Two op amps provide averaged absolute value

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, IC_1A, is a differential-output absolute-value converter. The second stage, IC_1B, 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 V_A and V_B. In addition, the capacitors ensure low ac-impedance points at nodes V_A and V_B 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.