1. Analogue Input/Output
Many sensors/transducers produce voltages representing physical data.
To process transducer data in a computer requires conversion to digital form. Examples:-
reading temperature from a thermocouple
processing speech from a microphone
Many output devices require variable control, not just two digital logic levels
To control these devices from a computer requires conversion from digital to analogue form.

2. Analogue Output Digital to Analogue Converter (DAC)
DAC Characteristics
resolution = 1/2n where n is the number of bits – step size
Max. digital output = 2n – 1
output voltage range – determined by reference voltage (Vref and AGND)
Step size in volts = resolution x voltage range
Max output voltage = (2n – 1)/ 2n x voltage range
uni-polar / bipolar types
slew rate – rate of change of output.
interface – parallel (fast) or serial (slower but uses fewer connections)

3. DAC principles – Example 4-bit DAC
Sum currents with operational amplifier R 2R
Vref/2 d3 d2 d0 d1 1
Vo = - Vref(Rf/Rinput) 4R
Vref/4 - Vo 8R
Vref/8 +
Vo = -Vref(digital value/2n) 1
Example: with 4-bit value = 1011
Vo = -Vref(d3/2 + d2/4 + d1/8 + d0/16)
Vo = -Vref(1/2 + 1/8 + 1/16)
Vo = -Vref(11/16)
16R
Vref/16 1
Vref
AGND

4. Digital to Analogue conversion
Previous design needs many different precise resistor values
Resisters need to have a tolerance less than the resolution. E.g. 8-bit resolution = 1:28 = 1/256 = resolution = %
Alternative is R-2R ladder arrangement
R-2R ladder - only requires 2 different resistor values.

5. Analogue Input Main types (methods) of ADC Characteristics
Successive approximation – good all-rounder
Flash – fastest type
Sigma-delta – good for audio
Dual slope integrating – slow but high resolution with good noise immunity
others – Sampling, ramp, charge balancing
Characteristics
resolution
conversion method
conversion time
input voltage range
interface – parallel (fast) or serial(fewer connections)

6. Typical ADC Block diagram
VAREF
VAGND
Conversion Control
Interrupt request
Mut iplexer
Sample & Hold
Converter
Result Register
AN0
AN1
ANn
Busy
Start conversion
Reference voltage

7. ADC – principle of operation
The voltage is presented to the ADC input.
The ADC is sent a signal to start conversion
While the conversion takes place the input voltage should remain stable.
The ADC outputs a signal to indicate that it is busy doing the conversion and should not be disturbed.
When the conversion is completed the ADC makes the result available and outputs a signal to indicate that the conversion has completed (e.g remove the busy signal)

8. Multiplexer To convert several analogue inputs
use an ADC for each input or …
use one ADC and switch the inputs through a multiplexer
requires selection of input before each conversion is started
short delay required before conversion started to allow switching to occur and signal to settle.

9. Sample and Hold Circuit
Sample and Hold (S&H)
while conversion takes place voltage must remain stable
sample voltage – input connected to S&H
voltage held on a capacitor
sample time – charging time of capacitor
input signal disconnected from S&H

10. Microchip PIC18F452 ADC

11. ADC Control Registers
The ADC is configured and controlled through two Special File Registers, ADCON0 and ADCON1. Additionally the PIR1 contains the ADIF bit (Analogue to Digital Interrupt Flag) which is set when a conversion is complete.
ADCON0
ADCON1

12. ADC Result Format
The 10-bit result can be placed in ADRESH and ADRESL in one of two ways depending on the ADFM bit in ADCON0
With left justified an equivalent 8-bit ADC can be obtained by simply using ADRESH and ignoring ADRESL.