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AMC1200: Issues with the amplifier model AMC1200SDUBR

Part Number: AMC1200

Hello everyone, a while ago I used the AMC1200SDUBR for a design to measure AC voltage applied to a single-phase motor with a capacitor. This is the circuit I'm using to measure the voltage:

However, I have a major issue, and it seems like the measurements I'm taking make no sense. When I turn on the equipment, it reads 100 volts, but when I turn it off and then back on after a couple of minutes for the same AC voltage, it reads 90, and then 103. It really doesn't make any sense. Did I design the circuit incorrectly? I reviewed an application note that was recommended to me, but I decided to use the circuit I showed you for my tests. What could be happening?

Note: I use a phase control, digital dimmer, to regulate the firing angle of a triac and thus control the voltage supplied to the motor.

         VDD = 3.3V and 5V_ANALOG = 5.0V.

         I'm using a B0505S to provide 5V_ANALOG to the chip.


  • Hi Martin,

    There could be a couple of reasons for the inconsistent voltage readings. The circuit could be oscillating. The 10nF differential output capacitor (C18) is quite large, this could be a cause for the oscillation. Possibly reduce the capacitor value or increase the resistor values (R22 and R23). 

    Another cause could be the power supply stability. A change in supply voltage would directly contribute to offset error in the output. The 1.65V on the single-ended common-mode voltage could be shifting. Try reading the voltage here as well. If there is inconsistency, try altering your filter. 

    Please let me know if this helps.



  • Hi ,

    Thank you for your interest in helping me. I tried to do what you recommended, but it still wasn't working properly. So, I decided to use the configuration from this application note:

    Now, everything is functioning correctly. I still have some issues with measuring accurately, but it consistently gives the same reading without any changes, even when I turn the device on and off.

    Best Regards,

    Walter Paz.

  • Hi Walter and Martin,

    I am glad you were able to find a solution!

    Best regards,