XC2000 datasheet(105/729 Pages) INFINEON

105 / 729 page

XC2000 Derivatives

Peripheral Units (Vol. 2 of 2)

Analog to Digital Converter

Preliminary

User’s Manual

16-11

V1.0, 2007-06

ADC, UM_V0.1D6

16.1.8

Electrical Models

Each conversion of an analog input voltage into a digital value consists of two

consecutive phases. During the sample phase, the input voltage is sampled and

prepared for the following conversion phase. A simplified model for the sample phase

describes the input signal path, whereas a second simplified model for the conversion

phase is related to the reference voltage handling.

16.1.8.1

Input Signal Path

The ADC kernel in the XC2000 is based on one switched capacitor field for

measurement with a total capacity represented by

CAIN and a small static capacitor at

each input pin. During the sample phase, the capacitor field

CAIN is connected to one of

the analog input CHx via an input multiplexer. The multiplexer is modeled by ideal

switches and series resistors

RAIN. Only the switch to the selected analog input is closed

during the sample phase. During the conversion phase or while no conversion is running

(ADC is idle), all switches are open. The voltage at the analog input channel CHx is

represented by VAINx.

Figure 16-4

Signal Path Model

A simplified model for the analog input signal path is given in Figure 16-4. An analog

voltage source (value VS) with an internal impedance of REXT delivers the analog input

that should be converted.

During the sample phase the corresponding switch is closed and the capacitor field CAIN

is charged. Due to the low-pass behavior of the resulting RC combination, the voltage

VC to be actually converted does not immediately follow VS. The value REXT of the

analog voltage source and the desired precision of the conversion strongly define the

required length of the sample phase.

To reduce the influence of R

REXT and to filter input noise, it is recommended to introduce

ADC_signal_path_model

ADC kernel

AIN

AGND

CHx

CH0

CH15

EXT

AINx

EXT

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