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AMC1300: AMC1300B - Connected to 16bit ADC of TMS320F28377D-EP

Guru 19460 points
Part Number: AMC1300
Other Parts Discussed in Thread: TMS320F28377D-EP

Hi Alexander-san,

 

We would like to sense the motor current by AMC1300B-Q1 and 16bit ADC of TMS320F28377D-EP.

I have two questions.

Could you please let us know if you have any concern and advice?

 

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[Q1: input filter]

Maximum frequency of motor current is 3.65kHz.

I understand that your recommended frequency range and CFLT, RFLT.

https://e2e.ti.com/support/amplifiers/f/14/p/971515/3589638#3589638

 

However, we would like to set the filter cutoff frequency 18.25kHz (3.65kHz * 5 times).

If we use CFLT=3.9nF, RFLT=1.2kohm values, does AMC1300B-Q1 become unstable?

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[Q2: output filter]

We would like to use 16bit ADC of TMS320F28377D-EP.

Sample window duration (set by ACQPS and PERx.SYSCLK) is 320ns(min).

 

Therefore, we set the output filter(ROUT and COUT) to be “less than 320ns” so ROUT=36Ω and COUT=330pF.

Could you please let us know if you have recommended(Other) design method and ROUT, COUT values?

 

<Circuit image>

 

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Regards,

Hide

  • Hello Hide-san,

    For the anti-aliasing filter, we typically recommend setting the cutoff to be at least 10x past the signal frequency of interest as the filter will start to attenuate the signal well before the -3dB point. I also calculate 17kHz not 18.25kHz with these values. 

    Another thing to consider with such a large resistance is the voltage drop that will be caused by having a large resistance for the anti-aliasing filter. This is not a problem as long as the expected voltage calculation in the TMS accounts for it.

    The differential input resistance is typically 22kohm, so there will be a voltage drop of approximately 1 - 22k/(22k+2*1.2k) = 9.8% from the shunt resistor to the input pins of the AMC1300B. 

    Depending on where the high side GND1 connection is made, the input bias current of typ 30uA will flow out of the positive input pin, through the positive filter resistor of 1.2k, then through the shunt resistor before reaching GND1, creating an offset voltage. If the high side GND1 connection is made at the shunt resistor, then the bias current flowing out of the negative input pin will flow through the negative filter resistor, offsetting a large portion of the offset error - however the offset voltage created by the positive bias current flowing through the shunt resistor will remain.

    Both of these error sources can be accounted for the TMS expected voltage calculation. 

    I am not an expert on the TMS device, I recommend posting to their forum for guidance. However, since it is a SAR adc, care must be taken in order to ensure that the sampling capacitor has enough time to fully charge before another sample is taken. This video series should help:

  • Hi Alexander-san,

     

    Thank you for the detail information.

    I greatly appreciate your cooperation.

     

    Regards,

    Hide

  • Happy to help Hide-san,

    Please let me know if you or the customer have additional questions. 

  • Hi Hide,

    can you please give a feedback and tell us what had helped?

    Thank you :-)

    Kai

  • Hi Team,

     

    Thank you for your response.

    I'm still considering.

    I will contact you again after I have new question.

     

    Regards,

    Hide