Program point of view understanding of ADC

• ADC conversion time

• Conversion time of ADC depend upon clock of crystal oscillator through which it gets clock, in modern microcontroller, provided separately in Ksps (Kilo sample per second). 
• ADC requires an input clock of frequency less than 200 KHz for maximum accuracy
• Lower the frequency higher the precision
• In AVR first conversion take 25 ADC clock cycle to initialize analog circuitory and pass the sample and hold circuit
• Then next each consecutive conversion take  13 ADC clock cycle

 

• ADC connection to ATmega32

 

• To get better accuracy of ADC, we must provide stable voltage source to AVcc.
• And we done throut it by 100nF capacitors and inductor of 10 uH.

 

• Register associated in programming of ADC are

ADMUX

 

• ADLAR – ADC left adjust result,In ATmega32 ADC is of 10 bit which mean result is of 10 bit. So it can’t see in single byte register and there for there is two register of each single byte named as ADCH (high byte) and ADCL (low byte). In this 16 bit only 10 bit are used rest are not used. By ADLAR bit we can use this 10 bit of left most or right most.
  If ADLAR = 1 , 10 bit are shifted left  in ADCH and ADCL
  If ADLAR = 0 , 10 bit are shifted right in ADCH and ADCL

REFS1 REFS0 Vref selection bit 0 0 AREF pin set externally 0 1 AVcc pin same as Vcc 1 0 Reserved 1 1 Internal 2.56 V (internally derived from Vcc)

 

• MUX  (DIFFERENTIAL  INPUT TO USE OPAMP GAIN)

Mux4 to Mux0 Differential input + Differential input – Gain 01001 ADC1 ADC0 10X 01011 ADC1 ADC0 200X 01101 ADC3 ADC2 10X 01111 ADC3 ADC2 200X 10000 ADC0 ADC1 1X 10010 ADC2 ADC1 1X 10011 ADC3 ADC1 1X 10100 ADC4 ADC1 1X 10101 ADC5 ADC1 1X 10110 ADC6 ADC1 1X 10111 ADC7 ADC1 1X 11000 ADC0 ADC2 1X 11001 ADC1 ADC2 1X 11011 ADC3 ADC2 1X 11100 ADC4 ADC2 1X 11101 ADC5 ADC2 1X

 

• MUX  (SINGLE  ENDED)- in differential mux select, we can also control a gain of op-amp upto 200x. in this mode , positive input of the opamp to ve ADC0 to ADC7 and negative input of the opamp be ADC0, ADC1 and ADC2. And in this mode of operation we can use single channel of  ADC as input and common ground as ground.

Mux4 to Mux0 Single ended input 00000 ADC0 00001 ADC1 00010 ADC2 00011 ADC3 00100 ADC4 00101 ADC5 00110 ADC6 00111 ADC7

 

ADCSRA

 

 

• ADEN  = 1, mean  ADC enable and ADEN = 0, mean ADC disable
• ADSC  = 1, mean  ADC start conversion and ADEN = 0 mean ADC conversion disable
• ADATE  =  ADC auto trigger enable 
• ADIF = ADC interrupt flag. 1 mean conversion complete
• ADIE = ADC interrupt enable when 1.
• ADPS

ADPS2 ADPS1 ADPS0 ADC clock 0 0 0 Reserved 0 0 1 Clk / 2 0 1 0 Clk / 4 0 1 1 Clk / 8 1 0 0 Clk / 16 1 0 1 Clk / 32 1 1 0 Clk / 64 1 1 1 Clk / 128