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Two op amps provide averaged absolute value

-October 30, 2003

The circuit in Figure 1 is useful when you need amplitude demodulation or an averaged absolute-value conversion. The circuit comprises two stages, the first of which, IC1A, is a differential-output absolute-value converter. The second stage, IC1B, is a traditional differential amplifier. The combination of the two stages performs single-ended absolute-value conversion but only if R3>>R2. The C1 capacitors integrate the current flow and yield averaged voltages VA and VB. In addition, the capacitors ensure low ac-impedance points at nodes VA and VB when the output diodes are reverse-biased. The additional C2 capacitors in parallel with R4 resistors impart a second-order-lowpass-filter characteristic to the circuit and remove the remaining ac signal. From a practical point of view, you can choose R3 to be five to 10 times higher than R2. The gain of the circuit is (R2||R3/R1)(R4/R3). In most applications, you would choose the filter time constants τ1=R2||R3C1 and τ2=R4C2 to be equal. The circuit in Figure 1 is simple, symmetrical, and cost-effective. It also makes it easy to calculate and adjust the gain using one resistor, R1. Other advantages are that the circuit has equal delay for positive- and negative-going signals and that it doesn't need matched diodes.

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