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

<>ez.analog.com/.../pastedImage_5.png

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

    •  Analog Employees 
    on Apr 2, 2018 10:51 PM over 2 years ago

    Hello Siva, 

    The AD629 has a typical offset voltage of 0.2mV for the A grade, and 0.1mV for the B grade. With +/-5V supplies, the offset can be as high as 1mV at 25C. 

    If you want better accuracy, you can calibrate out the offset error, either via hardware, software, or both. 

    To calibrate through software, ground the inputs of the amplifier then measure the output. This will be your offset, and can be subtracted through software when you take your actual measurements. It would be better to calibrate every so often so that you can correct for any drift on the offset. 

    If you want to do the calibration through hardware, you would still need to measure the offset with the inputs grounded, then invert this voltage and apply it to the reference pin. You could also use a potentiometer and a buffer to apply a voltage to the reference pin that will zero out your output voltage. 

    For best accuracy, you could do both. The hardware method can be used to zero out the output, and any residual offset can be subtracted through software. 

    I hope this helps. 

    Thanks, 

    Kris

  • Hi Kris,

    Thank you for your support.

    we measured offset voltage of differential amplifier is  0.3mV,but in this two different types of reference voltages one is 

    -Ref and +Ref .where i want to apply inverted 0.3mv voltage.

    •  Analog Employees 
    on Apr 4, 2018 5:45 PM over 2 years ago

    Hi Siva, 

    You can short the two reference pins and apply the inverted voltage to that node. 

    Thanks, 

    Kris

  • Hi Kris,

    we are facing one more issue with this differential amplifier(AD629BRZ) at temperature 60 C 

    we know already this amplifier having 0.3mV offset voltage at room temperature.

    At 60 C we observed more deviation in results as per datasheet deviation is less.

    For your information I am adding application circuit and test results

    can you please give me your inputs for solving this issue.

    Thanks,

    Siva