Hello Ankur

There seems to be two problems with your setup -

1) The 12V DC output and

2) The 50Hz power-line pickup noise.

First look at the second problem which is very common. You need to properly GROUND and SHIELD (note- both things are different) your circuit and the signal connections viz. input and output. This will help you REDUCE your noise pick-up considerably.

Now let's go back to your first problem - It is not clear whether you are using a classic 3 op-amp Instrumentation amplifier or a dedicated chip but it seems that your op-amps are saturated. Check for any open connections in the resistors. And also, initially check grounding your input - i.e. 0V as input and then see what  output you get.

Sometimes OFFSET voltages need also to be taken care of.

Please feel free if you need any further help.

Best Luck!

Thanks  Prof. Prasanna. I will look into it and let u know.

If you are not interested in DC you can elliminate the off-set: Just remove it with a Capacitor resistor high-pass filter, where RC determines the cut-off of the high-pass filter.

@Fernando,

Using a capacitor will only eliminate the DC offset-if any at the output and allow the AC signal to pass through it to the output. But the mail problem will remain - if there is a large OFFSET present at the output it will eventually limit your dynamic range or the signal swing at the output. So OFFEST voltages should be SET OFF (minimized) at the input side itself before being amplified by the gain and to appear as large DC at the output.

Dear Dr. Prasanna,

Just to give your answer yesterday, I am using classic 3 opamp instrumentation amplifier. I have ground one end of input as well but it is still showing same thing. If I do not put 10K Ohm resistor it will show 5~6V output with varying frequency from 25 to 50Hz. If I put 10K ohm resistor it will show +12V DC signal with no frequency. I am clue less. Should I check with signal generator as well?

Hi Ankur,

Yes, I think this is where you might be going wrong.

The Signal generator gives single-ended or Ground-referenced output. Whereas an "instru." has differential inputs - i.e. it has two signal inputs say "V1" and "V2" and both of these are referenced to the input GROUND. It means that you have 3 connections at the input as "V1", "V2" and "GND" and ONLY TWO connections at the output as "Signal Out" (or Vout) and "GND".

From your explanation above it seems that as you have connected either of the input (V1 or V2) to Ground. Therefore your output is saturated to 12VDC.

You need to feed your differential input from your input side, say for example a sensor, and connect it to V1 and V2. Use GND as SHIELD.

Try this and see what happens! If your signal is single-ended, then you can also try simple op-amp configurations as "Inverting" or "Non-inverting" amplifiers if you need to amplify a signal from generator.

Or you can send me more info about the input side so that I can guide you further.

Best Luck!

Hello Ankur,

If you want the support, please send us your circuit schematic together with IA  to instruments connection.

Dear Ioan Domuta,

Thanks for your mail. May I get your mail id. I would be happy to send it. Mine is [email protected]. You can drop your test mail there also.

Hi Ankur,

Any further developments?

Is that an IEPE sensor? if yes, you must configure it from the instrument to feed current to the sensor, otherwise you got NO input!