204
Atmel ATmega16/32/64/M1/C1 [DATASHEET]
7647K–AVR–12/13
18.6.1 Analog Input Circuitry
The analog input circuitry for single ended channels is illustrated in
Figure 18-8 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 resistance (combined resistance in the input
path).
The ADC is optimized for analog signals with an output impedance of approximately 10k
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 recommended to only use low 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 (fADC/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 18-8. Analog Input Circuitry
18.6.2 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 analog ground plane, and keep
them well away from high-speed switching digital tracks.
2.
The AVCC pin on the device should be connected to the digital VCC supply voltage via an RC network (R = 10
Ω max,
C = 100nF).
3.
Use the ADC noise canceler function to reduce induced noise from the CPU.
4.
If any ADC port pins (PB[7:2], PC[7:4], PD[6:4], PE[2]) are used as digital outputs, it is essential that these do not
switch while a conversion is in progress.
18.6.3 Offset Compensation Schemes
The gain stage 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 shortening both differential inputs using the AMPxIS bit
measurement results. Using this kind of software based offset correction, offset on any channel can be reduced below one LSB.
IIL
VCC/2
CS/H = 14pF
IIH
ADCn
1 to 100kΩ