Rainbow Electronics ATmega128L - Manual (Page 228)

228
ATmega128(L)
2467B–09/01
Note that the ADC will not be automatically turned off when entering other sleep modes
than idle mode and ADC noise reduction mode. The user is advised to write zero to
ADEN before entering such sleep modes to avoid excessive power consumption. If the
ADC is enabled in such sleep modes and the user wants to perform differential conver-
sions, the user is advised to switch the ADC off and on after waking up from sleep to
prompt an extended conversion to get a valid result.
Analog Input Circuitry The analog input circuitry for single ended channels is illustrated in Figure 112. An ana-
log source applied to ADCn is subjected to the pin capacitance and input leakage of that
pin, regardless of whether that channel is selected as input for the ADC. When the chan-
nel is selected, the source must drive the S/H capacitor through the series resistance
(combined resistance in the input path).
The ADC is optimized for analog signals with an output impedance of approximately
10 k
Ω or less. If such a source is used, the sampling time will be negligible. If a source
with higher impedance is used, the sampling time will depend on how long time the
source needs to charge the S/H capacitor, with can vary widely. The user is recom-
mended to only use high impedant sources with slowly varying signals, since this
minimizes the required charge transfer to the S/H capacitor.
If differential gain channels are used, the input circuitry looks somewhat different,
although source impedances of a few hundred k
Ω or less is recommended.
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.
Figure 112. Analog Input Circuitry
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 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 Figure 113.
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.
ADCn
I
IH
1..100 k Ω
C
S/H
= 14 pF
V
CC
/2
I
IL