Datasheet

145
ATtiny828 [DATASHEET]
8371A–AVR–08/12
15.9 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:
z Keep analog signal paths as short as possible.
z Make sure analog tracks run over the analog ground plane.
z Keep analog tracks well away from high-speed switching digital tracks.
z If any port pin is used as a digital output, it mustn’t switch while a conversion is in progress.
z Place bypass capacitors as close to V
CC
and GND pins as possible.
z The analog supply voltage pin (AV
CC
) should be connected to the digital supply voltage pin (V
CC
) via an LC
network as shown in Figure 61.
Figure 61. ADC Power Connections
Where high ADC accuracy is required it is recommended to use ADC Noise Reduction Mode, as described in Section
15.7 on page 144. This is especially the case when system clock frequency is above 1 MHz, or when the ADC is used for
reading the internal temperature sensor, as described in Section 15.12 on page 148. A good system design with properly
placed, external bypass capacitors does reduce the need for using ADC Noise Reduction Mode
15.10 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:
PB7
PB6
PB5
PB4
PB3
GND
PB2
AVCC
PB1
PB0
10mH
Analog Ground Plane
100nF
VCC
PA2
PA3
PA4
PA5
PA6
PA7
PA0
PA1