AD8479 as a shunt voltage amplifier in a large DC Motor

We need to monitor the voltage across a shunt in a large DC motor, the Motor voltage is 0 ~ 550VDC and the shunt voltage = 0 ~ 120mV to represent the 0 ~ 1200 Amps flowing into the armature, the signal from the shunt amplifier output will connect to a 3.3VDC analog input on a microprocessor (STM32 Arm Cortex Micro) where the logic is used to control the motor. The concern is that we need to be sure that the output from (for instance) a AD8479 will not damage the analog input on the Microprocessor. The resistance of the shunt is very low due to the current flowing through being very high. Should we be using an AD8217 instead?

We have VCC supply at 3.3VDC or 5VDC or 24VDC.

Are we suggesting the correct AD chip for this application? Is there a standard circuit for what we are trying to achieve?

Finally, we could alternatively use a chip with a serial SPI bus if there is such a chip available for measuring shunt voltages without the risk of the ground in the micro being affected by the 550VDC motor supply. The voltage when measured between the shunt and ground is floating, they are not connected in any way.

Any advice or assistance would be very much appreciated.



  • 0
    •  Analog Employees 
    on Jul 13, 2016 7:11 PM


    AD8479 fist your application given that you need 550V common mode voltage. But first you need to make sure that proper supply is implemented. The single supply operation of AD8479 affects the common mode range. Please kindly refer to Single Supply Operation section of the DS. For your concern on the output protection for your uController, AD8479 for instance operate out of your spec, the worst case output will clip to AD8479 supply.  Typical protection use in ADC inputs (which may also apply to your circuit) is to use a series resistor and a diode to supply please refer here.

    If you have 0 to 120mV diferential input in a gain of 1 AD8479, Do you think it is sufficient to be detected by your uController? If not, another amplifier can gain up the signal wihtin the required level of your uController and serve as protection like ADA4177.

    Please also take note that AD8479 cannot swing upto 0V (given you have single supply) output will swing within 0.3V of either rails.



  • Phil

    Thanks for your response. It is becoming most obvious that the AD8479 is

    not suitable for our purposes, so we will continue to search for a

    better fit.

    You would think that there would be a common product for measuring shunt

    voltages for large shunts that have high common mode voltage passing

    through them... There probably is but I am struggling to find it.

    Thanks again for taking the time to answer our question, your assistance

    is much appreciated