If this is for professional use (which it should be in these fora) and if you are not doing electronic design work ("mechanical" indicates that you are not) then the best bet is to use what you found.

There are "precision rectifiers" that are easily constructed and they convert AC to DC. An amplifier after the converter takes the level to where you want it.

One big question is: Is it rectified mean or RMS that you want to convert? What is the frequency of your AC signal? Required precision, temperature range, cost, number of units are also important considerations.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

Thanks for replying Gunnar,

The converter is to add a current logging functionality to my personal tools. AC current transducer>analogue data logger.

Rectified mean would be ok. The frequency of the AC signal is 50Hz. Precision +/- 2%, temperature range 10 to 45 deg C, cost as little as possible, number of units = 1.

http://sound.westhost.com/appnotes/an001.htm

Figure 4.

Change
R3 10K -> 1K
R4 20K -> 2K
R5 20K -> 100K

Then put a big capacitor across R5 to give the required filtering.  

Thanks logbook,

Just had a chance to build this circuit up and it works good. One question though, below 10mVAC input I get a negative VDC output   i.e. 6 mVAC in = -43mVDC out.   How can I tune the circuit so the output is calibrated to 0mVAC in = 0 VDC out?

seems to be easy to rectify via a opamp and then a zero and span circuit or even easier using a microcontroller instead the zero and span, just few lines of programming, thats all..

best

killabyte