Instrumentation amplifier gain, twice than expected

[keith1200rs]

Some signal generators assume 50 ohm load so if you set it to 100mV you will get 200mV if you don't load it with 50 ohms. It is always best to monitor both the input and output signals with an oscilloscope.

Keith.

 

[sudil99]

Hi. Thanx to everyone for your effort. But the problem is not solved yet.
Actually, I had already designed the PCB for that circuit after simulation. Its a part of ECG amplifier. The eagle schematics of this part is attached here. Since it was giving me double of expected gain, I checked in the breadboard. I tried using LT1167, INA129 and AD620, and got same result. I also checked my inputs.
So, right now, instead of 2.7k + 2.7k, I am using 4.7k + 4.7k = 9.4k, and getting a gain of 12, close to what I need.
I know its really strange.

 

[varunkant2k]

Gain should be 1+2*(R2/R1)
R1 = 2*2.7K or 2*4.7k
R2 as you already wrote 49.4K

 

[FvM]

R2 as you already wrote 49.4K

sudil99 didn't write this (and it isn't right anyway).

R2 is 25k for INA128 and 24.7k for INA129, most compatible industry standard instrumentation amplifier use the same value.

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[sudil99]

Today, I found out that this is due to the function generator/oscilloscope. For some reason, if I connect one channel of the oscilloscope at the IN-amp input where function generator is connected, it shows correct output. If I do not connect oscilloscope at input, it shows double the expected output. In either case I am connecting the other channel of oscilloscope on the IN-amp output to read the output.
Also with my arduino ADC, if I connect oscilloscope to input of ADC, ADC reads correct. If I do not connect, it reads double of value.
I rechecked the grounds of oscilloscope, function generator, power and circuit. They are all connected to same ground.

Does anyone know why it's behaving like this?