opamp rectification

[johnny4]

hi everyone,

i have sensor measuring current but i am having problem with conditioning signal. sensor makes output 0.5...4.5VDC (corresponds to input change -5 to +5A). this should be fine to bring directly to ADC (MCP3208) when measuring DC currents.

btw. i would really really like to stick with 5VDC single supply (ok, it can be up to 12V if needed).

the problem I have is if the current is AC. this 0.5...4.5V need to be rectified (using 2.5V as zero reference since this voltage represents zero current). I know that classic rectifier will not work because of forward drop on diodes. i did find "precission rectifier " or "super diode" circuit but quick test on a breadboard showed that is is trickier than it sounds. among other things, normal opamps like LM358 don't swing beyond 1.1..3.9V when powered by 5V.

can anyone offer ideas or possible solution? i'd like some slick circuit with just a jumper or dipsw. to select AC or DC circuit behavior.
i used this as reference but tested with LM358 which i had laying around:
http://www.freecircuits.net/circuit-61.html
other changes are:
- removed C1 (didn't think AC coupling is needed),
- single supply, created virtual ground using pair of 2k2 resistors and cap. to gnd.
- inserted switch in series with R5 (trying to select AC/DC)
so far i didn't have much luck with test circuit on a breadboard, will re-check...

johnny s.

 

[panic mode]

you may want to try rail to rail opamp like LMC6482

 

[ericgibbs]

hi j4,
This is one option, full wave precision peak rectifier, uses a MCP6002 dual rail to rail at 5V

 

[johnny4]

@panic

i know but didn't have ay at hand. have wait till my order arrives.

@eric
that looks nice, i like the low component count (i need to build bunch of such circuits on same board). btw. what software did you use? looks like some sort of spice.
also input should be 2Vpp sine in series with 2.5Vdc to get lower peak at +0.5V

 

[panic mode]

the software is called LT Spice (former Switcher Cad). it is freeware from Linear.com

https://www.linear.com/designtools/software/

 

[crutschow]

.............
@eric
that looks nice, i like the low component count (i need to build bunch of such circuits on same board). btw. what software did you use? looks like some sort of spice.
also input should be 2Vpp sine in series with 2.5Vdc to get lower peak at +0.5V

The interesting thing about Eric's circuit is that it will rectify an AC signal that goes negative (below ground) with absolute voltage as much as the positive supply rail and it will still be properly rectified.