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INA350: INA350ABSIDDFR has 2x gain

Part Number: INA350
Other Parts Discussed in Thread: OPA2837,

I have a circuit where I'm using the INA350ABSIDDFR part to amplify a signal. As per datasheet there are only two possible gains: 10x or 20x (selectable by the GS pin).

In my circuit I have jumpers to select the negative power supply (symmetrical negative or GND) and the reference voltage (GND or mid-rail).

In one test I selected symmetrical power (+/- 2.5V) with GND as the REF voltage. I supplied a 200mV pk-pk 1KHz input signal centered at GND. The circuit worked perfectly with a 20x gain.

However, in a different test I selected unipolar power (3V and GND). The REF voltage was generated at mid rail (1.5V - with a voltage divider buffered by an OPA2837). I supplied a 200mV pk-pk 1KHz input signal with a 100mV offset (i.e. 0V-300mV).

Even though the gain setting was 20x I got an approximately 2x amplification (should be impossible with this part). The output signal seemed noisy. I checked the scope probe settings: I used a 1:10 probe, but this was set correctly in the scope (verified by measuring the input signal with the probe) - so this is not the reason. I left the GS pin floating.

I have three identically connected INA350s on my board and they all behaved exactly the same, even after power cycling. I saw this 2x gain effect once before with this part in a different circuit, although it disappeared by itself.

Is there any known fault condition in which this part can have a 2x gain? E.g. GS in an intermediary state, REF loading, etc?