Datasheet

213
ATmega169A/PA/329A/PA/649A/P/3290A/PA/6490A/P [DATASHEET]
8284E–AVR–02/2013
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 23-8. Analog input circuitry.
23.7.2 Analog Noise Canceling Techniques
Digital circuitry inside and outside the device generates EMI which might affect the accuracy of analog measure-
ments. If conversion accuracy is critical, the noise level can be reduced by applying the following techniques:
1. Keep analog signal paths as short as possible. Make sure analog tracks run over the analog ground plane,
and keep them well away from high-speed switching digital tracks.
2. The AVCC pin on the device should be connected to the digital V
CC
supply voltage via an LC network as
shown in Figure 23-9 on page 214.
3. Use the ADC noise canceler function to reduce induced noise from the CPU.
4. 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
I
IH
1..100 kΩ
C
S/H
= 14 pF
V
CC
/2
I
IL