129
ATtiny25/45/85 [DATASHEET]
2586Q–AVR–08/2013
conversion is complete, that interrupt will be executed, and an ADC Conversion Complete interrupt request will
be generated when the ADC conversion completes. The CPU will remain in active mode until a new sleep
command is executed.
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.
17.8
Analog Input Circuitry
The analog input circuitry for single ended channels is illustrated in
Figure 17-8. An analog 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 channel is selected, the source must drive the S/H capacitor through the series resis-
tance (combined resistance in the input path).
Figure 17-8. Analog Input Circuitry
The ADC is optimized for analog signals with an output impedance of approximately 10 k
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 low impedant sources with slowly varying signals, since this minimizes the required charge
transfer to the S/H capacitor.
Signal components higher than the Nyquist frequency (f
ADC/2) should not be present to avoid distortion from unpre-
dictable 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.
17.9
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:
Keep analog signal paths as short as possible.
Make sure analog tracks run over the analog ground plane.
Keep analog tracks well away from high-speed switching digital tracks.
If any port pin is used as a digital output, it mustn’t switch while a conversion is in progress.
Place bypass capacitors as close to V
CC and GND pins as possible.
Where high ADC accuracy is required it is recommended to use ADC Noise Reduction Mode, as described in
Sec-tion 17.7 on page 128. This is especially the case when system clock frequency is above 1 MHz, or when the ADC
ADCn
IIH
1..100 k
Ω
CS/H= 14 pF
VCC/2
IIL