However, without the sawtooth generator and the accompanying CC coupling capacitors, the time drops in only five crude staircase steps compared to the continuously decreasing waveform of discharge time vs VIN. The addition of the sawtooth waveform into the comparators' reference inputs produces a linear approximation of a staircase, thereby improving the timeout precision.

thumbnailFigure 2 illustrates one case in which comparators 4 and 5 are continuously on, but comparator 3 is switching off and on with the duty cycle in Figure 2b. This situation creates a linear approximation of the staircase, which improves accuracy. You should adjust the sawtooth amplitude so that the teeth just touch each other, as Figure 2a shows. Coupling the sawtooth waveform with the CC capacitors reduces the effect of any dc drift in the reference string. Of course, the sawtooth frequency must be at least 10x higher than the fastest timeout.

The circuit in Figure 1 uses only five comparators, but you can also use more. Some bar-graph chips are also useful. If part of the waveform you wish to approximate is rising, you can use a comparator that injects current instead of one that sinks current. In any case, adding the sawtooth wave makes this method practical by reducing the number of necessary comparators. Instead of comparators, you can also use MOSFET switches to couple a waveform setpoint.

(Editors' note: You need not use the same values for all RRs and RDs; you can use different values to create a nonlinear response curve. Also, the circuit should have a resistor pulling up on CD.) (DI #1835)

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