MC9S08DZ60MLF Datasheet, PDF (197/416 Pages) Freescale Semiconductor, Inc – MC9S08DZ60 Series Features

Chapter 10 Analog-to-Digital Converter (S08ADC12V1)
10.6.2.3 Noise-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 V_REFH to V_REFL.
• There is a 0.1 μF low-ESR capacitor from V_DDAD to V_SSAD.
• If inductive isolation is used from the primary supply, an additional 1 μF capacitor is placed from
V_DDAD to V_SSAD.
• V_SSAD (and V_REFL, if connected) is connected to V_SS 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 wait
instruction or stop instruction.
— For stop3 mode operation, select ADACK as the clock source. Operation in stop3 reduces V_DD
noise but increases effective conversion time due to stop recovery.
• There is no I/O switching, input or output, on the MCU during the conversion.
There are some situations where external system activity causes radiated or conducted noise emissions or
excessive V_DD noise is coupled into the ADC. In these situations, or when the MCU cannot be placed in
wait or stop3 or I/O activity cannot be halted, these recommended actions may reduce the effect of noise
on the accuracy:
• Place a 0.01 μF capacitor (C_AS) on the selected input channel to V_REFL or V_SSAD (this improves
noise issues, but affects the sample rate based on the external analog source resistance).
• Average the result by converting the analog input many times in succession and dividing the sum
of the results. Four samples are required to eliminate the effect of a 1LSB, one-time error.
• Reduce the effect of synchronous noise by operating off the asynchronous clock (ADACK) and
averaging. Noise that is synchronous to ADCK cannot be averaged out.
10.6.2.4 Code Width and Quantization Error
The ADC quantizes the ideal straight-line transfer function into 4096 steps (in 12-bit mode). Each step
ideally has the same height (1 code) and width. The width is defined as the delta between the transition
points to one code and the next. The ideal code width for an N bit converter (in this case N can be 8, 10 or
12), defined as 1LSB, is:
1 lsb = (V_REFH - V_REFL) / 2^N
Eqn. 10-2
There is an inherent quantization error due to the digitization of the result. For 8-bit or 10-bit conversions
the code transitions when the voltage is at the midpoint between the points where the straight line transfer
function is exactly represented by the actual transfer function. Therefore, the quantization error will be ±
1/2 lsb in 8- or 10-bit mode. As a consequence, however, the code width of the first (0x000) conversion is
only 1/2 lsb and the code width of the last (0xFF or 0x3FF) is 1.5 lsb.
MC9S08DZ60 Series Data Sheet, Rev. 4
Freescale Semiconductor
197