This document discusses analog to digital conversion and interfacing an ADC with a PIC18 microcontroller. It describes the basics of A/D conversion including using transducers to convert non-electric quantities to voltages. It also discusses the characteristics of an ideal ADC and quantization error. The document provides an example of calculating digital codes from analog voltages for a 10-bit ADC. It then describes the registers and conversion process for the A/D converter module in PIC18 microcontrollers.

1. Lecture 13
ADC Interfacing
Fall 2014
Armaghan Mohsin
Department of Physics
CIIT Islamabad
Real Time
Embedded Systems
EEE446

2. 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

3. 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

4. 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

5. 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

6. Example
Suppose 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 V
1V + (3 * 80) ÷ (210 – 1) = 1.23 V
1V + (3 * 240) ÷ (210 – 1) = 1.70 V
1V + (3 * 900) ÷ (210 – 1) = 3.64 V

7. 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.

8. ADCON0 Register

9. ADCON1 Register

10. ADCON2 Register

11. 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

12. 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

13. 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

14. 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