[Diy_efi] I need to rectify a low ac voltage.

[Brian Dessent]

Jörgen Karlsson wrote:

> Can anyone please help me to rectify and filter the signal, I can't come up
> with a good way to do this.

If you want an ideal rectifier (i.e. no forward diode drop) you can put
the diode in the feedback path of an op amp.  I'm certain you can do
half wave, and pretty sure you can do full wave (worst case -- make two
half wave rectifiers, one with an inverted input, and sum the outputs.) 
Look at op amp cookbook websites or books for the details.  A quick
google search found this:

http://www.deas.harvard.edu/courses/es154/lectures/lecture_2/diode_circuits/diode_appl.html

You can probably combine filtering in the feedback loop and do it all in
one step.  If you provide details I can help more.

Also, depending on your application, you might try a ready-made
instrumentation chip.  For example I know there are chips out there that
take arbitrary AC input and output a (quite filtered) DC voltage
corresponding to the RMS value of the input.  These are intended for use
in meters and such.  I think a lot of them use bolometric sensing (i.e.
differential heating) but some might also use analog multiplication or
digital sampling.  For example, Analog Devices' AD536A / 636 / 637 / 737
/ 736.  I think the cheapest of these is under $5.

Brian

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