Datasheet

306
ATmega16U4/32U4 [DATASHEET]
Atmel-7766J-USB-ATmega16U4/32U4-Datasheet_04/2016
The ADC is optimized for analog signals with an output impedance of approximately 10k or less. If such a
source is used, the sampling time will be negligible. If a source with higher impedance 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 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 k
 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 24-9. Analog Input Circuitry
24.7.2 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:
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.
7. 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 24-10.
8. Use the ADC noise canceler function to reduce induced noise from the CPU.
9. 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