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DRV8703-Q1: Current Measurement Accuracy

Deepakji
Intellectual 300 points
Part Number: DRV8703-Q1

Hello,

The datasheet mentions "VIO is typically 5 mV (input referred)." but we are seeing considerable variations on this from one IC to another (all from same production batch / reel). Our observation shows that this varying from 2mV-5mV. Is this possible? The datasheet does not offer any range to this offset. Also please help understand "input referred".  

  • 0
    TI__Mastermind 37683 points

    Hi Deepakji,

    The 5mV input referred offset is the typical offset of the amplifier connected to SN and SP pins as shown in the block diagram of the device. Can you explain how you measured the offset?

  • 0 in reply to Pablo Armet
    Intellectual 300 points

    Pablo

    We are measuring the SO pin output with no current flowing through the current sense resistor (SP = SN = GND). Under this condition we are seeing output voltage variations from board to board. Our gain setting is (GAIN_CS = 10b)  39.4 V/V. 

    Under these settings the SO output should be 0.197V. 

    However, we are seeing this range from 0.096V to 0.160V. All measurements are made with a Fluke 179 TRMS DMM.(https://www.fluke.com/en-in/product/electrical-testing/digital-multimeters/fluke-179)

    There is no load  to the SO pin. It is connected to a ADC input directly. 

  • 0 in reply to Deepakji
    TI__Mastermind 37683 points

    Hi Deepakji,

    Thanks for the information.

    This variation is within acceptable range. However, one thing to note is that these values were taken with ambient temperature=25C, VM=13.5V and SN=SP=GND. If any of these parameters change when you did your measurements, it can lead to slightly different values. We only guarantee the accuracy of these specifications under the test parameters given in the datasheet.

  • 0 in reply to Pablo Armet
    Intellectual 300 points

    Pablo

    Thanks again for clarifying what we are seeing is expected variation as per the device. However, the datasheet does not say anything about what is MAX or MIN offset we can expect. If possible please clarify on the same, if that data is internally available at TI. 

    Again to reiterate my point in terms of actual currents - 

    At Zero Current the theoretical SO output is 0.197V however as per our observation we are seeing offset as much as 0.096V which implies a negative current of 0.640A (SP = SN = GND). FYI our sense resistor is a 4mOhms 1% resistor. 

    So at lower currents <1A this can lead to a 64% error in readings. Even at higher currents like 6A this will cause a approx 10% error. 

    Is this also acceptable as per your understanding? 

     

  • 0 in reply to Deepakji
    TI__Mastermind 37683 points

    Hi Deepakji,

    I'll check with our team internally if we have characterization data for the min and max values.

    Deepakji said:
    At Zero Current the theoretical SO output is 0.197V however as per our observation we are seeing offset as much as 0.096V which implies a negative current of 0.640A (SP = SN = GND). FYI our sense resistor is a 4mOhms 1% resistor.

    Question here: Are you shorting the terminals of the sense resistor to GND?

    Deepakji said:
    So at lower currents <1A this can lead to a 64% error in readings. Even at higher currents like 6A this will cause a approx 10% error. 

    At lower currents, the accuracy will be lower. There will also be other tolerances like sense resistor, added resistance from PCB traces, etc. that can decrease the accuracy.

  • 0 in reply to Pablo Armet
    TI__Mastermind 37683 points

    Hi Deepakji,

    I was able to confirm that the minimum VIO is 2.1mV. This value is around the same as the minimum value you measure.

    May I ask if this variation in the VIO is a major concern for your design?

  • 0 in reply to Pablo Armet
    Intellectual 300 points

    That was a major concern, however we have since then applied a firmware zero offset correction and this specific issue is handled now. However, it would be useful for other users if this information was available on the datasheet. That way user can be certain that the error is caused by the device and not the rest of the design. 

    It took is a while to trace this down to the IC. 

  • 0 in reply to Deepakji
    TI__Mastermind 37683 points

    Deepakji,

    Understood. I would to make a note to you and anyone else reading this post. The specification values in the electrical characteristics table of the datasheet are guaranteed by TI with some degree of error for the given test conditions. However, any min, max, typ value not specified in the datasheet is not fully guaranteed. In your example, the min VIO value is not specified. I obtained the min VIO from the characterization data collected for this device. 2.1mV is the average min value that was measured during characterization. However, this min value is not guaranteed to be always correct so please keep that in mind. Only the values given in the datasheet are guaranteed to be correct to some degree of error.