AD628 offset issue

HI,

I am using AD628 for my current sensing application .Current range 0 to 2.5amps through the R76 (0.01 current sense resistor )  . the gain for first stage is 0.1 and second stage is sate as 3.12  , supply voltage for the AD628 is +5V and GND (single supply). while measuring the 1st stage output at C512 (by varying the current from 0amps to 2 amps) , I am always getting 0.715V and second stage 2.174V . whereas the expected voltage level should be as below .

  

Current(amps) Expected output @ 1st stage of AD628 . Expected output @ 2st stage of AD628.
0.5 0.05V 0.156V
1 0.1V 0.312V
1.5 0.15V 0.468V
2 0.2V 0.624V
2.5 0.25V 0.78V

Kindly address the  same issue .

Regards ,

Wasim

  • 0
    •  Analog Employees 
    on Jan 5, 2016 1:13 AM

    Hi Wasim,

    Thank you for the details. What kind of supply is the 15Vdc that you are monitoring here and what kind of load is it supplying? What application will this be going into?

    Normally when the AD628 is used with a single 5V supply, VREF is connected to 2.5V, as in table 2 of the datasheet. For both amplifiers in the AD628, A1 and A2, the minimum output voltage swing is 0.9V. The output of your first amplifier, A1, is saturated just below this at 0.715V, which is why you can't see any change in the output voltage. See the "Input Voltage Range" section of the datasheet for a complete explanation of the operating voltage ranges.

    Also, I disagree with your expected voltages. 0.5A * 0.01 ohms = 5mV across the shunt (also the differential AD628 input), so the calculated A1 output would be 0.5mV and the A2 output would be 1.56mV, and so on for the higher currents. You will need to take more gain to see the current changes and also raise Vref above 1.2V (as in the datasheet figure 29) so you can measure zero input current. You might also consider a dedicated current-sense amplifier such as AD8418A.

    I hope this helps.

    Best regards,

    Scott

  • Hi Scott,

    Thanks for your support . As per the present plan I don't want to redesign the circuit with new components . I did some re-work for a current from 0.856 to 1.8Amps and measured the voltage at the pin-4 (output of the first stage ) . While measuring I made the second the stage saturation mode by mounting R430 as 300K and R431 as 10.2K . While changing the current from 0.856 to 1.8Amps , Observation shows the output of the 1st stage (i.e Pin-4) decreasing from 601mV to 599mV . Can you please confirm the point weather it is  a feasible way , by providing one gain stage from the pin-4 to measure the current.

    I want to say that the load is resistive fro Current sink(@15V output ).

    Regards,

    Wasim

  • 0
    •  Analog Employees 
    on Jan 13, 2016 1:10 AM

    Hi Wasim,

    I don't understand your re-work. Did you only change R430 and R431? But you didn't change the supplies or the voltage at VREF (pin 3)? If so, that doesn't help as the A1 output is still saturated. Why do you want the second stage amplifier to be saturated? You can't measure a signal with a saturated amplifier. The input voltage and output voltage have to be within the linear range. For example, see MT-041. The two op amps inside AD628 have the same limitations.

    If you can't change VREF, the other option is to remove ground from pin 2 (-VS) and apply a negative supply such as -5V to pin 2 instead. -5V is more than enough to satisfy the voltage ranges.

    Once the input and output ranges are satisfied, you can optimize the gain and signal range. Check pin 4 and pin 5 to see how the output changes with current. 1.8 amps should be 54mV (1.8A * 0.01Ohms * 0.1 * (1 + 300k/10.2k)) at the output pin 5 with the new R430 and R431 values. If you changed -VS, the 54mV is referenced to ground, but if you changed VREF, the 54mV is referenced to that voltage. You may still find you want to increase the AD628 gain more. If you reduce R431 to do this, make sure to add a matching resistor Rext3, as shown in figure 29 and described in the "Gain Adjustment" section of the AD628 datasheet.

    You might be able to increase your shunt resistor. 2.5A and 10mOhm is only 1/16W. Then increase the AD628 gain until your signal is large enough to measure. Table 5 only shows gains up to 10, but AD628 is specified up to G=100.

    But just to be clear, the AD628 will not work in your application unless you either:

    1) increase VREF (both pin 3 and the termination of R431, as in figure 29) to at least 1.2V

         OR

    2) decrease -VS (pin 2) to lower than -1.2V (with enough margin for power supply variation)

    I hope this helps.

    Best regards,

    Scott

  • 0
    •  Analog Employees 
    on Aug 2, 2018 3:19 PM
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