Full wave rectifier using opamp - 0 to 5v ac (for measuring voltage drop across  - Page 1
Forum section: electronics / projects

author: Rakesh.Pundir
author_rank: Newbie
timestamp: June 11, 2025, 03:58:38 pm
content: I'm working on a system that reads the voltage drop across test probes measuring AC voltage in the range of 0.05V to 5.00V RMS. I need help to design a precision full-wave rectifier circuit using an op-amp that can accurately rectify these low-level AC signals, so they can be digitized by an ADS1115 ADC and further processed using an Arduino.+------------------------+| 230V AC Mains |+------------------------+|v+--------------------------------+| Step-down Transformer || (230V AC to 5V AC) || 25A |+-------------------------------+|v+----------------+| Test Probes |+----------------+|v+------------------------+| Op-Amp Precision || Full-Wave Rectifier |+------------------------+|v+------------------------+| ADS1115 ADC || (16-bit Resolution) |+------------------------+|v+----------------+| Arduino |+----------------+|v+----------------+| Display || (LCD |+----------------+My ultimate goal is to design the ECRT tester (Earth contact resistance tester)

author: JacobPilsen
author_rank: Regular Contributor
timestamp: June 22, 2025, 05:50:13 pm
content: This is best known precision rectifier:5 Resistors, 2 diodes, 2 OpAmp, nothing less.http://www.eeeguide.com/precision-full-wave-rectifiers/https://www.allaboutcircuits.com/worksheets/precise-diode-circuits/Did you Search for "precision rectifier" here on Forum ???

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 23, 2025, 09:14:14 am
content: more examples here, including single rail solutions.https://cdn.hackaday.io/files/1524906524132160/PrecisionRectifiers.pdf

author: Terry Bites
author_rank: Super Contributor
timestamp: July 04, 2025, 12:34:44 pm
content: Check this saucy circuit:https://www.edn.com/peculiar-precision-full-wave-rectifier-needs-no-matched-resistors/If the sample rate is high enough I'd consider level shifting the AC and feeding it into the ADC. The rectifying could be done in code with an Abs() function.You can get the RMS, peak or what have you.

author: JacobPilsen
author_rank: Regular Contributor
timestamp: July 23, 2025, 11:13:39 am
content: "needs no matched resistors" = needs matched transistor ...Now did I realize the question:Does this mean that the resistors in the schematic I linked must be matched ?

author: ledtester
author_rank: Super Contributor
timestamp: July 23, 2025, 11:51:33 am
content: Here's a stack exchange question that seems relevant:https://electronics.stackexchange.com/questions/602433/read-negative-voltage-using-an-ads1115What's the frequency of the signal you are trying to measure?Most answers suggest level shifting but there's also this comment about trying to measure a 50Hz signal with the ADS1115:Quote

author: zapta
author_rank: Super Contributor
timestamp: July 24, 2025, 11:58:40 pm
content: One option is to use an RMS to DC converter. It will do the rectifying and RMS integration and will give you a DC that is proportional to the RMS voltage.For examplehttps://www.analog.com/media/en/technical-documentation/data-sheets/AD8436.pdf