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AMC3301: Shunt based current measurement and CM noise in a fast switching inverter

Arun Ashok
Prodigy 90 points
Part Number: AMC3301

Hi Team,

We are planning to use AMC3301 in a SiC inverter for phase current measurements (fsw till 40kHz). The 250mV signal at the input across a shunt is converted to a single ended signal using a differential amplifier as shown in the datasheet. 

           

We are using R1= R3 = 4.7k and R2 = R4 = 3.74k, Vref = 1.65V to get a gain of 0.79 so that we are able to get close to the max range of the 3.3V ADC of the C2000 MCU. We also use C1=C2=47pF to get >500kHz bandwidth.

1) Do you see any issues with the R or C selection or do you think we should go higher with C values? 

2) We are concerned with CM noise in the current measurement and In EVM datasheet (sbrua330C) we see that on page 3, Caps C10 and C13 are mentioned to improve CM noise. Where should we add CM noise filter caps in the different amplifier based circuit above and what are the ideal values ? 

Thank you

  • +1
    TI__Guru 50301 points

    Hi Arun,

    For the resistors, you may want to reduce the gain slightly to allow for up to ~55mV of headroom from the supplies of the differential to single-ended conversion operational amplifier. Consult the operational amplifier's datasheet to confirm this ~55mV value as it may be different for each operational amplifier. 

    For the capacitors, in general these values are fine and selecting the common-mode noise filter caps depends on your desired cutoff frequency as listed in the table. Please note that the bandwidth of the AMC3301 is minimum 290kHz. Additionally, it is often best to rely on the common-mode rejection ratio of the devices to filter out noise that is in the system. I recommend placing the differential to single-ended operational amplifier close to the input of the C2000 to limit the amount of noise present in the datasheet. 

    Lastly, I recommend selecting a shunt resistor with low inductance as this is a high frequency application. 

    This document may help as a reference: https://www.ti.com/lit/an/sbaa347/sbaa347.pdf

  • 0 in reply to Alexander Smith
    Prodigy 90 points

    Thanks Alex, Which TI datasheet parameter exactly talks about this 55mV headroom ?

  • +1 in reply to Arun Ashok
    TI__Guru 50301 points

    Hi Arun,

    Happy to help! From the TLV900x datasheet on page 17: