Absolute-value circuit delivers high bandwidth

-May 15, 2003

Most absolute-value circuits have limited bandwidth and high component count, and they require several matched resistors. The circuit in Figure 1 uses three fewer components than most absolute-value circuits require, and only two of the resistors must have 1% tolerance to obtain 1% accuracy. This circuit's output voltage is an accurate representation of the absolute value of the input signal, and it is accurate for input signals containing frequencies as high as 10 MHz. Another advantage of this circuit is that it has a positive-voltage output, thus saving an analog inverter in most applications. When the input voltage is positive, the negative output voltage of IC1 cuts off the diode, thereby preventing signal propagation through IC1. Virtually no signal propagates through R2, because the resistor connects to ac ground through the output of IC2. The only signal path is through R3 to buffer IC2, and the output of the buffer is a positive voltage. When the input voltage is negative, the positive output voltage of IC1 forward-biases the diode, thus providing an ac short circuit for R3 to ground. IC2 is within IC1's feedback loop, so the output voltage is positive because of IC1's configuration as an inverting op amp.

This design uses a dual op amp to minimize parts count. Two op amps in a feedback loop tend to be unstable. Select an op amp that has sufficient phase margin to prevent oscillation when the input voltage is negative. The circuit's dynamic range is from the op amp's input offset voltage to the maximum output voltage. This dynamic range is from 1 mV to 4.1V for the TLC072 with ±5V power supplies. The excellent bandwidth performance results from combining the high-frequency TLC072 op amp with a fast Schottky-barrier diode. You can use higher frequency op amps to obtain better bandwidth results, but you must take care in the op-amp selection to avoid oscillation or reduced dynamic range.

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