ATtiny13A datasheet(89/174 Pages) ATMEL

89 / 174 page

8126D–AVR–11/09

ATtiny13A

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 imped-

ance 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.

Signal components higher than the Nyquist frequency (fADC/2) should not be present 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.

14.9 Analog Noise Canceling Techniques

Digital circuitry inside and outside the device generates EMI which might affect the accuracy of

analog measurements. When 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 VCC and GND pins as possible.

Where high ADC accuracy is required it is recommended to use ADC Noise Reduction Mode, as

described in Section 14.7 on page 88. This is especially the case when system clock frequency

is above 1 MHz. A good system design with properly placed, external bypass capacitors does

reduce the need for using ADC Noise Reduction Mode

14.10 ADC Accuracy Definitions

An n-bit single-ended ADC converts a voltage linearly between GND and VREF in 2

n steps

(LSBs). The lowest code is read as 0, and the highest code is read as 2n-1.

Several parameters describe the deviation from the ideal behavior:

ATtiny13A Datasheet(PDF) 89 Page - ATMEL Corporation