Circuit designers rely on data sheet specifications to estimate the accuracy of a cell measurement circuit. Other real-world effects often dominate the measurement error. Factors affecting the measurement accuracy are:
- Initial tolerance
- Temperature drift
- Long-term drift
- PCB assembly stress
- Noise rejection
A good technology must take into account all of these factors in order to deliver very high performance. Measurement accuracy of the IC is primarily limited by the voltage reference. Voltage references are sensitive to the mechanical stress. Thermal cycling during PCB soldering stresses silicon. Humidity is another cause of silicon stress as water is absorbed in the package. Silicon stress relaxes over time, leading to long term drift of the voltage reference.
Battery measurement ICs use either a band gap voltage reference or a Zener voltage reference. IC designers use an NPN emitter-base junction operating in reverse breakdown as a Zener reference. Breakdown occurs at the surface of the die, where the effects of contamination and oxide charge are most pronounced. These junctions are noisy and suffer from unpredictable short- and long-term drift. The buried Zener places the junction below the surface of the silicon, well away from contamination and oxide effects. The result is a Zener with excellent long-term stability, low noise, and relatively accurate initial tolerance. For that reason, Zener references are far superior for mitigating real world effects over time.