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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.
5-1
Analogue Output


Digital to Analogue Converter (DAC)
DAC Characteristics
= 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)
 resolution
5-2
DAC principles – Example 4-bit DAC

Sum currents with operational amplifier
1
d3
2R
R
Vref/2
Vo = - Vref(Rf/Rinput)
4R
Vref/4
0
d2
8R
Vref/8
+
1
d1
1
Vref
16R
Vo
Vo = -Vref(digital value/2n)
Example: with 4-bit value = 1011
Vref/16
Vo = -Vref(d3/2 + d2/4 + d1/8 + d0/16)
Vo = -Vref(1/2 + 1/8 + 1/16)
d0
AGND
Vo = -Vref(11/16)
5-3
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 = 0.00390625
resolution = 0.390625%
Alternative is R-2R ladder arrangement
 R-2R
ladder - only requires 2 different resistor values.
5-4
Analogue Input

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
noise immunity
 others – Sampling, ramp, charge balancing
 Successive

Characteristics
 resolution
 conversion
method
 conversion time
 input voltage range
 interface – parallel (fast) or serial(fewer connections)
5-5
Typical ADC Block diagram
Interrupt request
Conversion
Control
AN0
AN1
ANn
M
u
t
i
p
l
e
x
e
r
Busy
Start
conversion
Sample
& Hold
Converter
VAREF
Result
Register
VAGND
Reference voltage
5-6
ADC – principle of operation
1.
2.
3.
4.
5.
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)
5-7
Multiplexer

To convert several analogue inputs
1. use an ADC for each input or …
2. 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.
5-8
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
5-9
Microchip PIC18F452 ADC
5-10
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
5-11
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.
5-12