Lecture 13ADC Interfacing

Fall 2014

Armaghan Mohsin

Department of PhysicsCIIT Islamabad

Real Time Embedded Systems

EEE446

Basics of A/D Conversion• Can convert only electrical voltages to digital values• A transducer is needed to convert a non-electric quantity

into an electrical voltage• Different names of transducers are used for different

physical quantities• A data acquisition system is used to referred to those

systems that perform A/D conversions

Analog Voltage and Digital Code• Characteristic of an Ideal A/D Converter

– Needs infinite number of bits to encode the A/D conversion result

– Unachievable and impractical

Characteristic of an Ideal n-bit ADC• The area between dotted line and staircase is called the

quantization error.– The resolution of this A/D converter is VDD/2n.– Average conversion error is VDD/2n+1.– A real A/D converter has nonlinearity

Optimal Voltage Range for ADC• A/D converter requires a low reference voltage (VREF-)

and a high reference voltage (VREF+) to perform conversion.– Most A/D converters are ratiomertic:

• An analog input of VREF- is converted to digital code 0.• An analog input of VREF+ is converted to digital code 2n-1.• An analog input of k V is converted to digital code k

• The A/D conversion result k corresponds to the following analog input:

VK = VREF- +((VREF+ - VREF-) * k) ÷ (2n – 1)• Most systems use VDD and 0V as VREF+ and VREF-,

respectively.• The output of a transducer should be scaled and shifted

to the range of 0V ~ VDD in order to achieve the best accuracy

ExampleSuppose that there is a 10-bit A/D converter with VREF- = 1 V and VREF+ = 4V. Find the corresponding voltage values for the A/D conversion results of 25, 80, 240, and 900.Solution:VK = VREF- +((VREF+ - VREF-) * k) ÷ (2n – 1)The corresponding voltages are as follows:1V + (3 * 25) ÷ (210 – 1) = 1.07 V1V + (3 * 80) ÷ (210 – 1) = 1.23 V1V + (3 * 240) ÷ (210 – 1) = 1.70 V1V + (3 * 900) ÷ (210 – 1) = 3.64 V

PIC18F ADC Module• The PIC18 has a 10-bit A/D converter.

– The number of analog inputs varies among difference PIC18 devices.

– The A/D converter has the following registers:• A/D Result High Register (ADRESH)• A/D Result Low Register (ADRESL)• A/D Control Register 0 (ADCON0)• A/D Control Register 1 (ADCON1)• A/D Control Register 2 (ADCON2)• The contents of these registers vary with the PIC18 members.

– Early PIC18 (PIC18FXX2) members have only ADCON0 and ADCON1 registers.

ADCON0 Register

ADCON1 Register

ADCON2 Register

Successive Approximation• The A/D converters in most PICmicro MCUs are of the

Successive Approximation type• Converts one bit at a time• Converts MSB first, LSB last• One A/D clock time required for each bit

Conversion Time• Each bit requires one A/D clock period (TAD) for

conversion• Two to three additional A/D clocks required for settling

time• After conversion the final value is written to the result

register

A/D Clock• Multiple Sources

– 2TOSC (FOSC/2)– 8TOSC (FOSC/8)– 32TOSC (FOSC/32)– RC (dedicated internal) 4 or 6uS typical

• Must meet minimum time

A/D Conversion Steps• Configure I/O Pins• Select the Channel to Convert• Configure and Enable the A/D• Wait the Acquisition Time• Initiate the Conversion• Wait for the Conversion to Complete• Read the Result