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DRV425EVM: Clarification on Output Voltage section

Jonathan Geronga
Expert 7930 points

Part Number: DRV425EVM
Other Parts Discussed in Thread: ADS1256, DRV425

Hi team,

Our customer would like to get clarification in the information on the Output Voltage section in page 3 of the DRV425EVM user guide.
"The output voltage of the DRV425EVM is located on J1 pin 2 and can be referred to the reference at J1 pin 2 or the power supply ground at J1 pin 3."

Base on the customer:
"I was confused by the statement found in the user guide. It basically said that I could either measure Vout - Refout or Vout - Vout (Vout referred to Refout and Vout). My other concern was that I tried measuring Vout -Refout as suggested in the datasheet and I got a negative value. I won't be able to measure this negative value with an ADC (the value was around -0.5V)."

Please let me know if you can answer the customer inquiry in DRV425EVM.

Thanks,

Jonathan

  • 0
    TI__Mastermind 33530 points

    Hello Jonathan,

    The way the output works it that it is referenced to the VREF voltage.  For example if the field being measured is positive the Vout pin would show a voltage above Vref.  If the field is negative the voltage would then be below VREF.  For the most accurate measurement it would be best to measure the voltage as described with respect to the VRef.

    If not measuring with respect to Vref then an error based on how accurate VREF is being driven.  If using VREF driven by VREFOut and selecting VDD/2 you need to account for the variation of VDD along with the specified range 45% to 55% of VDD and not 50% exactly.  If selecting 2.5V or 1.25V then you need to account for the variation in the VrefOut as stated in the datasheet.  For example 2.5V Vref selected it can range from 2.45V to 2.55V at 25°C.  

    If you have two channels of the ADC that is single ended you can measure Vref and then measure Vout to determine the difference between the two.

    Also if the ADC is differential you could make the measurements as suggested.

  • 0 in reply to Javier Contreras4
    Prodigy 190 points

    Hi Javier,

    I was wondering where I can find the information regarding the dependency of VDD on temperature. You stated as an example that " 2.5V Vref selected it can range from 2.45V to 2.55V at 25°C". I couldn't find this information in the user's guide. I also don't quite understand what you mean by "if the ADC is differential you could make the measurements as suggested". Are you saying that I can measure Vout-Refout using a differential ADC or should I measure Vout and Refout and take the difference. I tried measuring Vout-Refout with a voltmeter and I got a value close to -0.5V. I don't think I would be able to read this value with an ADC. The ADC I'm planning on using is the ADS1256 and its absolute dynamic range is -0.3V to VDD+0.3v with buffer OFF. 

    I was also confused about equation [1] found on page 4 of the user's guide: B = Vout /(4 * 12.2 *R1). Are the units of B and Vout in Tesla and Volts respectively? Is Vout in the equation referred to GND or Refout?

    Thanks,

  • 0 in reply to user6152136
    TI__Mastermind 33530 points

    The specific DRV425 specifications are in the DRV425 datasheet.  Below are the specifications for Vref for convince, but please view the datasheet of the DRV425 for further information.

    The ADC you mentioned you can make differential measurements.  You have so many choices.  If you drive the ADC VREFN to GND (same as DRV425) and you have VREFP you can connect that voltage the the VREF of the DRV425 then they are the same.  This would require you to provide your own reference.  

    For the calculation it is in B (Tesla).  The example below in the users guide give and example calculation.  So with 5V supply and Vref = 2.5 the max Vout -Vref  = +/-2.5V.  So from the example full scale  B = Vout /(4 * 12.2 *R1)  = 2.5/(4/12.2/100) ~ 512µT.

    Vout is always referenced to VREF because the device make both positive and negative measurements.