I agree withVasile but really to solve this problem I would need the exact circuit in order to see where the offset is coming from.  For example are you using an AD 633 multiplier ic for the modulator? If so, then you could add a small DC at the  z input. However, if the DC is coming from an opamp then use a nulling DC at the terminals is the correct approach. Putting the complete circuit into PSpice is a great way of quickly solving problems like this.

regards

paul

Awais, you should add a sketch of your circuit...

Massimo I added the block diagram of my circuit.

well i dont know anything about the electronic you are using... its hard to say at this point but a capacitor should so it. add a capacitor before output then rearrange your circuit and calculations accordingly 

I would try with a decoupling capacitor.

I agree withVasile but really to solve this problem I would need the exact circuit in order to see where the offset is coming from.  For example are you using an AD 633 multiplier ic for the modulator? If so, then you could add a small DC at the  z input. However, if the DC is coming from an opamp then use a nulling DC at the terminals is the correct approach. Putting the complete circuit into PSpice is a great way of quickly solving problems like this.

regards

paul

Paul I am using AD630 for modulation demodulation, First stage amplifier is AD8421, and Second is an instrumentation amplifier LT1229

Hi Muhammed

Ok, I normally use the AD633 as a modulator as its very easy to simulate and build circuits. Connecting a 50 k pot across pins 5 and 6 will null the DC offsets in the AD630. Have you considered using an ac source instead of a DC source, across the bridge? There are many advantages to this but it's a long time since I did it. However, I'm sure good old Google will come to the rescue! Better still look at the AD630 PDF. Include an active filter in the circuit will remove residual signals (I can send you some pages from my PSpice for Filters and Transmission lines, that might be of use, but you might have it in your university library)

Regards

Paul

yes i am using AC source across Bridge.

I thought you might have! Have you simulated the complete circuit in PSpice?

2 ways: either a capacitor and even better, a capacitor in series with a known active impedance (capacitor + OP amp based stage) or a second: right before the PSD add a low gain summing OP amp base stage or a low gain non-inverting stage with variable bias on the inverting input. The bias of the inverting input will allow the offset adjustment (eg: cancelling the offset). Let me know if further details are useful!

I simulate it in Multisim.

Hi Muhammad,

The offset at the output signal may be due to an unbalance of the bridge. Despite of all extensometers have the same nominal value (e.g. 350 Ohm), in fact, they are all different, i.e. there are slight differences of resistance between all them, which contributes for the offset in the output signal.

You may confirm this hypothesis performing a simple test. You can excite the bridge with a DC signal (e.g. 5VDC) and measure the output signal with a high resolution voltmeter.

If this hypothesis is confirmed, you can decrease the offset by adding resistances to the extensometer arms (in parallel configuration, and not necessarily in all arms). These resistances must have a temperature coefficient similar to that of extensometer.

I hope to have contributed to solve your problem. If you feel I can help you, please send me feedback.

Best regards.

Carlos

Hi Muhammad,

The offset at the output signal may be due to an unbalance of the bridge. Despite of all extensometers have the same nominal value (e.g. 350 Ohm), in fact, they are all different, i.e. there are slight differences of resistance between all them, which contributes for the offset in the output signal.

You may confirm this hypothesis performing a simple test. You can excite the bridge with a DC signal (e.g. 5VDC) and measure the output signal with a high resolution voltmeter.

If this hypothesis is confirmed, you can decrease the offset by adding resistances to the extensometer arms (in parallel configuration, and not necessarily in all arms). These resistances must have a temperature coefficient similar to that of extensometer.

It is not sure that simulation may give you the right answer for your problem. The real conditions are always slightly different from the simulated ones, mainly if you don't know which element of your system is the source of offset. I would say that there are several sources of offset and you just need to find the strongest.

I hope to have contributed to solve your problem. If you feel I can help you, please send me feedback.

Best regards.

Carlos

Inorder to remove an constant amount of voltage from an input signal, develop a subtraction amp op topology then put in non invester input side your signal and connect a potentiometer in to the non inverted side of the amp op. Remember the value of potentiometer must be less than the other resister in subtractor topology in a considerable amount.

I prefere to opt all the resistors the same value eg. 100k.

Now you put the oscilloscope pin in the out put and see how much you have to turn the potentiometer to have the adequate out Put.

in this case I do not recommend you to use capacitor since it removes important information.