SPMCK16Z4MPV20 datasheet(207/500 Pages) FREESCALE

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M68HC16 Z SERIES

ANALOG-TO-DIGITAL CONVERTER

USER’S MANUAL

8-21

8.8.6 Analog Input Pins

Analog inputs should have low AC impedance at the pins. Low AC impedance can be

realized by placing a capacitor with good high frequency characteristics at the input

pin of the part. Ideally, that capacitor should be as large as possible (within the practi-

cal range of capacitors that still have good high frequency characteristics). This capac-

itor has two effects. First, it helps attenuate any noise that may exist on the input.

Second, it sources charge during the sample period when the analog signal source is

a high-impedance source.

Series resistance can be used with the capacitor on an input pin to implement a simple

RC filter. The maximum level of filtering at the input pins is application dependent and

is based on the bandpass characteristics required to accurately track the dynamic

characteristics of an input. Simple RC filtering at the pin may be limited by the source

impedance of the transducer or circuit supplying the analog signal to be measured.

Refer to 8.8.6.2 Error Resulting from Leakage. In some cases, the size of the capac-

itor at the pin may be very small.

Figure 8-10 is a simplified model of an input channel. Refer to this model in the follow-

ing discussion of the interaction between the user's external circuitry and the circuitry

inside the ADC.

Figure 8-10 Electrical Model of an A/D Input Pin

ADC SAMPLE AMP MODEL

AMP

CDAC

VSRC

INTERNAL CIRCUIT MODEL

EXTERNAL CIRCUIT

= SOURCE VOLTAGE

= INTERNAL CAPACITANCE (FOR A BYPASSED CHANNEL, THIS IS THE CDAC CAPACITANCE)

VSRC

CDAC

= FILTER IMPEDANCE (SOURCE IMPEDANCE INCLUDED)

= FILTER CAPACITOR

= DAC CAPACITOR ARRAY

= INTERNAL VOLTAGE SOURCE FOR PRECHARGE (VDDA/2)

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