Most
of AVR microcontrollers have Analog to Digital Converter (ADC)
integrated in to chip. Such solution makes embedded designers life much
easier when creating projects and programming them. With no need
external ADC PCB takes less space, easier to create programs – it saves
time and money. As an example lets take Atmega8 microcontroller which
have up to 8 ADC inputs most with 10-bit resolution(excluding ADC4 and
ADC5 inputs that are 8-bit). All features of AVR internal ADC can be
found on official ATMEL AVR datasheets, but most important to mention
are:
-
±2 LSB accuracy – so measurements aren't very accurate. If AREF voltage is 5V then error may reach ±0.04V but this is still good results for most of tasks;
-
Integral nonlinearity ±0.5 LSB;
-
Conversion speed up to 15kSPS at maximum resolution. This is far not enough for 20kHz audio signal sampling.
ADC unit is powered with separate power supply
pins AVCC with AGND, but AVCC must not differ ±0.3V of VCC. Also ADC
unit can have different voltage reference sources selectable in ADMUX
register. References may be taken from AREF pin, AVCC with external
capacitor or internal 2.56V voltage reference. All ADC inputs are
multiplexed via multiplexer. Each channels can be selected by changing 4
bits in ADMUX register. ADC unit can operata in two modes:
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