Help with OP Amp circuit

Thread Starter

pattom12

Joined Sep 1, 2006
3
I am trying to build a simple voltage follower OP Amp circuit using an old AD542 a friend of mine gave me. I constructed a circuit as shown in the attachment but it does not work and I do not know why. Following are the measurement I get:

I have a pH probe connected to the input and I know it is putting out a +mv signal.

Measured output is 2.5 v with or without the probe connected,.
Measured input is 1.25 v with or without the probe connected.

I removed the 1 meg resistor from the output to see if that had any effect and it does not. Does anyone have any suggestions as to how I might make this circuit perform as a unity gain voltage follower?

I am really new to this so please excuse me.

Regards

Pat
 

Attachments

beenthere

Joined Apr 20, 2004
15,819
Hi,

Your circuit can't work with the 1K resistor on pin four. That is one of the power supply pins for the op amp. Just remove the resistor and tie the pin to ground. It should look more like a voltage follower.

By the way, the 2M resistor to ground off the output is probably unnecessary.
 

Ron H

Joined Apr 14, 2005
7,014
The minimum supply voltage for AD542 is +/-5V. It won't work with a single supply if your input voltage is near zero. The output of a pH probe is about 60mV per pH unit.
 

Thread Starter

pattom12

Joined Sep 1, 2006
3
I removed both resistors and tied pin 4 to ground as suggested. What I now get is 1.9v out and .7v in regardless of the probe pH. Any ideas what I should try next to make this unity gain follower op-amp work properly?

Thanks again

Patrick Thomas
 

hgmjr

Joined Jan 28, 2005
9,029
Greetings pattom12,

Though you have not labeled your diagram as a top view or bottom view, in looking at the ad542 datasheet the view in you diagram appears to be a top view.

Is it possible that you have interpreted the pinout as a bottom view instead of a top view?

hgmjr
 

Ron H

Joined Apr 14, 2005
7,014
I removed both resistors and tied pin 4 to ground as suggested. What I now get is 1.9v out and .7v in regardless of the probe pH. Any ideas what I should try next to make this unity gain follower op-amp work properly?

Thanks again

Patrick Thomas
Read my lips: :)
As I said above,
The minimum supply voltage for AD542 is +/-5V. It won't work with a single supply if your input voltage is near zero.
You will need two power supplies, one at +5V (or more) and one at -5V (or more).
 

Thread Starter

pattom12

Joined Sep 1, 2006
3
I used an old computer power supply to get +12v and -12v which I hooked to pin 7 and pin 4 respectively. The op-amp then performed as it should. Thanks for the help.
 
Top