Rainbow Electronics ATmega128RFA1 - Manual (Page 421)

421
8266A-MCU Wireless-12/09
ATmega128RFA1
Figure 27-11. Analog Input Circuitry
A D C n
I
I L
I
I H
C
S /H
= 1 4 p F
V
A V D D
/2
2 k
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.
27.7.2 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:
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. Use the ADC noise canceller function to reduce induced noise from the CPU.
3. If any ADC port pins are used as digital outputs, it is essential that these do not
switch while a conversion is in progress.
27.7.3 Offset Compensation Schemes
The differential amplifier has a built-in offset cancellation circuitry that nulls the offset of
differential measurements 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 then be subtracted in software from the measurement results. The
offset on any channel can be reduced below one LSB using this kind of software based
offset correction.
27.7.4 ADC Accuracy Definitions
An n-bit single-ended ADC converts a voltage linearly between 0V and V
REF
in 2
n
steps
(LSB’s). 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.