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BQ76PL455A-Q1: PCB Effect on Measurement Accuracy

Shinichi Yokota
Genius 9715 points
Part Number: BQ76PL455A-Q1

Hello,

The bq76PL455A-Q1 data sheet lists total channel measurement accuracy under two conditions - with and without PCB in Sections 6.13 and 6.11 respectively. How can PCB create additional measurement errors? In other words, what can cause the additional measurement errors on PCB?

Best regards,
Shinichi Yokota

  • 0
    TI__Genius 11480 points
    Hi Shinichi,

    There are several ways, some are very in depth. In many ways, it is similar to any precision ADC layout.

    A proper GND is very important. Without it, you can get parasitic resistance and inductance that can have adverse effects on your measurement. This is why we strongly recommend a 4 layer pcb with a pure GND plane as opposed to a 2 layer PCB with routing on both sides.

    Proper decoupling of the reference and supplies are also very important. In particular, the reference should be noise free and stable.

    Parasitic's in your measurement path (higher R from an RC filter, or higher cable/connector resistance) will play a role by introducing I*R drop in the measurement path from leakage current.

    One challenge is that physical strain on the reference can cause the voltage to drift, due to changing the geometry of the BJTs that are generating the reference voltage. This is true for any reference.

    I hope you find this useful.
  • 0 in reply to David Wiest Jr
    TI__Genius 9715 points

    David,

    I briefly understand what you answered for me, but I'd like to dive into details. Ideally, if the input impedance (VSENSEn) is high enough, its CMRR is good enough, and the device's PSRR is good enough, I guess there should be no difference in measurement accuracy between with and without PCB as long as a value captured by its ADC is well averaged, in other words, AC components are well removed. Or, if a ground shift occurs by PCB resistance and the device's operating bias current, which can cause an offset error in measurement accuracy, for example, I guess the offset is somewhat predictable and can be calibrated in the system, which will result in as good measurement accuracy as described in the Section 6.11 ADC: VSENSEn Cell Measurement Inputs.

    Could you elaborate specifically on what kind of the device's elements affects the measurement accuracy on PCB and how?

    Best regards,
    Shinichi Yokota