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What is the maximum common mode voltage that can be applied to the input of AD7403? Can AD7403 be used to measure current on the high side of the motor phase?

How can i use the AD7403 to measure current on the high side of the motor phase.?

I have used  current sense resistor in series with motor phase windings. My DC Link voltage is 170V.

And I have connected the two ends of the current sense resistor to the input pins of AD7403 (vin+ and vin-).

The IC is getting damaged when I am switching on the DC link voltage. I have used isolated power supply for the analog side of the IC(VDD1) ?

  • This is an example on how to use the AD7403 for motor phase current measurement.

    Low Voltage Drive Board [Analog Devices Wiki] 



  • This should not happen. Is the isolated supply ground referenced to the phase switching node? If you can attach a schematic that would help

  • Hi Sir,

    I have attached my schematic. The voltage drop is sensed acrossR402, R403 and R404. ICs U83, U84, U85 and U86 are not mounted, These  hall effect based current sensor ICs I have provided in case I have not mounted the resistors R402 to R404.. One more point I want to mention that, I had not connected the motor to the board. I just  wanted to check if the AD7403 is giving some output. the voltage on the motor phases was 170V. The ground of isolated power supply of AD7403 is shorted to the DC Link voltage return.(AGNDISO shorted to DCLinkRET shorted to GNDISO).

    I have gone through the schematic as provided by  Mr. ACozma (above reply) and I found that the current sense resistor is different than what i used.

  • The problem is that you need a separate isolated supply for each motor phase and one for the low side. i.e. 4 isolated supplies. The isolated supply negatives should be referenced to U,V,W and DC_link return or PHC-, PHB-, PHA- (it doesnt really matter which side of the sense resistor you reference to). The common mode voltage at each of the current sense resistors is completely different and is jumping between DC bus and 0V, so you will damage the ICs the way you have it. You cannot implement high side current sensing with shunt resistors unless you provide individual isolated supplies for each measurement circuit, ground referenced to that high side switching node. 

  • Is it OK if I use four terminal resitsor(WSL3637R0100FEA from vishay/Dale) and connect the terminals E1 and E2 to to the input of the AD7403 without any further change in the schematic. (or i connect the terminal E1 to +ve input and E2 terminal of resistor and -ve input of ad7403 to GNDISO)? Even after changing the two terminal resistor to four terminal resistor, do i still need isolated power supply for each phase?

  • yes you do, these resistors are electrically the same as 2 terminal resistors. They just enable you to get a more accurate reading. The need for isolated supplies on each phase is fundamental. 

  • Thank you sir for the information. I will try to incorporate the changes as suggested by you.I also thanks to Mr. ACozma for providing the reference design.