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140
8183F–AVR–06/12
ATtiny24A/44A/84A
In order to avoid distortion from unpredictable signal convolution, signal components higher than
the Nyquist frequency (f
ADC/2) should not be present. The user is advised to remove high fre-
quency components with a low-pass filter before applying the signals as inputs to the ADC.
Figure 16-8.
Analog Input Circuitry
Note:
The capacitor in the figure depicts the total capacitance, including the sample/hold capacitor and
any stray or parasitic capacitance inside the device. The value given is worst case.
16.9
Noise Canceling Techniques
Digital circuitry inside and outside the device generates EMI which might affect the accuracy of
analog measurements. When conversion accuracy is critical, the noise level can be reduced by
applying the following techniques:
Keep analog signal paths as short as possible.
Make sure analog tracks run over the analog ground plane.
Keep analog tracks well away from high-speed switching digital tracks.
If any port pin is used as a digital output, it mustn’t switch while a conversion is in progress.
Place bypass capacitors as close to V
CC and GND pins as possible.
Where high ADC accuracy is required it is recommended to use ADC Noise Reduction Mode, as
is above 1 MHz, or when the ADC is used for reading the internal temperature sensor, as
bypass capacitors does reduce the need for using ADC Noise Reduction Mode
16.10 ADC Accuracy Definitions
An n-bit single-ended ADC converts a voltage linearly between GND and V
REF in 2
n steps
(LSBs). The lowest code is read as 0, and the highest code is read as 2
n-1.
Several parameters describe the deviation from the ideal behavior, as follows:
ADCn
IIH
1..100 k
ohm
CS/H = 14 pF
VCC/2
IIL