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Developing a cell-monitoring system

Posted: 12 Dec 2013     Print Version  Bookmark and Share

Keywords:cell measurement system  cell-monitoring  analogue front-ends  analogue-to-digital converter  AFE 

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 reference drift will typically scale with the precision of the ADC. Nonetheless, it is always good to make sure that the reference offers relatively good accuracy.

The component selection
Once again, the MAX14921 AFE is useful here where cost reduction diminishes the accuracy of the rest of the system.

This architecture eliminates the external reference, introducing the need for an ADC with an internal reference. Here the MAX11165 ADC offersoffset error, gain error, and INL values equal to the MAX11163 used in the high-accuracy architecture (figure 2); its internal reference provides an initial accuracy of ±4mV with a temperature coefficient of 17ppm/°C (maximum).

Again, using the Error Measurements spreadsheet shows that the maximum six-sigma error for this system is 0.178% (7.305mV) and the maximum three-sigma error is just 0.122% (5.014mV). Anecdotal evidence from the performance of the MAX14921 evaluation (EV) kit shows that this system should have much better performance than even these maximum errors indicate. Note: such anecdotal evidence serves as a framework for a possible outcome, but actual system design should rely on maximum error.

Cost-optimised architecture
There are times when costs trump accuracy in battery-monitoring applications. In this solution, accuracy is a trade-off that designers typically must accept. Now it is advisable to eliminate any possible external components and use a microcontroller with integrated ADC (figure 5). This architecture significantly reduces costs, and the lower component count allows precision cell-monitoring systems to be implemented in space-constrained applications. This design is a viable cost-optimised solution for lithium battery chemistries with moderate-to-steep discharge curves that do not require high precision measurement.

Figure 5: This cost-optimised architecture again features the precision MAX14921 AFE and a microcontroller with integrated ADC and reference.

There are two caveats with this design: ADC accuracy and reference voltage. First, an ADC integrated in a microcontroller typically does not have the accuracy performance of a discrete ADC.

The issues with the microcontroller's internal reference are more complex. A 3.3V microcontroller may have a reference around 1.195V. As a result, the integrated ADC can only accept a full-scale voltage of 1.195V, but most cell-stack monitoring applications require a full-scale voltage of 4V or higher. This situation requires a more significant accuracy trade-off for the designer. To decrease the full-scale voltage at the input to the ADC, a voltage-divider must be used. The converted value output from the ADC must then be multiplied by the IN/OUT ratio of the voltage-divider to regain the original scale. Multiplying the output magnifies the error caused by the first difficulty, and the voltage-divider introduces error in three ways that will be discussed in the Important Parameters section below.

Important parameters
When selecting a microcontroller, it is advised that at least one 12bit (minimum) ADC be available for conversion of the output from the MAX14920/MAX14921. ADC characteristics will typically scale with the cost of the microcontroller, so selecting a microcontroller within the budget is the most optimisation needed here.

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