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Original question:

AMC1200-Q1: Input side DC power supply grounding

AMC1200: AMC1200BDWVR DC Link Voltage measurement application GND1 to be connected to DC link -Ve terminal.

SHIVAKUMAR T N
Prodigy 40 points
Part Number: AMC1200

Hi,

Pls find the below AMC1200 schematic of our DC Link voltage measure application. Here, after inverter turn on, the AMC1200 differential input (VINP-VINN) will oscillating to maximum voltage & output also changed to maximum. Due to this our inverter is going DC link over voltage & Brake chopper firing. find the attached wave forms for reference.

1) C1 & C2 DC Link Voltage, C3 is AMC out put & C4 is AMC input differential voltage.

Q1. What is the issue in below waveform, Any changes is required in circuit, pls let me know. 

2) below waveform is circuit modified to -VDC side resistors shifted to +VDC side (8 resistor in Series) & GND1 (AMC1200) connected to -VDC. Here AMC1200 input oscillation is reduced. 

C1 is DC Link Voltage, C3 is AMC out put & C2 is AMC input differential voltage.

Q2. After connecting the GND1 pin -VDC oscillation is reduced. Let me know GND1 pin should be connected to -VDC or not.

Q3. same circuit can be used AC voltage measurement or not.

Thanks & Regards

Shiva Kumar T N

  • 0
    TI__Mastermind 49071 points

    Hi Shiva Kumar T N,

    Can you please re-post your pictures? 

  • 0
    Prodigy 40 points

    Hi,

    Pls find the below AMC1200 schematic of our DC Link voltage measure application. Here, after inverter turn on, the AMC1200 differential input (VINP-VINN) will oscillating to maximum voltage & output also changed to maximum. Due to this our inverter is going DC link over voltage & Brake chopper firing. find the attached wave forms for reference.

    1) C1 is DC Link Voltage, C3 is AMC out put & C2 is AMC input differential voltage.

    Q1. What is the issue in below waveform, Any changes is required in circuit, pls let me know. 

    2) below waveform is circuit modified to -VDC side resistors shifted to +VDC side (8 resistor in Series) & GND1 (AMC1200) connected to -VDC. Here AMC1200 input oscillation is reduced. 

    C1 is DC Link Voltage, C3 is AMC out put & C2 is AMC input differential voltage.

    Q2. After connecting the GND1 pin -VDC oscillation is reduced. Let me know GND1 pin should be connected to -VDC or not.

    Q3. same circuit can be used AC voltage measurement or not.

    Thanks & Regards

    Shiva Kumar T N

  • 0 in reply to Alexander Smith
    Prodigy 40 points

    Hi,

    Pls find the below AMC1200 schematic of our DC Link voltage measure application. Here, after inverter turn on, the AMC1200 differential input (VINP-VINN) will oscillating to maximum voltage & output also changed to maximum. Due to this our inverter is going DC link over voltage & Brake chopper firing. find the attached wave forms for reference.

    1) C1 & C2 DC Link Voltage, C3 is AMC out put & C4 is AMC input differential voltage.

    Q1. What is the issue in below waveform, Any changes is required in circuit, pls let me know. 

    Attachment2

    2) below waveform is circuit modified to -VDC side resistors shifted to +VDC side (8 resistor in Series) & GND1 (AMC1200) connected to -VDC. Here AMC1200 input oscillation is reduced. 

    C1 is DC Link Voltage, C3 is AMC out put & C2 is AMC input differential voltage.

    Q2. After connecting the GND1 pin -VDC oscillation is reduced. Let me know GND1 pin should be connected to -VDC or not.

    Attachment 3

    Q3. same circuit can be used AC voltage measurement or not.

    Thanks & Regards

    Shiva Kumar T N

  • 0 in reply to SHIVAKUMAR T N
    TI__Mastermind 49071 points

    Hi Shiva Kumar T N,

    Where is your high side power being generated from? 5V2 and GND2? 

    GND1 of the AMC1200 should certainly be tied to one of the inputs such that the inputs signals remain within the common-mode range of the AMC1200. 

    I would also recommend increasing the size of the differential input filter capacitor, shown as 330pf. This yields a cutoff frequency of 20MHz, which is 20x too high for the AMC1200. Try and adjust the anti-aliasing RC filter cutoff frequency to be 1MHz and see if that helps reduce noise of the input signal. 

    Two resistor divider strings would not be necessary for an AC voltage input. Since the input of the device is psuedo-differential, simply tie VINN to GND1 and use a resistor divider to scale down the input voltage connected to VINP. However, the same input structure could also be used for AC voltage sensing, as long as the power supply is capable of floating with the input voltage. 

  • 0 in reply to Alexander Smith
    Prodigy 40 points

    Hi Alexander Smith,

    Thanks for your feedback, Pls find the below feedback from my side.

    Where is your high side power being generated from? 5V2 and GND2? 

    Ans: We using 5V to 5V Isolated DCDC converter (1W,3KV) for High voltage side power supply, input taken from low voltage side 5V (in given circuit it is 3.3V mention,but actually it is 5V).

     

    GND1 of the AMC1200 should certainly be tied to one of the inputs such that the inputs signals remain within the common-mode range of the AMC1200.

    In our circuit this GND1 is connected to differential input (R7 resistor 2nd pin) & After R7 resistor we have connected 4nos 39K resistor in series to -VDC. is it ok or GND1 & R7 resistor 2nd pin need to connect -VDC. If we connect GND1 & R7 resistor 2nd pin need to connect -VDC, the differential input & output signal are smooth.

     

    I would also recommend increasing the size of the differential input filter capacitor, shown as 330pf. This yields a cutoff frequency of 20MHz, which is 20x too high for the AMC1200. Try and adjust the anti-aliasing RC filter cutoff frequency to be 1MHz and see if that helps reduce noise of the input signal. 

    Yes, We have changed the filter frequecy to 1MHz & 500KH, but input signal noise is remain same, find the below waveform.

    approximate 1MHz filter

     

     

    approximate 500KHz filter


     

    Pls suggest further for suppress the noise.

     

    Thanks & Regards

    Shiva Kumar T N.

  • 0 in reply to SHIVAKUMAR T N
    TI__Mastermind 49071 points

    Hi Shiva Kumar T N,

    Thank you for trying the different anti-aliasing filter frequencies, it looks like there was some benefit but it did not fix the issue. 

    What is the high level voltage that is being stepped down? Is 4x 39k sufficient to step down this voltage? 

    I would recommend splitting the resistor shown as 75E into two resistors, then connecting GND1 between them such that a center-tap connection is made. 

    An example of this configuration can be seen in the attached file. 6254.8865.VLL_sense_AMC3301_AS.TSC 

    Is it possible for you to access the board with an external power supply? It may be that there is some interference from your ISO DC/DC converter. Removing the connections and powering the device externally may help isolate the issue.