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I have an issue with the PGA207 instrumentation op-amp.

Basic connections of the PGA207

I connected it as a voltage buffer, as seen in the following image:

Voltage buffer in design

When I connect a 1 kHz, 2Vpp sine wave to the non-inverting input (pin 5) and measure the output on pin 11, the signal amplitude is precisely halved.

I measured the rails and each is within the minimum range (datasheet says minimum ±4.5V, and I measured ~4.9V and ~-4.9V on the rails). The input (pin 5) and output (pin 11) are disconnected from the rest of the circuit (I put 0 Ω links in to remove for testing).

Currently, I am not driving the two gain control pins (15 and 16), so I should be getting a gain of 1. I also tried pulling the gain control pins high one after the other and it does not seem to reliably change the gain according to the schematic. "01" lifts the amplitude slightly and "10" makes a big jump to outside the Pico range. By default, as mentioned before, the programmable gain of "00" gives me an actual gain of 0.5.

Any other debugging tips or tricks I can try would be appreciated.

ocrdu
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Adriaan Sadie
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    an instrumentation amplifier has the feedback circuitry on the die. So by connecting V- to the output, you've altered the feedback network. – Rahmany Aug 30 '22 at 08:19
  • How would I be able to make a summing amplifier using this chip? – Adriaan Sadie Aug 30 '22 at 17:47
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    Assuming the signals are referenced to the same potential, then it's the same way you would make a normal summing amplifier but without the feedback. https://en.wikipedia.org/wiki/Operational_amplifier_applications#Summing_amplifier i.e. without Rf. Signals should be fed to the V+ pin and their reference to V- – Rahmany Aug 30 '22 at 22:36
  • Thanks. Will try that! – Adriaan Sadie Aug 31 '22 at 06:28

2 Answers2

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You should not be feeding the output back to the Vin(-) input- this is an instrumentation amplifier- the feedback is all internal.

Connect the Vin(-) terminal to a quiet ground if you don't need it.

Now you have:

Vout = Vin(+) - Vin(-) (G=1)

But you've connected Vin(-) to Vout, so Vout = Vin(+) - Vout,

or Vout = Vin/2.

Spehro Pefhany
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I connected it as a voltage buffer

You made a circuit that would be totally suitable for an op-amp but unsuitable for an instrumentation amplifier given it's main use as a differential amplifier.

Andy aka
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