282
7679H–CAN–08/08
AT90CAN32/64/128
1.
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.
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
.
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.
Figure 21-9.
ADC Power Connections
21.6.3
Offset Compensation Schemes
The gain stage has a built-in offset cancellation circuitry that nulls the offset of differential mea-
surements as much as possible. The remaining offset in the analog path can be measured
directly by selecting the same channel for both differential inputs. This offset residue can be then
subtracted in software from the measurement results. Using this kind of software based offset
correction, offset on any channel can be reduced below one LSB.
21.6.4
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 0.5 LSB). Ideal value: 0 LSB.
VCC
GND
100nF
Analog Ground Plane
(ADC0) PF0
(ADC7) PF7
(ADC1) PF1
(ADC2) PF2
(ADC3) PF3
(ADC4) PF4
(ADC5) PF5
(ADC6) PF6
AREF
GND
AVCC
52
53
54
55
56
57
58
59
60
61
61
62
62
63
63
64
64
1
51
NC
(AD0) PA0
10uH
Содержание AVR AT90CAN128
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