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I have an instrumentation amplifier, INA 126. I am using it to amplify the physiological signals from the eye movement. The circuit I have constructed has 2 stages: 1st stage has INA 126 with a gain of 1000 and the 2nd stage has another INA 126 also with a gain of 1000.

My question is that the 1st stage only has one output so where do I connect this output?I have tried connecting it to Vin+ and grounded the Vin- of the 2nd INA 126, is this the right thing to do? I have read the datasheet and it does not mention of having one input only.

P.S I am using dual supply.. +12V and -12V and I have connected the Ref pin of the INA126 to ground as mentioned in the datasheet.

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  • \$\begingroup\$ That sounds fine. \$\endgroup\$ – Andy aka Apr 13 '15 at 14:22
  • \$\begingroup\$ So theoretically the Vout from the 2nd stage is Vout=Gain(Vin+) right? \$\endgroup\$ – Herbert Apr 13 '15 at 14:28
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    \$\begingroup\$ There's no reason to use another instrumentation amp as the second stage - use a regular opamp instead. \$\endgroup\$ – Nick Johnson Apr 13 '15 at 14:43
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Yes you can do that. There's not much point in using an instrumentation amplifier when the input is single-ended but there's no harm in it (other than to your pocketbook and likely inferior noise performance). Since you're preceding it with a gain of 1000, noise is not going to be an issue, so just cost.

I do hope your circuit is AC coupled, otherwise you're going to have problems- the 250uV offset of the INA126 multiplied by 10^6 is 250V which will rail the output amplifier.

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  • \$\begingroup\$ Forgive me for my ignorance sir but how do you do AC coupling? \$\endgroup\$ – Herbert Apr 13 '15 at 14:39
  • \$\begingroup\$ @Herbert One method is to provide a resistance to ground for the bias current and connect a capacitor to the input voltage. You can do that between stages. The input might be a bit trickier- here is one method. \$\endgroup\$ – Spehro Pefhany Apr 13 '15 at 14:45
  • \$\begingroup\$ @herbert, It's probably enough to put the AC coupling between the first and second stage. There a simple RC will be fine... You need to know what frequencies you are interested in... 0.1 uF with 10k ohm to ground starts responding above ~ 100 Hz. If your signal is at or near DC you may have to change your approach. \$\endgroup\$ – George Herold Apr 13 '15 at 15:01
  • \$\begingroup\$ @Herbert: I strongly recommend that you do not proceed. You plan to use this device attached to a human, but you don't even know AC coupling? that is not good My feeling is that you should learn a lot about electronics before progressing..... \$\endgroup\$ – Sascha Jan 5 '17 at 13:31

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