Electronic – Why is the instrumentation amplifier’s gain non-linear

gaininstrumentation-amplifieroperational-amplifier

I am using an AD8226 instrumentation amplifier as the pre-amp in a circuit which uses a micro fuel cell oxygen sensor to measure oxygen content in a gas stream. The pre-amp is configured in single ended supply mode, with a 5 V supply. Gain resistor was chosen to give a gain of ~80.

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The output of the in-amp is then passed through an active low pass filter and to a microcontroller's ADC, but I don't believe this is relevant to the problem. Everything discussed here was done with the preamp outut (O2_PRE node in the diagram) floating and only connected to a multimeter.

When testing this circuit with the sensor I found that the gain was nice and linear – up until a certain point where the gain dropped off (this point was at about 20mV input/1.6V output).

To eliminate any issues with the sensor I replaced the sensor with a voltage divider made up of a fixed reisistor and a multi-turn linear pot:

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With the resistor network I observed the same problem (see further down for a plot of the inamp output).

To further eliminate any issues with the surrounding circuitry I wired directly up to the AD8226, using my bechtop supply as VCC. Giving the circuit seen here:

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This showed the same behaviour as seen in this plot:

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('mainboard' refers to the output when the voltage divider is used to drive the original circuit, 'breadboard' gain is slightly lower as I used a 630R gain resistor)

The AD8226 data sheet specifies an output voltage swing of 0.1 V to +VS−0.1 V in single ended mode. I am measuring the output into a high quality multimeter (ie. high impedance), although I did observe the same results when I added a load resistor of 20 K. I have repeated the results with multiple AD8226 devices.

Can someone explain why the gain would be falling off above a differential input of ~20 mV given that I seem to be staying well within the output limits?

Best Answer

Most likely a common-mode voltage violation. See Figure 15 of the datasheet. Unfortunately it does not give you the limits for a single power supply, but shows you that common-mode and output voltage limits go hand in hand.

Actions that may solve your problem:

  • Use dual power supply.
  • Lift inputs above ground.
  • Use a lower gain setting.

EDIT:

As pointed out by Dave Tweed, the datasheet does have information on common-mode and output voltage ranges for single supply. See Figure 13 on page 10 of the datasheet (your case is the blue triangular area). Your common-mode input is practically ground, so as soon as your output exceeds about 0.7 V, the limit is exceeded, and your device will not behave as specified. This will happen even if you change Vref to 2.5 V: the output limit will be 3.2 V for a common-mode input voltage of ground to become invalid.

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