Design a cell-monitoring system to optimize accuracy, lower costs, or both: Page 4 of 10

December 02, 2013 //By Jeremy Georges
Design a cell-monitoring system to optimize accuracy, lower costs, or both
Jeremy Georges, MTS, Maxim Integrated discusses cell-measurement architectures for cell balancing and battery-measurement applications and presents example designs that meet diverse accuracy and cost requirements.
The ADC
resolution and the reference accuracy are high enough to allow for an LSB of less than 100µV.
 
Quick calculations using the Error Measurements spreadsheet produced a six-sigma error of about 0.042% and a three-sigma error of about 0.027% for the complete solution. This translates to about 1.714mV maximum six-sigma error or 1.087mV maximum three-sigma error. Figure 3 shows actual cell measurements taken in the lab over the full temperature range for various cell voltages on the MAX14921EVKIT, which uses the accuracy-optimized architecture shown in Figure 2. Measurements show a maximum change in error of only 0.009% (368µV) over temperature, with a maximum cell-voltage measurement error of 0.017% (696µV).
 


Figure 3. System measurement error over temperature.
Test results are for the system in Figure 2.

Accuracy-Optimized, Cost-Reduced Architecture

Cost reduction - we have all had to manage this in new designs. It often forces designers to trade-off reduced accuracy for a lower solution price. Yet, this trade-off need not greatly reduce system accuracy. A properly selected ADC with an integrated reference, and the same precision battery-monitoring AFE and microcontroller used above are an excellent compromise when costs matter greatly (Figure 4) .

 

Figure 4. Diagram of an accuracy-optimized architecture for a limited budget. In this design costs are reduced by using an ADC with integrated reference. The precision MAX14921 battery-monitoring AFE is still used.
 

This architecture still uses an ADC separate from the microcontroller to keep accuracy high, but the ADC has an internal reference. The reduced component count lowers cost, but accuracy is somewhat diminished with the internal reference. In this configuration, the accuracy-for-cost trade-off is typically acceptable.

The Important Parameters

Because the reference is integrated in the ADC in this design, there will be less flexibility with the reference parameters. As such, focus on the ADC’s offset error, gain error, INL, and the gain and offset error temperature coefficients. The reference error and

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