The tolerance of the resistors introduces the second source of error. Assuming 0.1% tolerance on both resistors, a maximum error of 1.67mV is introduced through the tolerance alone. When added to the error discussed above, the total maximum error is as high as 9.73mV. The error increases significantly with increased tolerance.
Lastly, the voltage divider introduces error because of the need to multiply the output by the IN/OUT ratio. This error affects two areas:
- The errors introduced by the voltage-divider, as mentioned above, are introduced at the input to the ADC. This means that the error is included in the value output from the ADC. This value is then multiplied by the IN/OUT ratio. As such, the true error introduced at the output by the voltage-divider is the IN/OUT ratio multiplied by the sum of the imperfect resistor error value and the maximum error due to resistor tolerance.
- The IN/OUT multiplier also amplifies the ADC and reference errors. If the total error introduced by the ADC and reference is 1mV, the error from the ADC after the multiplier is 1mV x IN/OUT.
Note: The high value of R2 = 2.37MΩ along with the 10pF (maximum) input capacitance to the ADC forms an RC time constant that must be accounted for in the system. Waiting 5 times the RC value, or about 120µs in this example, before beginning the ADC conversion will allow the input capacitance to charge before sampling the signal.
The Component Selection
This architecture depends on a low-cost microcontroller like the Freescale Semiconductor K10P64M72SF1. The integrated SAR ADC offers 12-bit, single-ended conversions with