MC33771 ADC sampling issues

[__ALIYA_H]

We do not perform battery voltage balancing. Will this affect the accuracy of battery voltage sampling?

We use two MC33771 chips in a daisy-chain configuration. The first MC33771 measures 12 battery cells, and the second one measures 11 cells. We have removed the RBAL resistors and only kept RBAL_C, without performing cell voltage balancing. However, the last four cells measured by the second MC33771 show significant inaccuracy. In addition, we measured an obvious voltage drop (approximately 0.3 V) across the RLPF‑1 and RLPF‑2 resistors, especially on the last channel of the second chip. What could be the cause?

[RafaR · Accepted solution]

Hello Alex

Good Day!

Yes, removing the RBAL resistors does affect voltage‑measurement accuracy, and what you are seeing (large error on the last channels of the second MC33771 and ~0.3 V drop across the RLPF resistors) is a known failure mode when the balancing network is partially removed.

Leakage currents in the IC: The way the internal circuit of the IC is designed results in some leakage currents at the cell terminal inputs. Together with the anti-aliasing filter, this leakage current may cause an error in the voltage measurement . The lower the filter resistance, the lower the voltage measurement error.

Removing RBAL but leaving only RBAL_C leaves parts of the input network floating, especially at high cell numbers.

Re‑installing RBAL resistors should restore accuracy.

I hope this information has helped you, please let me know if you need help with anything else.

Have a great day and best of luck.