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DRV425: Output behaviour at flux density higher 2 mT

Michael Pichler
Prodigy 110 points
Part Number: DRV425

Dear TI team,

based on the datasheet, I could not find any description of the behaviour of the output voltage at higher flux densities. What I understood is that the current sense amplifier overrange flag will be set, as soon as the current sense amplifier clips to the rails and at even higher flux densities the fluxgate will saturate, hence does not provide a valid output signal. 

Is the behaviour of the output signal defined (means it is wrong, but always wrong in the same way: e.g. the output clamps to the last linear output value)? 

Thank you very much.

Kind regards

Michael

  • 0
    TI__Mastermind 26475 points

    Hello Michael,

    Thanks for using the forum to get your questions answered.

    The DRV425 will attempt to compensate the field through the comp pins and will be limited by temperature, Rshunt and VDD voltages.  The value it saturates could be higher or lower than 2mT.  I would use the DRV425 System Parameter Calculator to help determine the max field before saturation of the DRV425.  Saturation of the sensor does not mean the Vout will rail and vice-versa.

    There are two outcomes that are possible during saturation.  Case #1 is most likely to happen.  

    1. Most likely is the DRV425 continues to try to compensate for the field internally and goes to that max full scale (Full scale is dependent on Rshunt).  T
    2. The saturation happens fast enough that it blinds the sensor before it determines the correct direction of the flux and goes to either max full scale and the DRV425 does not know if the compensation is in the correct direction. This case is unlikely.

    For both cases the Error Flag will be set, and depending on your Rshunt you could have your OverRange flag trip as well.  

    Once the saturation condition is removed the part will return to its normal operation.

  • 0 in reply to Javier Contreras4
    Prodigy 110 points

    Hello,

    thank you very much for your answers.

    1. Is there any risk of damaging/destroying the part by this condition or will the shunt resistor (with a certain resistance) protect the device? As I assume, rather high currents will flow into the device pins.

    2. Can you give me a rough estimation which slew rate of the external field would be required to blind the sensor? What would happen if the magnetic field (high enough to saturate) is applied before powering on the device? My guess is, it would go to either max. or min. of the full scale range (so it would be blind) - is my understanding correct?

  • 0 in reply to Michael Pichler
    TI__Mastermind 26475 points
    Michael,

    1. You should not damage the part with higher flux. I have heard of someone applying about 1T and then removing the field and it came back. The issues is it will take some time to recover maybe even tens to hundreds of µS. Besides the Rshunt resistor the internal compensation of about 100Ω will also limit the current and the max voltage will be determined by the supply voltage.

    2. You are correct that it is full scale but by full scale I mean max voltage at DRV1 and DRV2 pins. The impedance will limit the current and slow it down. There are two factors in the slew rate. One is the sample rate of our sensor which is approximately 250KHz. The second is the response time to drive the internal compensation which is determined by Rshunt and the impedance of the compensation coil.

    What slew rate are you trying to deal with?
  • 0 in reply to Javier Contreras4
    Prodigy 110 points
    Dear Javier,

    thank you for response.

    1. That is good to hear, that there is basically no upper limit for the flux density.

    2. There is currently no defined slew rate of the external magnetic stray field (e.g. 4 mT/us), so the whole examination is of theoretical nature. The application is a differential current sensor application in which we place two sensors in a bus bar. We use both sensors as single absolute field sensors (because of the flux density of approx. 1.6 mT at the maximum primary current) and substract both signals with a differential amplifier to get the differential signal. The common mode signal will be derived by summing up the outputs of both sensors. As the external magnetic fields (3000 A/m - could be even more) can easily exceed the measurement range of 2 mT, there is the idea of using an external compensation coil and a control loop (e.g. an analog control loop) to shift the common mode stray field into the measurement range. The questions regarding the output behaviour are directly related to this approach. In case the output signal of one or both DRVs is wrong, the control loop gets are wrong input signal, hence could shift the fluxgates even further into saturation. So it would be helpful to know, where the limitations are, e.g. maximum slewrate or the output behaviour in case of power-on into saturation, then it is possible to define a procedure to recover from those situation.

    Kind regards
  • 0 in reply to Michael Pichler
    TI__Mastermind 26475 points

    Michael,

    I have not myself every powered up the device once it was already saturated.  I will attempt this in my lab.  I will also attempt to get a transient response but I am not sure what lab limitations wiil be.  I have as most measurements with a Helmholtz coil and the response time is dependent on the impedance of the coil and the driving source.  Most of my measurements are based on the accuracy so I need to make some changes to get high slew rate.  I will keep you updated on my process of both of these experiments.  Keep in mind this will be a small sample size.

  • 0 in reply to Javier Contreras4
    Prodigy 110 points

    Thank you very much for the experiments you are going to do. I'm curious about the results.

  • 0 in reply to Michael Pichler
    TI__Mastermind 26475 points

    Michael,

    I was able to saturate the DRV425 while it was off and then turn on the power.  In this condition the DRV425 device I tested always railed high regardless of the direction of the magnetic field.  This is because the DRV425 fluxgate is saturated and the direction of the field cannot be determined.

    During a large transient and with the DRV425 not initially saturated it would saturate in the correct direction.  I was not able to verify the speed of the transition. 

  • 0 in reply to Javier Contreras4
    Prodigy 110 points

    Hello Javier, thank you very much for the test that you have performed. Did the DRV425 raise an error flag in these conditions?

    Thank you very much.

  • 0 in reply to Michael Pichler
    TI__Mastermind 26475 points

    Michael,

    Yes the DRV425 did raise the ERROR flag and OR flag.  In my test condition I used a DRV425EVM with the default 100 ohm shunt which will have a full scale range of about 500uT.