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BQ76920: bq78350<->bq76920 Factory Gain/Offset

Jeffrey Wright
Expert 2875 points
Part Number: BQ76920
Other Parts Discussed in Thread: BQ78350

Hello;

The 920 datasheet says "Each differential cell input is factory-trimmed for gain or offset, such that the resulting reading through I2C is
always consistent from part-to-part and requires no additional calibration or correction factor application."

However, the 920's datasheet lists only one offset and one gain value (ADCOFFSET 0x51)?

So the equation that shows how to convert the 14-bit ADC reading into an analog voltage - "V(cell) = GAIN x ADC(cell) + OFFSET" seems to confuse the calibration statement (since each cell would have its own offset, or otherwise why would each cell require offset calibration?)

Also, I "assume" then, that the 78350 does this V(cell) = equation, and so the cell voltages my host u/C reads are already computed (?)

Thanks

  • 0
    TI__Guru 54715 points

    Hi Jeffrey,

    Each input of the BQ76920 is factory trimmed with adjustments which are not visible. The result of that trim is not ideal, a gain and offset are calculated and stored for the device as a whole.  A typical characteristic is shown in the data sheet http://www.ti.com/document-viewer/BQ76920/datasheet/typical-characteristics-slusbk25062#slusbk25062 figure 2.  Notice there is variation in the cells in the figure shown, and some should be expected in each individual device.  The gain and offset are for the device as a whole, not the best for each or any one individual cell.

    The BQ78350 uses the stored gain & offset and applies these to each cell voltage reading from the BQ769x0.  Because of the cell to cell variation in the BQ769x0, the BQ78350 provides for an offset adjustment to align the cells at a voltage point of interest.  You might think of it as shifting the curves in figure 2 so that all go through one point.