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Original question:

DRV425EVM: RShunt Placement in DRV425 and Impact on Design

DRV425: DRV425 in Differential Mode

Sid
Prodigy 20 points
Part Number: DRV425
Other Parts Discussed in Thread: TIDA-00463

Tool/software:

TI Team,

I am working on a proto setup with two DRV425 in Differentail mode to measure electrical activity of an insulated AC electrical cable (3-Conductor cable, ~10kV). The goal is to find if the cable is energized or not by keeping the DRV425 setup about 1cm away from the cable.

When measured with commerical EMF meters, the flux coming out of the insulated cable is around 10uT to 300uT depending on the load. The two DRV425s are separated by about 10mm on the PCB and the axis of sensitivity are aligned and are perpendicular to the current flow direction of the cable. DRV's are powered by 3.3V and using VDD/2 for VREF internal.

What are the best approaches for selecting the RSHUNT value to make it sensitive for the ~10uT readings - which is the challenging part.

  • 0
    TI__Mastermind 19485 points

    Sid,

    Thanks for using E2E! 

    We have a great resource in our R_SHUNT calculator that allows you to enter your design values and determine sensitivity amongst several other parameters in the design. I have posted a link here. 

    A challenge here for 10uT operation is going to be the earth's magnetic field, but if you run the second device off of DRV2 to take the inverse of the field, this should be cancelled unless shielding or other effects potentially create differences between the parts.

    Typically, this can be 60uT or greater based on orientation. I would look at the DRV425 busbar application, as this contains a lot of information pertinent to your application, although the point of measurement is taken at the center of the excitation, not off to the side. 

  • 0 in reply to Carolus Andrews
    Prodigy 20 points

    Thank you. What are your thoughts on implementing a solution similar to TIDA-00463 for detecting presence of magnetic field flux around the insulated high voltage AC cable and cancelling out the ambient geo-mag fields? Could you please explain the pros and cons?

    In this config, the two units are in the same orientation and are separated by 10mm. 

  • 0 in reply to Sid
    TI__Mastermind 19485 points

    Sid, 

    Let me examine TIDA-00463 and I will follow up with you with my thoughts and recommendations tomorrow. 

  • 0 in reply to Sid
    TI__Mastermind 19485 points

    Sid, 

    Yes, examining this TIDA, I believe this orientation should work. I would recommend using the calculator tool to select your RSHUNT value, and targeting a value of 300uT + max external field as your total field target for the design. If the maximum ambient is only the earth field, you can simply add 60-70uT in addition to your given 300uT above. 

    Beyond this, I would also caution you to make sure that you calibrate out the device offset for each device given how low you are looking to measure, and make sure you take noise into account. Please see this thread for more information on that (and check out the thread Javier points to inside that one as well for even more information on how to make lower measurements with this device. 

  • 0 in reply to Carolus Andrews
    Prodigy 20 points

    Hi thank you for your time and the insights. This is very helpful. I am considering BSEL = GND that gives a BW of 32kHZ. Since the variable of interest is magnetic field from a 60HZ AC line, I am assuming this BW selection is more than enough. Any suggestions on this item?

  • 0 in reply to Sid
    TI__Mastermind 19485 points

    Sid, 

    I am in agreement here. For such a small signal frequency vs. the frequency BW,  BSEL = 0 should be fine.