Differential amplifier drift at output

Hi,

In our design we are using differential amplifier( AD629BRZ) but it is behaving some what different.

my application is measuring current,for your reference i am adding schematics

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my current is 48mA

I have measured voltage across the resistor is 4.8mV,but output of differential amplifier is 4.4mV,there is a drift of 0.4mV

why this much of variation in input and output?

I want exact  input voltage in output what i will do ,can you please guide how to resolve this issue

Thanks in advance..

Thanks,

Siva Nageswara Rao

Mobile: +91 9030346190