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I am building an ECG sensor based on the AD620 instrumentation amplifier.For the purposes of testing , i have built a simple AD620 circuit with pin 2(-) and pin 3(+) connected to a 2.82mV(peak to peak) sinusoidal signal generator. pin 7(+Vs) is connected to +4.5,pin 4(-Vs) is connected to -4.5. a 33k Ohm resistor is connected between pin 1 and pin 8 which theoretically should give a gain of around 2.5. Pin 5(Vref) is grounded and pin 6(output) is connected to an oscilloscope with ac coupling. 1)My first problem is the signal on the scope is 282mV peak to peak which implies a gain of 100, how can this be explained and solved? 2) if I change the gain resitor from 33k, to 37.7k, to 70.3k, I still get the same output. 3)if I change the input signal from 2.82mV peak to peak to 0.91mV I stil get the same output swing, however if i switch off the signal generator the output goes to zero. I have also tried to connect 33k resitors between each input and the ground but the result is the same.circuit diagram

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  • \$\begingroup\$ Measure the INPUT waveform with your scope to make sure your numbers make sense. In particular, pay attention to the channel probe settings and any impedance switches on the probe. \$\endgroup\$ – Scott Seidman May 8 '18 at 15:54
  • \$\begingroup\$ Also, measure your resistor. \$\endgroup\$ – Scott Seidman May 8 '18 at 15:55
  • \$\begingroup\$ As Scott says, make sure your input voltage from the function generator is actually that low, some function generators can't directly produce signals that small. \$\endgroup\$ – Spehro Pefhany May 8 '18 at 16:02
  • \$\begingroup\$ @ScottSeidman_, The input signal is too small for the scope to properly pick I think, I have tried to connect it to the scope but it is displayed superimposed on noise which makes it really difficult to distinguish it. I am new to the oscilloscope, so you can help with a better way of using it. the signal generator i am using is for biomedical purposes,it produces a signal in the range of 0-2.82mV peak to peak. \$\endgroup\$ – Major Beey May 8 '18 at 16:02
  • \$\begingroup\$ If you can't see 2.8mV, you'll probably have issues with 2.8mVx2.5 -- I recommended you do this just to confirm that you're not having issues with a scope or probe setup. Have you measured your resistor? \$\endgroup\$ – Scott Seidman May 8 '18 at 16:07

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