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AMC1100: Using TL031 to converter AMC1100 differential unipolar output to single ended bipolar output.

Part Number: AMC1100
Other Parts Discussed in Thread: TL031,

Dear TI E2E community, 

We are implementing a 3 phase voltage measuring circuit using the AMC1100 isolation amplifier. Our DAC unit requires a bipolar input of +-10V range, so I am using the TL031 operational amplifier in differential mode to get rid of the common mode voltage set by the AMC1100 and to amplify the signal up to the DAC full range. 

The main problem is that this design is not as precise as we require, and the voltage output suffers from high common mode voltage (200mV or more), gain error (slightly reduced) and distortion of the waveform when zero crossing. We tried using high precision resistors (0.1% tolerance) and although the output improved, the problem still persists, particularly at low input signals. 

Are we doing something wrong? In case no, is there any other way to achieve this configuration with improved accuracy? 

Many thanks in advance and Kind Regards,

Bran Alfeiran 

  • Hi Bran,

    Welcome to the e2e forum!

    Can you grab a screen shot of the VOUTP and VOUTN signals from the AMC1100? It would be interesting to see what they look like compared to your VOUT signal from the TL031. That might also lend some insight to the offset issue you are having. The gain error is most likely due to the 1K shunt that you are using. The AMC1100 has a switched cap input which looks like ~28K input impedance (see Figures 31 and 32 in the datasheet). For the zero crossing discontinuities, can you try a little experiment for me and (using a signal generator across your shunt) try adding a little offset to the input while monitoring your VOUT and see if you can impact the behavior of the zero crossing errors.
  • Hi Tom,

    Many thanks for your reply.

    The circuit I am testing differs slightly from the diagram shown above. First difference is that the high voltage ground (GND_HV on the diagram) and the low voltage ground (GND_LV) are the same.  Second difference is that I am not using the voltage divider network. Instead I am using the signal generator capability of the oscilloscope to create the input to the AMC1100 by means of direct connection (no shunt resistor at the amplifier's inputs).

    The oscilloscope has only 2 ports of which one can be used as signal generator. Thus I cannot show you on the same screen the input and the output signals, but I can tell you the amplitude, frequency and offset of the input signal.

    Applying a 50mV amplitude, 50Hz with 100mV offset sinusoidal signal between the VINP and VINN of the ACM1100 provides the following results:

    AMC1100 signal output measured between VOUTP and VOUTN:

    TL031 signal output measured between VOUT and GND:

    It looks to me that both the AMC1100 and TL031 are amplifying the offset. Could this be caused by the way the oscilloscope works? Or is it because I am not isolating the high and low voltage grounds? 

    Also, I think the zero crossing distortion still at the TL031 output, although it is difficult to spot due to the low resolution. 

    The oscilloscope I am using is a Picoscope 2204A, which has 'differential' probes for measuring. 

    Hope you can shed some light onto this issue. 

    Thanks again,


  • Hi Tom,

    I think I found some explanation for the zero crossing glitch. 

    First, I implemented the circuit with two separate and isolated supplies and grounds for the input and output sides of the AMC1100. That seemed to solve the offset issue, and now the amplified signal from the TL031 output is centred around 0V (in reality the offset still there but is only 15mV or so). 

    I tested the circuit with the signal generator and produced a triangle waveform of 10mV amplitude, 50Hz with 0V offset, applied at the VINP and VINN of the AMC1100. When measuring the output at the TL031, the result looks as follows:

    The glitch seems to come from the AMC1100 output, probably because the input signal near 0V is too small to trigger the amplifier, thus producing a flat response of 4 pulses duration. Could that be the case? 

    Best Regards,

    Bran Alfeiran

  • Hi Bran,

    Yes, that could be the issue. Please try adding offset, a little at a time, to the input signal and see if the output from your DAC changes.