Datasheet

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 impedance 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 kΩ 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 26-8. Analog Input Circuitry
ADCn
I
IH
1..100k Ω
C
S/H
= 14pF
I
IL
V
CC
/2
26.6.2. 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 ground plane,
and keep them well away from high-speed switching digital tracks.
2. The AV
CC
pin on the device should be connected to the digital V
CC
supply voltage via an LC
network as shown in the figure below.
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.
Atmel ATmega32A [DATASHEET]
Atmel-8155I-ATmega32A_Datasheet_Complete-08/2016
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