Datasheet

ATmega164A/PA/324A/PA/644A/PA/1284/P
 2018 Microchip Technology Inc. Data Sheet Complete DS40002070A-page 251
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
measurements. 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.
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