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

    •  Analog Employees 
    on May 6, 2018 6:26 PM over 2 years ago

    Hi Siva,

    Can you tell me what is the part number of the shunt resistor that you used? If the value of the shunt resistor change due to temperature, it will reflect as gain error on the measurements.

    How did you expose the parts on 60°C? May I know if all your circuit was exposed to this temperature or only the AD629? I just want to check all variable that could contributes on the error of your measurements.

    May I also know what is the common mode voltage across the shunt resistor? I have experience that CMRR also drift with respect to temperature but since the AD629 is almost 20 years old part, this information is not available on its TPC. But we have the newer version of the AD629 which is the AD8479, it has better overall accuracy specs than the AD629 which you might also consider.

    Are you working with a new development design? May I know why you chose the AD629 on your applications?

    Best regards,

    Emman

  • Hi Emman,

    Please see my response in below

    Can you tell me what is the part number of the shunt resistor that you used? 

    ERJ-B1CFR10U

    How did you expose the parts on 60°C?

    we keep the enclosed box inside the temperature chamber , AD629 is also exposed similar to all other componets

    May I also know what is the common mode voltage across the shunt resistor?

     

    Voltage at pin 2(-Ve terminal)=0.3731 V

                     pin3(+Ve terminal)=0.3931V

           Voltage across shunt resistor=19.9652mV

           output voltage= 20.03mV

    Are you working with a new development design?

     Yes

     

     

     

    Thanks,

    siva

    •  Analog Employees 
    on May 7, 2018 5:41 PM over 2 years ago

    Hi Siva,

    Thanks for your answer. Drift due to the tempco of the shunt resistor can cause you around 0.7 mA error. May I know other components that is exposed on high temp? Note that since you are exposing your whole system on high temp, the error that you are getting is not only coming from the AD629 but from all the components that also drift with respect to temperature.

    You mentioned that common mode voltage is around 0.37V but looking at your schematic, having a 200mA input current and R206 (10M), it will give you a lot of common mode voltage. Is R206 connected to your circuit? Are you using current source or voltage source when testing?

    Is your input source coming from the equipment or it is included on your circuit inside the chamber? Can you get measurements of the following for me to check possible source of error.

    1. At 25C, with 200 mA input current, measure the voltage across TP110, TP11, R212 (shunt resistor), TP94(Vout)

    2. At 60C, with 200mA input current,  measure the voltage across TP110, TP11, R212 (shunt resistor), TP94(Vout)

    What is the model of your current source and the multimeter that you are using?

    Also, the computation you showed for the -20C, the computed value from the datasheet specs should be subtracted from the 197 mA (197mA - 4.5mA) if the reference value is at 25C.

    Best regards,

    Emman

  • Hi Emman,

    please see my comments in below 

    1.May I know other components that is exposed on high temp?

     Yes

    2.Is R206 connected to your circuit?

    It's connected

    3.Are you using current source or voltage source when testing?

    Current source

    As per application field device generate 200mA current ,it is given to circuit.

    we observed device is giving 199mA(connected ammeter in series) 

    4.Is your input source coming from the equipment or it is included on your circuit inside the chamber?

       Input is coming from external equipment 

    5.What is the model of your current source and the multi meter that you are using?

       Current source:  Process/Hygienic Master Transmitter 610(Field device)

        Multimeter : Keysight 34461A

    6.The computation you showed for the -20C, the computed value from the datasheet specs should be subtracted from the 197 mA (197mA - 4.5mA) if the reference value is at 25C.

    yes,you are correct

    197mA-4.5mA=192.5 but we are geting  190mA-12mA=185mA

    i will share  TP110, TP11, R212,TP94 values at 25 C and 60 c ASAP

    Thanks,

    siva