Datasheet

188
ATmega8A [DATASHEET]
8159E–AVR–02/2013
23.6.2 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:
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 Figure 23-7.
3. Use the ADC noise canceler function to reduce induced noise from the CPU.
4. 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 Two-wire Interface (ADC4 and ADC5) will only affect the
conversion on ADC4 and ADC5 and not the other ADC channels.
Figure 23-7. ADC Power Connections
23.6.3 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:
• Offset: The deviation of the first transition (0x000 to 0x001) compared to the ideal transition (at 0.5LSB). Ideal
value: 0LSB.
GND
VCC
PC5 (ADC5/SCL)
PC4 (ADC4/SDA)
PC3 (ADC3)
PC2 (ADC2)
PC1 (ADC1)
PC0 (ADC0)
ADC7
GND
AREF
AVCC
ADC6
PB5
10μH
100nF
Analog Ground Plane