Precision rectifier blues

[puretube]

maybe try this (after having turned around the diodes):
use all 1% resistors (I suppose you already did so...),
make that 47k+100k-trimpot a fixed 100k (=exact half value
of the 2nd opamp`s resistors),
and hook one of the opamp`s Vref not to exactly Ub/2, but adjustable by trimpot +/- 10%, to compensate for any offset.

:idea:

[gez]

use all 1% resistors (I suppose you already did so...),

Yup!  

make that 47k+100k-trimpot a fixed 100k (=exact half value
of the 2nd opamp`s resistors),

I did this to begin with but there was a difference in amplitude between the two peaks so thats why I lowered the input resistor and incorporated a trimpot.  Don't quite see how reversing the diodes will help matters, but I'll give it a try.

and hook one of the opamp`s Vref not to exactly Ub/2, but adjustable by trimpot +/- 10%, to compensate for any offset.

Yeah, I should think I need to incorporate offset resistors.  Was going to use fixed resistors, but trimpot is a good idea.  Thanks!

Won't have time to do anything today, but will experiment on the weekend.

[puretube]

http://www.play-hookey.com/analog/full-wave_rectifier.html
this features "your" diode orientation, but the impedances in the 2 branches are different.
Furthermore the values are 10x smaller: this maybe taken into account with "your" values, when thinking about offset;
Maybe for this reason too, you should put resistors from the non-inverting inputs to Vref, instead of direct connections.

heads up, you`ll get there!

[Boofhead]

Reversing the diodes will reverse the polarity of the output.  It way or may not speed things up depending on the specfic opamp.

Some ways to speed it up are:

- use Schottky diodes
- use faster opamps

Someone already mentioned putting a filter cap on the second opamp.  This can help too.  If you have a filter following the rectifier you will be better off transferring this to the feedback network of the second opamp.

If you are dealing with small input signals then DC offsets can affect the circuit.  The best solution here is to simple amplify the signal before the rectifier then drop the DC down (if necessary).

[gez]

PT, the next time you’re in London I owe you a beer, this works beautifully!



There was a slight offset, and it was enough to be causing the problem.  The offset seems to be a result of a mismatch between devices within this individual chip and nothing to do with the bias networks.  As I said earlier, the chip I’m using is CMOS and has miniscule input bias (leakage) current (2fA!!!) which is why I don’t usually bother with offset resistors.  It’s also the reason why I used resistors 10X larger than you’d normally see for this circuit, it means I can get away with a smaller input cap and save some space on a PCB.

I tried offset resistors from +ve input to Vbias and it didn’t even make a dent in the bias, even with resistance in to the Meg range!  So, I tried PT’s trim idea and it works like a charm - square in, sine out at double the frequency (whole circuit that is, not just the bit shown Paul)  

I still have a load of work to do before I get anything resembling a working circuit (for guitar), but I’ve got past the first hurdle.

Huge thanks go out also to RG and everyone else who replied….also to the director…makeup artist…oh, and not to forget the lighting man…but... above all (blub)….my MOM AND DAD!  (led off the stage to the sound of embarrassed applause)

[puretube]

CHEERS!

you alternatively could also quote Michael Jackson (double pitched):
"I love you all"