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Issue measuiring differential voltage accurately.

Category: Hardware
Product Number: AD8227

Hello,

I am having trouble with a current sensing circuit that uses a shunt resistor and the AD8227 Differential Op-amp. 

I find that I have a differential voltage of +/-10mV at the inputs. I am using a gain of 85V/V. I am using a 1.65V VREF so that I can measure the current across the shunt resistor in both directions. 

with these parameters I expect an output voltage of 0.8V (when input is -10mV) to 2.5V (when input is +10mV). 

Spice Agrees with this calculation. 

(Schematic:)

Differential Input:

Output:

In the real world here is my schematic:

What I am seeing is that when the differential input voltage is -10mV, The output is 0.9V as expected, however, when the differential input voltage is +10mV, the output starts at 2.5V as expected, but then slowly ramps down to 1.65V over about a minue. 

I confirmed that the input voltage is not changing from the 10mV, and also confirmed the current is not changing during this time. 

I measured this ramp down output voltage both at the output of the AD8227 (U27) and at the output of the buffer op amp (U28).

Any help would be appreciated!

Thank you,

Aaron

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Top Replies

    •  Analog Employees 
    •  Super User 
    Mar 31, 2024 +1 verified

    Hi  

    Thank you for reaching out and sharing your inquiries with the team.

     Apologies for the late response, as we had a long holiday last week.

     Regarding your inquiry, I tried the AD8227 in…