Transcript Microprocessor Engineering
Many sensors/transducers produce voltages
representing physical data.
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
control these devices from a computer requires
conversion from digital to analogue form.
Digital to Analogue Converter (DAC)
= 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
DAC principles – Example 4-bit DAC
Sum currents with operational amplifier
Vo = - Vref(Rf/Rinput)
Vo = -Vref(digital value/2n)
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)
Digital to Analogue conversion
Previous design needs many different precise
Resisters need to have a tolerance less than the
resolution. E.g. 8-bit
resolution = 1:28 = 1/256 = 0.00390625
resolution = 0.390625%
Alternative is R-2R ladder arrangement
ladder - only requires 2 different resistor values.
Main types (methods) of ADC
approximation – good all-rounder
Flash – fastest type
Sigma-delta – good for audio
Dual slope integrating – slow but high resolution with good
others – Sampling, ramp, charge balancing
input voltage range
interface – parallel (fast) or serial(fewer connections)
Typical ADC Block diagram
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
To convert several analogue inputs
1. use an ADC for each input or …
2. use one ADC and switch the inputs through a
requires selection of input before each conversion is
short delay required before conversion started to allow
switching to occur and signal to settle.
Sample and Hold Circuit
Sample and Hold (S&H)
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
Microchip PIC18F452 ADC
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.
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.