An rms-to-dc converter requires an output lowpass filter, which sometimes leads to overly long settling times. Paralleling two converters reduces the circuit's settling time signal without increasing conversion errors Fig 1.
The circuit in Fig 1 monitors 50-Hz mains. IC1, IC2, and their associated circuitry compose a phase shifter. The phase shifter has a 90° delay for 50-Hz inputs and constant gain. The second-order, lowpass, state-variable filter, IC5, sums and filters the outputs of rms-to-dc converters IC3 and IC4.
The circuit's settling time to 1% of final value is 20 msec.
F (p) = 0.5 / (a2p2 + 2kap + 1), where k = 0.7 and a = 0.5 msec.
If the circuit were to have only one rms-to-dc converter, its settling time would be 100 msec.
To reduce the circuit's settling time further, you could parallel more converters, each having a different phase shift. The trick is to set up the phase shifts so as to cancel ripple in the summed signal. (DI #1424)