SLUUCJ0 November 2023 BQ76907
The BQ76907 device includes an on-chip temperature measurement and can also support measurement of an external thermistor on the TS pin. The device includes an internal trimmed 20-kΩ pullup resistor to bias the thermistor during measurement. The TS pin can be selected for thermistor measurement or general purpose ADCIN measurement using the Settings:Configuration:DA Config[TSMODE] data memory setting.
When the pin is selected for thermistor measurement, the internal pullup resistor is used to bias the pin during the measurement. To provide a high precision result, the device uses the same 1.8 V internal LDO voltage for the ADC reference as is used for biasing the thermistor pullup resistor, thereby implementing a ratiometric measurement that removes the error contribution from the LDO voltage level. Because the pullup resistor is only enabled during the pin measurement, it is recommended to limit the capacitance at this node to less than 4 nF to reduce the effect of incomplete settling when the pullup resistor is biased.
If the pin is selected for general purpose ADCIN measurement, the pullup resistor is not enabled during measurement, and the ADC uses VREF1 for its reference when measuring the pin.
The data is reported using the TS Measurement() command, in units of 16-bit ADC counts. The full-scale digital value reflects an analog input level of its reference × 5 / 3. So when the TS pin is measuring a thermistor in ratiometric mode using the 1.8V internal regulator for its reference, the 16-bit LSB is 1.8 V × 5 / 3 / 32768 ≅ 92 μV. When the TS pin is measuring in ADCIN mode using the VREF1 reference, the 16-bit LSB is 1.1962 V × 5 / 3 / 32768 ≅ 61 μV. The processing performed on the ADC data before reporting is shown below, using the Calibration:Temperature:TS Offset data memory parameter.