Even though we publish new blogs daily from your intrepid editor and a number of well-known writers, sometimes it's good to have a look back at our well-received articles from the recent past. Here is one that discusses the precision full-wave rectifier.
A precision full-wave rectifier is used in signal processing, typically with audio signals or as part of a data acquisition system. Complex bipolar waveforms are rectified so that only positive voltages are present.
There are many appnotes that cover precision full-wave rectifier circuitry, but they show versions powered from both positive and negative supplies. That makes sense, since the circuit is processing bipolar signals. However, in this blog, we see a special version of the circuit. This one uses RRIO (rail-to-rail input and output) op-amps and is powered from a single (or unipolar) supply.
The author discusses previous methods used in similar circuits. These rely on the op-amp's output swinging all the way to the supply rails and then going into saturation. This works as long as you are processing fairly low frequency signals and don't care about the time it takes for the op-amp to come out of saturation. And as long as you don't have stringent accuracy requirements.
The author then shows how to use an active clamp circuit to effectively change an op-amp from an amplifier with a gain of +1.00V/V to an amplifier with a gain of -1.00V/V. Accuracy is a function of resistor matching, op-amp offset voltage, and op-amp bias current. You must also consider the frequency content of the signals being processed and compare that to the gain-bandwidth product of the op-amp.
The article, part of our SIGNAL CHAIN BASICS series, was written by Rick Downs, at the time an Applications Engineering Manager for Texas Instruments.