Freescale Semiconductor MCF51QE128RM manual Sources of Error, Sampling Error, Pin Leakage Error

Analog-to-Digital Converter (S08ADC12V1)

For proper conversion, the input voltage must fall between VREFH and VREFL. If the input is equal to or exceeds VREFH, the converter circuit converts the signal to 0xFFF (full scale 12-bit representation), 0x3FF (full scale 10-bit representation) or 0xFF (full scale 8-bit representation). If the input is equal to or less than VREFL, the converter circuit converts it to 0x000. Input voltages between VREFH and VREFL are straight-line linear conversions. There is a brief current associated with VREFL when the sampling capacitor is charging. The input is sampled for 3.5 cycles of the ADCK source when ADLSMP is low, or 23.5 cycles when ADLSMP is high.

For minimal loss of accuracy due to current injection, pins adjacent to the analog input pins should not be transitioning during conversions.

11.6.2Sources of Error

Several sources of error exist for A/D conversions. These are discussed in the following sections.

11.6.2.1Sampling Error

For proper conversions, the input must be sampled long enough to achieve the proper accuracy. Given the maximum input resistance of approximately 7kΩ and input capacitance of approximately 5.5 pF, sampling to within 1/4LSB (at 12-bit resolution) can be achieved within the minimum sample window (3.5 cycles @ 8 MHz maximum ADCK frequency) provided the resistance of the external analog source (RAS) is kept below 2 kΩ.

Higher source resistances or higher-accuracy sampling is possible by setting ADLSMP (to increase the sample window to 23.5 cycles) or decreasing ADCK frequency to increase sample time.

11.6.2.2Pin Leakage Error

Leakage on the I/O pins can cause conversion error if the external analog source resistance (RAS) is high. If this error cannot be tolerated by the application, keep RAS lower than VDDAD / (2N*ILEAK) for less than 1/4LSB leakage error (N = 8 in 8-bit, 10 in 10-bit or 12 in 12-bit mode).

11.6.2.3Noise-Induced Errors

System noise that occurs during the sample or conversion process can affect the accuracy of the conversion. The ADC accuracy numbers are guaranteed as specified only if the following conditions are met:

•There is a 0.1 μF low-ESR capacitor from VREFH to VREFL.

•There is a 0.1 μF low-ESR capacitor from VDDAD to VSSAD.

•If inductive isolation is used from the primary supply, an additional 1 μF capacitor is placed from

VDDAD to VSSAD.

•VSSAD (and VREFL, if connected) is connected to VSS at a quiet point in the ground plane.

•Operate the MCU in wait or stop3 mode before initiating (hardware triggered conversions) or immediately after initiating (hardware or software triggered conversions) the ADC conversion.

—For software triggered conversions, immediately follow the write to ADCSC1 with a stop instruction.

MCF51QE128 MCU Series Reference Manual, Rev. 3

238 Freescale Semiconductor

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