AD8223 gain issue

Hello every one!

In my design, I am using a AD8223 as a current sense amplifier on a +5V 10A power supply.

Here is the AD8223 wiring:

-Vs = GND (0V)

+Vs = +15V

REF = GND

RG = 1.23kohm

IN+ and IN- are connected to the sense resistor, which is in series with the +5V output rail.

The sense resistor is 0.005ohm.

The power supply I am sensing runs in constant-current mode and can be set form 0A to 10A. Its output voltage can vary from 0V to 5V depending on settings and load.

If I correctly understand the datasheet, this configuration is possible. However, the voltage gain of the AD8223 is not 70 as expected but 80 according to my measurements.

I checked the resistors values and the power supplies and everything seems OK.

Do you know where the problem comes from?

Thank you!

  • 0
    •  Analog Employees 
    on Sep 21, 2017 11:27 PM

    Hi Pascal, 

    Are you measuring a gain of 80 for the entire range of inputs? I can see that this configuration should work.

    Instrumentation Amplifier Diamond Plot Tool | Analog Devices 

    The only thing I would note for now is that the output does not go all the way to ground. Because of this, you might run into the rails when sensing lower currents out of the supply (currents less than 1A). 

    Could you provide some of your measurements? When measuring the output, could you also check the voltage across the sense resistor - just to see whether the output to input ratio is accurate. 

    Let me know how it goes.

    Thanks, 

    Kris

  • Hi Kris,

    In the link you provided, Vref is set to 350mV. But if I set it to 0V, like in my design, I get an error "The AD8223 is not operating within the supply limits".

    Here are the measurements made on 2 identical circuits. The expected I-V gain is 0.005 * 70 = 0.35. I did not measure the sense resistor voltage, I don't think my multimeter is acurate for this voltage range. For information, the resistor is also used by the power supply driver (LT3741) for current regulation. The load is a 0.2ohm 25W resistor + wires.

    Measurements below 1A don't need to be accurate for my application.

    Measured output current AD8223 output voltage Calculated I-V gain
    0,082 0,025 0,304878049
    0,133 0,039 0,293233083
    0,203 0,068 0,334975369
    0,314 0,115 0,366242038
    0,454 0,176 0,387665198
    0,667 0,271 0,406296852
    0,988 0,42 0,425101215
    1,492 0,646 0,432975871
    2,185 0,867 0,396796339
    3,3 1,374 0,416363636
    4,71 1,845 0,391719745
    6,83 2,788 0,408199122
    9,91 4 0,403632694
    Measured output current AD8223 output voltage Calculated I-V gain
    0,013 0,005 0,384615385
    0,012 0,005 0,416666667
    0,056 0,017 0,303571429
    0,168 0,051 0,303571429
    0,311 0,111 0,356913183
    0,524 0,206 0,393129771
    0,852 0,36 0,422535211
    1,359 0,592 0,435614422
    2,054 0,807 0,392891918
    3,179 1,336 0,420257943
    4,6 1,828 0,397391304
    6,71 2,68 0,399403875
    9,79 3,912 0,39959142

    Here are measurements made on another design with -Vs = -5V, +Vs = +15V, REF = GND and Rg = 842ohm. The expected gain is 0.5.

    Measured output current AD8223 output voltage Calculated I-V gain
    0,006 0,013 2,166666667
    0,265 0,123 0,464150943
    0,511 0,253 0,495107632
    1,238 0,64 0,516962843
    1,982 1,005 0,507063572
    4,94 2,528 0,511740891
    10,12 5,25 0,518774704

    I guess setting REF to -Vs is the origin of my problem. I thought this was possible according to the datasheet.


  • Pascal,

      Schematic?  Board layout?

      Are you using a commercial four wire Kelvin resistor?

    One inch of pc board trace can easily be 50 milliohms.

    Very difficult to work with a five milliohm sense resistor.

    See:

    http://www.analog.com/media/en/analog-dialogue/volume-46/number-2/articles/optimize-high-current-sensing-accuracy.pdf 

    Harry