Design a cell-monitoring system to optimize accuracy, lower costs, or both: Page 6 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 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 12-bit (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 optimization needed here.
 
The resistor values are the most controllable parameter in this architecture. Consider an application with a maximum cell voltage of 4V and a 1.195V reference on the ADC. The 4V input must be divided down to 1.195V to match the full-scale voltage capabilities of the ADC. Once the conversion is completed, the output value must then be multiplied by the IN/OUT ratio to regain the original 4V scale. The division of the input, conversion, and subsequent multiplication of the output introduces three sources of error.
 
The first form of error introduced by the voltage-divider is the error caused by imperfect resistor values because calculated values are unavailable. For example, selecting R 1 = 1MΩ requires that R 2 = 2.347MΩ. Such precise values are not available, so the designer must settle for a close value that is available for purchase. In this case both 2.32MΩ and

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