Using the Error Measurements spreadsheet, the MAX14921 AFE and the integrated ADC in the Freescale microcontroller produce a 12-bit output with a maximum six-sigma error of 251.307mV (861.17 LSB). The maximum three-sigma error is 116.849mV (400.42 LSB).
Cost-Optimized, Accuracy-Enhanced Architecture
A cost-optimized, accuracy-enhanced architecture leverages the previous cost-optimized architecture (Figure 5) and improves accuracy by using a microcontroller without the integrated reference (Figure 6). An external reference not only greatly improves the accuracy, but also reduces the amount by which the analog signal from the MAX14921 AFE needs to be divided down.
Figure 6. This cost-optimized, accuracy-enhanced architecture uses the MAX14921 AFE, a MAX6034B precision voltage reference, 4 and a microcontroller with integrated 12-bit ADC.
By using the external reference, initial accuracy improves and temperature drift is lower. The IN/OUT ratio of the voltage-divider drops with a higher voltage reference, so the inaccuracies associated with that multiplying factor drop as well. Ultimately, a designer must decide if the additional cost of the external reference is sufficiently offset by the increased accuracy of this system.
The Important Parameters
This microcontroller must integrate at least one 12-bit (minimum) ADC capable of providing the desired accuracy. Also important for accuracy is the selection of R 1 and R 2 in the voltage-divider. 5
Selection of the external reference depends both on the budget and the capabilities of the microcontroller. Many microcontrollers operate from a 3.3V supply and can only handle a 3.3V reference. For maximum accuracy, select