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PGA112: Current input

Part Number: PGA112


One of my customers is using PGA112 and noticed the following:

Both the Analog and Digital part of the PGA112 are being supplied with 5V. As soon as the output goes into saturation [Vin x Gain >= 5V] CH1 stops being high impedance, instead current starts flowing. Also, the bigger the amplification, the more current that flows:

VIN = 1V / GAIN = 1 -> I_IN = 0mA

VIN = 1V / GAIN = 2 -> I_IN = 0mA

VIN = 1V / GAIN = 4 -> I_IN = 0mA

VIN = 1V / GAIN = 8 -> I_IN > 0mA

VIN = 4V / GAIN = 1 -> I_IN = 0mA

VIN = 4V / GAIN = 2 -> I_IN > 0mA

VIN = 4V / GAIN = 128 -> I_IN = 0,6mA

This device is a non inverting amplifier according to the DS. In Common Mode range the input should be high impedance and independent from the output. 

While this behavior does not seem to have an impact on the application, it would still be great to understand why this is happening. On the DS I couldn't find anything that might explain this.

Is this behavior expected?

Can you please explain why this happens?