Differential Op Amp bipolar to unipolar circuit help !!!

[ranaya]

Hello......

I nee to interface a bipolar signal to adc and interface should be differential. Using LM324 single supply op amp I came up with this cct which is simulated using multisim.



I received the expected output. But does it work for the real op amp ???

Thanx alot

 

[FvM]

This isn't a differential input, because one input terminal is grounded. The OP inputs are connected wrongly (interchanged).

You don't tell about intended input and output voltage ranges, thus we can't say if it "works". To achieve full 0..5 V output range, an OP with true rail-to-rail output would be needed, LM358 is limited to 0..3.8V.

 

[rfredel]

Hello ranaya,

the first fault I can see is, that you have made a positive feedback with R16. So the circuit will be an oscillator.

Your negative input is with resistor R diectly coupled to ground. Better is it to do it with a capacitor, so can eliminate the offset voltage.

Look to the attached diagram:



Regards

Rainer

 

[ranaya]

Hi thanx both of u for quick responses.....

Seems my image is not clear enough, Apologize for that. here I uploaded it again. As rfredel says, I made a mistake connecting R16 to non inverting pin. But i hvnt connectd the invrting input to the ground. The circuit is used to interface -/+10 bipolar signal to adc. R25 is connected to ground. Im using LT1079 precision op amp. (Just use LM324 because of the similar footprint)



Is this right ???

Thanx Alot

 

[rfredel]

Hello ranaya,

now, there are 2 faults in drawing.

1. R16 must be directly connect to PIN 13 of OP.

2. X7-2, R31 and R14 must connected to ground.

Please compare it with my drawing in last post. If you use C8 in my drawing or not, is no problem, but you need a connection to ground.

Regards

Rainer

 

[godfreyl]

Hi Ranaya

If you want balanced input, the circuit below is probably the easiest. The output is biased to half the supply voltage.

Gain = Vout/Vin = 1/10, where Vin is the difference between the two input voltages. It's low on purpose so the opamp doesn't clip with high input voltages. If you want higher gain, just reduce R1 and R2. e.g. If you make them both 10K, gain will be 1.

Cheers - Godfrey