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DRV425: Constant VOUT

Greg
Prodigy 20 points
Part Number: DRV425

I've made up a board to test and play around with the DRV425 and found that Vout is a constant 1.27 V irrespective of the magnetic field (tested by rotating sensor N-S then E-W and moving various magnetic objects nearby). My first thought was a short somewhere, but having buzzed it all out, I haven't found anything obvious. Any thoughts would be greatly appreciated.

My setup is:

Rshunt = 270 Ohm

Vcc is 3.266 V

Vref is set to 1.65V (1.635 V actual) via Rsel0 = 1 and Rsel1  = 0

Both OR and ERROR are high (2.68 V each)

Vout = 1.27 V

I've attached my schematic. Note only R5+R6 (ERR and OR pullups) and R10 (shunt) are populated, other resistors are either zero-ohms or open (R8) (it's set up for testing various filters for noise reduction).

  • 0
    TI__Mastermind 31030 points

    Hello Greg, 

    Please check if removing C8 from the output and maybe even C7.  Those output cannot drive a direct capacitance at the output of VrefOut or Vout.  

    Placing C7 at VrefIn will be a better option after resistor.  Also Placing C8 after R3 if it is not 0Ω.

    Try these first and let me know if this does not fix the issue.

  • 0 in reply to Javier Contreras4
    Prodigy 20 points

    Hi Javier

    Sorry, I forgot to say only C9 and C10 are populated, all other caps are not placed. Again this was to be for testing later

    Cheers

    Greg

  • 0 in reply to Greg
    TI__Mastermind 31030 points

    Greg,

    I cannot see anything that stands out.  I don't understand "Both OR and ERROR are high (2.68 V each)" as these should be at your VS.  Have you tried an DRV425EVM in the same environment?

  • 0 in reply to Javier Contreras4
    Prodigy 20 points

    Hi Javier,

    I have breadboard the DRV425EM and got some nice results. I switched out the default Rshunt to 470 ohm to limit the max field and improve adc resolution and by playing with the ADC measurement averages was about to readily get 50 nT peak to peak noise - impressive for its size and cost. Sadly, I can't probe at my breadboarded EM until next week.

    With my new setup, the OR and ERROR pins have 10k pullup resistors (as per the datasheets), which drop the missing 0.57 V from Vcc.

    I've switched out Rshunt to 470 ohm (to match the EM setup) so the max field should be ~ +/-60µT (local geomagnetic field here max. is ~50 µT). I've tried moving strong magnets nearby, which should definitely over range and and probably saturate the sensor driving OR and ERROR low, but neither pin changes.

    Cheers

    Greg

  • 0 in reply to Greg
    TI__Mastermind 31030 points

    Hello Greg,

    For the OR and ERROR pins the value you state means there is a leakage of 57µA which is much more than I would expect.  I would change that 10k pullup and verify something is not driving that OR or ERROR pin to about 2.7V.  If you change resistors higher or lower and the voltage does not change then something else could be driving those pins.  I think the evidence points to this as the High should be ~3.3V and the low should go down to about 0V or at least below the 0.3V listed on the datasheet.

    Please note how the OR and ERROR work.  When I took a look at this the first time I had issues.  With your setup it should alarm as you are expecting.

    OR-- This is pulled low only when the Output Difference amplifier is saturated.  You can have a condition that this could not alarm.  I do not believe you have this case.  For the case of Rshunt = 470Ω and VS = 3.3V your max range is ~1.65V/4(V/V)/470Ω = 0.878mA max current on compensation for OR alarm.  0.878mA /12.2mA/mT  = 72µT

    ERROR-- 1.6mT above max compensation current will this alarm activate.  So for example if you have your 470Ω and the internal compensation resistance is 100Ω and we have a 3V supply.  The max compensation current is 3.3V/570Ω = 5.79mA. 

    So max compensation field is 5.79mA / 12.2mA/mT = 0.475mT.  

    So the ERROR flag will activate when field is above ~ 2.075mT

  • +1 in reply to Javier Contreras4
    Prodigy 20 points

    Hi Javier,

    I've made some comparison between my DRV425EM and the one on my board. The main thing I noticed was a big difference in Comp1-Comp2 resistance. On the EM this was about 200 KOhm, but on my board it was just short of1 MOhm, suggesting a poor/broken connection. 

    So I re-mounted the chip and it seems to have solved the issue. Vout varies with field and give the correct approximate values (not calibrated yet). Both OR and ERROR drop low (~0.02 V) with high fields.

    The only remaining thing is that both ERROR and OR sit at ~2.7V with 10 KOhm pull-ups (on the EM with 10k pull-ups they are at ~3.11 V with Vcc @ 3.112V). 

    When I switch the the 10k to 1k pull-ups, ERROR and OR sit at ~3.2 V, which still corresponds to ~58 µA (the corresponding EM current is only 0.02 µA)

  • 0 in reply to Greg
    TI__Mastermind 31030 points

    Greg,

    The Comp1 to Comp 2 resistance should be 100Ω.  Either way it is good you found the main problem.

    When you place the 1K.  The ERROR and OR sit at 3.2V are you still at VCC and pull up at 3.112?  The reason I ask is the outputs are open drain and should not supply current.  If the part is supplying current it could have got damaged somehow.  The only time I wolud expect that pin to supply current is if the voltage is greater than Vsupply.  This is due to the internal ESD diodes getting activated.  Could the Pico_3.3V and the +3.3V nodes be different Voltage?  Also what else is connected to that node of ERROR and OR.

  • 0 in reply to Javier Contreras4
    Prodigy 20 points

    Hi Javier,

    I get 192.7 kohm probing the pins on the DRV425EM and 174.8 kohm on my build. Are these pins straight into the coil or are there any internal working attached?

    The schematic image is clipped a little. The DRV425 Vcc is tied to PICO_3V3, the +3.3V was a power flag to stop the KiCad design rules check complaining. So, the ERROR and OR pull-up are tied to the same rail as Vcc. ERROR and OR are tied to GPIO pins of Pico MCU.

    I have some other DRV425 ICs is can try to see if there is a difference.

    Thanks for the help so far.

  • 0 in reply to Greg
    TI__Mastermind 31030 points

    Greg.  

    Maybe there is some switches that are active only during operation.  I will take a look at in the lab. I hope to look at it today or tomorrow but I don't think it is a big item since the functionality is there.

  • +1 in reply to Javier Contreras4
    TI__Mastermind 31030 points

    Greg,

    I appreciate your patience.  I did the same test you performed verified when the part is off the connection is high impedance.  When active the internal connections are made to the compensation coil.  If you have other questions please let us know.

  • 0 in reply to Javier Contreras4
    Prodigy 20 points

    Hi Javier,

    Sorry for the slow response. Thanks for help. I've got the device working really well now. Peak-to-peak noise is on the order of 40-60 nT (averaging to a sample rate of 5-10Hz), but this hitting the limit of what the ADC can resolve. A great little sensor.

    Cheers

  • 0 in reply to Greg
    TI__Mastermind 31030 points

    Greg,

    Glad to hear it is working out.  FYI, the 40-60nT is in the expected noise of the DRV425 at that frequency.  See this post if interested.  Let us know if you need anything else.