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DRV425: Using multiple shunt resistors

Bas van Duijnhoven
Prodigy 30 points
Part Number: DRV425

Hello,

Im thinking of using the DRV425 to measure magnetic fields in the surroundings.
The device should distinguish between earths magnetic field and magnetic fields from 50/60Hz power distribution.
The later is typically much smaller than earth magentic field.

This device will have a position to measure DC-fields and a postiion to measure AC-fields from 50/60HZ equipment below say 150Hz (I dont need harmonics)

I would like to have multiple measuring ranges bij switching/bridging multiple shunt resistors.
Switching these would ideally be done by an MCU based on the ERROR and/or OVERRANGE pins of the DRV425.

My question is could i use for instance a MOSFET (with known Rdson) to switch the shuntresitors?

Furthermore:
Im using a second order low-pass filter (Fc approx. 150Hz) amplifier circuit after the DRV425 to amplify and filter the DRV425 noise.

Would this be adequate to measure a 100nT field with some sort of accuracy (say +/-20%)
Any tips on how to filter the DC component (due to earth magnetic field) from the DRV425 signal?
Ideally this will be switched on and of with an signal from the MCU.

Any input would be greatly appreciated.

Thanks in advance

Bas

  • 0
    TI__Mastermind 26485 points

    Hello Bas,

    Thanks for using our E2E forum.

    I would not see an issue using a MOSFET as long as you meet all the requirements from the DRV pins and IComp pins.  The DRV pins will create a voltage to drive a current on the IComp proportional to the magnetic field seen at the sensor.  Adding any resistance path will limit the range.  The ICOMP resistance is 100 Ω at room temperature and you need to take that into account along with the change over temperature.  Our DRV425 System Parameter Calculator is a tool that is useful in determining if you will have issues.  A concern I would have is the Rdson changing.  If you are only using the MOSFET to switch the resistors and connect the difference amplifier across a independent Rshunt.  Please include worst case resistance in calculations.

    I believe the 100nT will be difficult especially in an uncontrolled environment.  From my calculations/simulations I assume you would get 500nT peak to peak noise.  In the datasheet we also specify the noise typically 17nTrms~112nT peak to peak at frequency of 0.1 to 10HZ.  If you are still averaging this cold help but you need many averages to get the Peak to Peak close to 100nT. 

    If the AC measurement and DC measurements are not separated I do not believe you can accomplish 100nT measurement.

    Removing earths magnetic field or any stray fields you do not want to measure can be done in two ways.  One is of course shielding the fields away.  This is sometimes impractical for some applications.  Another way of doing this is having two DRV425 and measuring a delta similar to our Bus Bar Current sensor reference design application or Fluxgate displacement sensor design.  These reference designs are briefly described in our datasheet and there is more information in the DRV425 product folders.

    DRV425_TechnicalDocuments

    DRV425_Tools&Software

  • 0 in reply to Javier Contreras4
    Prodigy 30 points

    Hi Javier,

    Thank you for your fast and concise response, very very helpfull.

    The range switching circuit I have in mind consists of three resistors in series, in which the resistors for the more accurate ranges are bridged by a mosfet with low Rds-on (50mOhms). The MCU will switch the mosfets according to the OR-pin of the DRV425, which will begin at the highest range and switch to more accurate ranges. I'm finetuning the ranges, but for they are 18-160-1600 Ohms, where the latter two can be bridged by a mosfet. So effective resistances in will be approx. 18-180-1800 Ohm, giving ranges of about 1600uT/160uT/16uT (calculated with the spreadsheet)

    The response time of the measurement is not very critical (can be 1-2 seconds) so averaging is very much something to do. This will be done in the MCU.
    Based on this article where averaging is done in the oscilloscope for testing and over 32 or 64 samples, this looks very promising. As i will probably be sampling with something like 5kHz at (maybe even higher if needed) with 12 bits accuracy, I have a lot of samples to average over.

    The aim of the measurement is not to measure current flowing or detecting an object, but just a measurement with some reasonable precision of the magnetic fields in de vicinity of the user. For this I need to distinguish between low-frequency AC fields (which can be somewhere between 300uT and below 1 uT at 50 or 60Hz) and the earths magnetic field.

    Thanks again for you helpfull suggestions, this really helps me understand the DRV425 sensor better.

    Regards,
    Bas

  • 0 in reply to Bas van Duijnhoven
    TI__Mastermind 26485 points

    Hello Bas,

    Good luck and please post any further questions you have.