• State
  • Locked Locked
  • Replies 2 replies
  • Subscribers 46 subscribers
  • Views 931 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

AMC1200 accuracy problem

Wendelin Steiner
Prodigy 20 points

Other Parts Discussed in Thread: AMC1200

Hi

I'm using the AMC1200 isolation amplifier for voltage measurement. I use it once for measuring the grid-tension and once for measuring a DC-Bus (0V up to 300V). Unfortunately I have a big accuracy on the amplification gain (up to 3.4%).

To measure the gain I did a test with a 300VDC input voltage. You'll find an extract of the schematics and the measurement results in the appendix (Marked in red: lines I cut on the circuit; marked in orange: lines I added, marked in blue: measurements and calculations).

So I put 300VDC on the Grid and the same 300VDC on the DC-Bus (U_BUS_PRIM).

The amplification gain for the grid is 1/3600 -->expected value is 300V/3600=83.3mV, the measured value is 83mV (Fluke 175)

The amplification gain for the DC-Bus is 1/1600 --> expected value is 300V/1600=187.5mV, the measured value is 189mV (Fluke 175)

Then I measure on the output of the AMCs:

The output voltage of the AMC for the grid is 675mV --> total gain from AMC = 675mV/83mV = 8.133

The output voltage of the AMC for the DC-Bus is 1563mV --> total gain from AMC = 1563mV/189mV = 8.270

Do you have an idea, why I have such a big accuracy on the amplification gain? What could influence the measurement?

If something isn't clear enough, feel free to ask for more information.

Thanks for your response.

Kind regards

Wendelin

4087.problem AMC1200.pdf

  • 0
    TI__Genius 9975 points

    Hello Wendelin,

    Thank you very much for your complete explanation and the schematic.

    There are a couple of things I would suggest to try:

    1)    Corroborate that the voltage near pin 3 of U21 (upper AMC1200) is 2.5 V above its pin 4. Similarly for U22, confirm that pin 3 of U22 (lower AMC1200) is 2.5 V above its pin 4.

    2)    Would it be possible for you to disconnect pin 2 of U21 from pin 1 of U20:A? If so, could you then connect pins 2 and 3 of U21 and measure the output again?

    In your previous measurements you found a combination of gain error and offset voltage; shorting the inputs will give you a rough idea of the offset voltage.

    Hope this helps and let me know what you find out. Best regards,

    Jose

  • 0 in reply to Jose Duenas
    Prodigy 20 points

    Hi Jose


    My response is a bit late, but it might help others who got the same problem.


    I just put a capacitor of 330pF between VNN and VNP and now it works with the desired accuracy.

    Best regards,

    Wendelin