Half wave rectifier diodes

Thread Starter

sealdogfish

Joined Nov 15, 2011
15
Hi all!

I'm working on a project, I have a pressure sensor with a -50mv to +50mv output, I have 2 AD8666 op amps set up, one inverting one non inverting, what sort of diodes should I use on the outputs for half wave rectification?
I've looked all over google and the forums and all they say is diode..
Gain is 40 btw, I want the output from the non inverting to be 0V to 2V when the pressure sensor is 0 to 50mv and the output from the inverting to be 0 to 2V when sensor is 0 to -50mv

(Both op amps are set up correctly and working, it's just the half wave rectifiers..)

Thanks!
 

SgtWookie

Joined Jul 17, 2007
22,220
You should look up "precision rectifier".
If you don't use a precision rectifier, the forward voltage of the diode will introduce an offset. Precision rectifiers compensate for that voltage drop.

See this page:
http://sound.westhost.com/appnotes/an001.htm
Look at figure 3.
Note that you will need to use a dual-rail supply (both negative and positive supplies).
 

Thread Starter

sealdogfish

Joined Nov 15, 2011
15
Here is the schematic:


That second ADC1 should be ADC2 btw,
So what diodes should I use and where to place them? I have read that website but again it does not mention the particular diodes to use
 

SgtWookie

Joined Jul 17, 2007
22,220
You need to have a negative supply, or else the negative portion of the waveform will be truncated (clipped off) at 0v.

After that, you can use 1N914 or 1N4148 diodes.

[eta]
If you really don't want to have a negative supply, you might use a pull-up resistor on your input signal to get the entire signal above 0v; but you will wind up with some attenuation.
 
Last edited:

crutschow

Joined Mar 14, 2008
25,450
Here's a full-wave precision rectifier that requires no diodes and no negative voltage supply (it actually won't work properly with a negative supply). It does require a rail-to-rail type op amp.
 

SgtWookie

Joined Jul 17, 2007
22,220
Carl, I looked at that exact article a few years ago, but the output left a good bit to be desired. There was quite a bit of distortion when it got near ground.
 

SgtWookie

Joined Jul 17, 2007
22,220
I don't remember; it's been a good while. I just remember that it didn't behave well around the ground rail. I'll have to do some digging to see if I can find it.
 

SgtWookie

Joined Jul 17, 2007
22,220
Well, I found my simulation of it, and I'm wondering what I saw over a year ago that caused me to not like it?

Looks pretty good from what I can see. I really didn't change anything.
V(out) is being overlaid by V(in) on the positive swings; it's there.

Perhaps I'm getting that one confused with another similar circuit I was simulating at the time. I don't know ... it's late and I'm mighty tired.
 

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