Analogue Input/Output

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
1
Vo - Vref(Rf/Rinput)
4R
Vref/4
0
-
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 128 1/256
  0.00390625resolution 0.390625
• Alternative is R-2R ladder arrangement
• R-2R ladder - only requires 2 different resistor
  values.

5
Analogue Input

• Main types (methods) of ADC
• 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
Reference voltage
7
ADC principle of operation

1. The voltage is presented to the ADC input.
2. The ADC is sent a signal to start conversion
3. While the conversion takes place the input
   voltage should remain stable.
4. The ADC outputs a signal to indicate that it is
   busy doing the conversion and should not be
   disturbed.
5. 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 (SH)
• while conversion takes place voltage must remain
  stable
• sample voltage input connected to SH
• voltage held on a capacitor
• sample time charging time of capacitor
• input signal disconnected from SH

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.