ATmega128 datasheet(237/386 Pages) ATMEL

237 / 386 page

237

2467S–AVR–07/09

ATmega128

sleep modes and the user wants to perform differential conversions, the user is advised to

switch the ADC off and on after waking up from sleep to prompt an extended conversion to get a

valid result.

Analog Input Circuitry

The Analog Input circuitry for single ended channels is illustrated in Figure 113. An analog

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

less of whether that channel is selected as input for the ADC. When the channel 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 k

Ω or

less. If such a source is used, the sampling time will be negligible. If a source with higher imped-

ance 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 recommended to only use low impedant sources

with slowly varying signals, since this minimizes the required charge transfer to the S/H

capacitor.

If differential gain channels are used, the input circuitry looks somewhat different, although

source impedances of a few hundred k

Ω or less is recommended.

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 113. Analog Input Circuitry

Analog Noise

Canceling Techniques

Digital circuitry inside and outside the device generates EMI which might affect the accuracy of

analog measurements. If conversion accuracy is critical, the noise level can be reduced by

applying the following techniques:

Keep analog signal paths as short as possible. Make sure analog tracks run over the

ground plane, and keep them well away from high-speed switching digital tracks.

The AVCC pin on the device should be connected to the digital V

CC supply voltage

via an LC network as shown in Figure 114.

Use the ADC noise canceler function to reduce induced noise from the CPU.

If any ADC port pins are used as digital outputs, it is essential that these do not

switch while a conversion is in progress.

ADCn

IIH

1..100 k

CS/H= 14 pF

VCC/2

IIL

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