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I am using AD8231 instrumentation amplifier in unity gain configuration. I used the inbuilt op-amp to provide reference voltage as the datasheet suggests. A 4.4kHz sine wave generated with my professional signal generator (PC sound card) is fed into the non-inverting input of the in-amp.

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Channel one is the input signal and channel two is the output of the in-amp. For some reason the output clamps near the reference voltage (1.6V). enter image description here

The circuit is built on a breadboard and I haven't used decoupling and filtering capacitors since I'm prototyping. Here is a photo of the breadboard in case it helps:

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Table 3 on page 5 of the linked datasheet gives a minimum input voltage of 0.05 V. It looks like you are driving the input voltage at +INA to below ground, so you are violating this requirement.

Probably there are esd protection diodes on the input pins that are short-circuiting the input to ground when you try to drive it negative.

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  • \$\begingroup\$ Damn, it seems to be the case. I thought that setting the reference voltage to mid-supply will offset the input signal and allow the inputs to go negative. \$\endgroup\$
    – Ashton H.
    Commented Apr 22, 2015 at 22:31
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    \$\begingroup\$ If you offset the -INA input and also center the input signal around that voltage (for example, by ac-coupling), you should be able to get the results you want. \$\endgroup\$
    – The Photon
    Commented Apr 22, 2015 at 22:38
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    \$\begingroup\$ Alos, the REF input probably only affects the last stage of the amplifier. The input stages most likely are not affected at all by what you do wit the REF input. \$\endgroup\$
    – The Photon
    Commented Apr 22, 2015 at 22:39
  • \$\begingroup\$ I don't really "get" a single supply instrument amp. The sample circuit in the spec sheet are all for split supplies. \$\endgroup\$ Commented Apr 22, 2015 at 23:39

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