Op Amp Gain help

Op Amp is wired like this.

Data sheet:


The power is from a 9v battery reading 8.62v (I was using it for temp testing)

The goal was to get a x50 gain. The pressure transducer (PT) has an output of 0-100mV. So the op amp output should be 0-5v (well 5.1v actually). Right now I have a 7.48v reading on the output (labeled UNO). I double checked everything twice (data sheet, online gain calculator, etc.) I was worried about the non-inverting input being too high, so I grounded the non-inverting pin and it still reads 7.48v. Any ideas?

ALSO... That darn pressure transducer has a 4 wire configuration (Excite +, Excite -, Output +, Output -). I called the manufacture to find out if I could share a ground between the excite circuit and the output and they said "Yes this is fine". When I connect the PT to the 9v battery and share the ground, I get a reading of more than 2 volts @ 0psi. The published output range is 0-100mV for 0-100PSI!!! When I do not share the ground and read the output across the output + and -, I get 0.001mV (the correct reading). My question is what to do with the output - when connected to the op amp???

Pressure Transducer Data sheet (422 H2):


A little bit of wiring info...


Thank you for any help.

Many op amps need both a positive and negative supply to work correctly. Can't quite make out the markings on yours, but if it is not a single supply model, then you'll have trouble without the negative rail. (EDIT: I overlooked the data sheet. That is not a single supply amp, so you will need the negative rail).

I would also suggest using a decoupling capacitor (0.1uF) between the supply pins, as close as possible to the op amp. I've cured mysterious op amp problems more than once by adding a cap.


I'm a bit confused. A negative rail is not the same as a grnd?

No, not the same. I'm far from an op amp expert, but I think the main issue is that your negative rail needs to be a couple of volts below the inverting input in that circuit.

Try adding a second battery in series with the first, and tie GND to the common terminal.


OH WAIT!!!!... can I use the output - from the sensor to the inverting input, then the grnd to the Vcc -??? Can it be so simple?

edit... From your post, I guess it is not that simple.

I don't think so, but give it a try :cold_sweat:

I think at least one problem will be that the inputs to the op amp are very high impedance so there will be no current path for the sensor.


How can you tell from the data sheet if it's a single supply amp or not.

It doesn't brag about being "single supply."

Compare your data sheet to http://www.ti.com/lit/ds/symlink/opa2340.pdf


lol... fair enough.

Ok... The op amp problem looks to be as simple as adding a Neg rail.

Now what do I do with that darn Output - from the transducer? Could I tie that to the inverted input? I don't think I can leave it not connected because it will float, but I'll have to check that.

It is hard to see from the data sheet but I think your sensor needs a differential input amplifier. You can make one with a few op amps not just one.
Back in 86 I wrote an article about interfacing pressure transducers. Here is the text:-
I have attached the diagrams.

RobDrizzle: Now what do I do with that darn Output - from the transducer?

I don't see why you can't leave it connected to GND (i.e, the common connection between the series batteries). But based on what GrumpyMike has offered, this might be a moot point.


Because when it's connected to ground the output voltage goes from 0-100mV to 2.02volts... So I'm assuming there is some sort of voltage leak.

Holy crap Mike.... That is a lot of in depth information! I'll have to re-read cause I'm still a little foggy on how to get done, but thanks.

I'm also reading this, which seems to be close to your right up at points.


I'm going to go home tonight and do some experiments :)