AD8275 Accuracy Problem

Hi,there

I'm using AD8275 for my project to scale the +/-10V bipolar singal to unioplar 0-4V so that it could be accepted by a 5V ADC. According to the typical appication circuit of the data sheet,i built a minimum one based on this chip.During testing,i found that it worked well for positive signals,but the scaled output was not so accurate as i expected when input was switched to negative range.Can someone help me figure out what the probelem lies in? I attach some measured and calculated value here. 


** The measured reference voltage(VREF) is about 4.1V
** VOUT = VIN / 5 + VREF / 2

  VIN     Measured Value     Calculated Value
+9.5V          3.95V                       3.95V
+7.5V          3.55V                       3.55V
+5.0V          3.05V                       3.05V

  0.0V          2.05V                        2.05V

-5.0V          1.04V                        1.05V
-7.5V          0.54V                        0.55V
-9.5V          0.12V                        0.15V

It's clear that the accuracy is not satisfied when VIN goes to negative.

Thank you in advance.

--Adam

  • Adam,

    Usually this kind of problem results from operating the input or the output too near the supply voltages. For instance the AD8275 claims to swing from -Vs + .048V to +Vs - 0.1 V. For your condition this is a swing of 5 V. - .148 V. = 4.852 V.

    However when you look at the specified range for good nonlinearity it is only 4 V. (with a heavy load).

    You are likely to get a better result if you perfectly center your output swing. That means raising your Vref/2 up to 2.5 V.

    And if your Vsupply is likely to dip below 5 V. you should set your Vref/2 to (Vsupply_minimum)/ 2.

    Hope this helps,

    Chris

  • 0
    •  Analog Employees 
    on Aug 2, 2018 3:49 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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    EZ Admin