Rainbow-electronics Atmega169L Manuel d'utilisateur télécharger pdf (Page 199)

199

ATmega169V/L

2514A–AVR–08/02

Analog Input Circuitry The analog input circuitry for single ended channels is illustrated in Figure 89. An analog

source applied to ADCn is subjected to the pin capacitance and input leakage of that

pin, regardless of whether that channel is selected as input for the ADC. When the chan-

nel is selected, the source must drive the S/H capacitor through the series resistance

(combined resistance in the input path).

The ADC is optimized for analog signals with an output impedance of approximately

10 kΩ or less. If such a source is used, the sampling time will be negligible. If a source

with higher impedance is used, the sampling time will depend on how long time the

source needs to charge the S/H capacitor, with can vary widely. The user is recom-

mended to only use low impedant sources with slowly varying signals, since this

minimizes the required charge transfer to the S/H capacitor.

Signal components higher than the Nyquist frequency (f

ADC

/2) should not be present for

either kind of channels, to avoid distortion from unpredictable signal convolution. The

user is advised to remove high frequency components with a low-pass filter before

applying the signals as inputs to the ADC.

Figure 89. Analog Input Circuitry

ADCn

1..100 kΩ

S/H

= 14 pF

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Rainbow-electronics Atmega169L Manuel d'utilisateur Page 199