User Manual

249
ATmega48A/PA/88A/PA/168A/PA/328/P [DATASHEET]
8271E–AVR–07/2012
mended to only use low impedance 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 24-8. Analog Input Circuitry
24.6.2 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:
a. 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.
b. The AV
CC
pin on the device should be connected to the digital V
CC
supply voltage via an LC network as
shown in Figure 24-9.
c. Use the ADC noise canceler function to reduce induced noise from the CPU.
d. If any ADC [3:0] port pins are used as digital outputs, it is essential that these do not switch while a con-
version is in progress. However, using the 2-wire Interface (ADC4 and ADC5) will only affect the
conversion on ADC4 and ADC5 and not the other ADC channels.
ADCn
I
IH
1..100 kΩ
C
S/H
= 14 pF
V
CC
/2
I
IL