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.
• 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 (usually
  an analogue voltage).
• The conversion from analogue signals to digital
  values is performed by Analogue to Digital
  Converter (ADC)
• The conversion from a digital value to an
  analogue signal is performed by a Digital to
  Analogue Converter (DAC).

2
DAC function
Parallel DAC
Serial DAC would typically have a single data
input line and a clock input signal which would
be used to clock in the serial data stream. Adv.
- Fewer pins. Disadv. Slower data transfer.
3
Analogue Output

• Digital to Analogue Converter (DAC)
• DAC Characteristics
• resolution 1/2n where n is the number of bits
• Max. digital value 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)

4
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

1
Vo -(Vref-AGND)(digital value/2n) Vo a digval/
2n
16R
Vref/16
AGND
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)
Vref
5
Output from DAC
Output Voltage
VMAX Maximum output voltage where n is
number of bits
MAXVAL Maximum digital value 2n -1 where n
is number of bits
VMAX
Digital Value
MAXVAL
6
Example DAC device

• M AX5722 dual,12-bit, low-power, buffered voltage
  output, digital-to-analog converter (DAC) is
  packaged in a space-saving 8-pin µMAX package
  (5mm ? 3mm).

Ultra-Low Power Consumption 112µA at VDD
3.6V 135µA at VDD 5.5V Wide 2.7V to 5.5V
Single-Supply Range 8-Pin µMAX Package 0.3µA
Power-Down Current Guaranteed 12-Bit
Monotonicity (1LSB DNL) Safe Power-Up Reset to
Zero Volts at DAC Output Three
Software-Selectable Power-Down Impedances (100kO,
1kO, Hi-Z) Fast 20MHz, 3-Wire SPI, QSPI, and
MICROWIRECompatible Serial Interface
Rail-to-Rail Output Buffer Amplifiers
Schmitt-Triggered Logic Inputs for
Direct Interfacing to Optocouplers Wide -40C to
125C Operating Temperature Range
7
Audio LPC 23xx DAC example

• Recreate audio

• What resolution?
• What sampling rate ?

8
Analogue Input

• Analogue to Digital Converter (ADC)
• ADC Characteristics
• resolution 1/2n where n is the number of bits
• Max. digital value 2n 1
• input voltage range determined by the reference
  voltages (Vref and AGND)
• Step size in volts resolution x voltage range
• uni-polar / bipolar types
• interface parallel (fast) or serial (slower but
  uses fewer connections)
• often integrated into microcontrollers.

9
General ADC function

• The analogue input voltage is converted into a
  value.
• The value is dependent on the reference voltages
  and the number of bits n.

10
Analogue Input

• Main types (i.e.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)

11
MCP3208

• Features
• 12-bit resolution
• 1 LSB max DNL
• 1 LSB max INL (MCP3204/3208-B)
• 2 LSB max INL (MCP3204/3208-C)
• 4 (MCP3204) or 8 (MCP3208) input channels
• Analog inputs programmable as single-ended or
• pseudo-differential pairs
• On-chip sample and hold
• SPI serial interface (modes 0,0 and 1,1)
• Single supply operation 2.7V - 5.5V
• 100 ksps max. sampling rate at VDD 5V
• 50 ksps max. sampling rate at VDD 2.7V
• Low power CMOS technology
• 500 nA typical standby current, 2 µA max.
• 400 µA max. active current at 5V
• Industrial temp range -40C to 85C
• Available in PDIP, SOIC and TSSOP packages

12
Example8-bit ADC with Vref 5v and 0v VAgnd

• Number of steps (values) 2n 28 256
• steps are numbered 0 to 255
• step size reference voltage range / number of
  steps (5v 0v) / 256 19.53125 x10-3v ?
  20mv
• number range 0 to 255 corresponds to voltage
  range of 0 to ? 5v
• ADC value (Vin / (Vref Agnd)) 256
  remember max ADC value is 255 so max input
  voltage that can be converted accurately is less
  than 5 volts.
• What is the maximum convertible input voltage?

13
8-bit ADC with 5v reference
ADC value
255
ADC value
0
Volts
?4.98
14
Cont.
ADC value
4
3
2
1
Volts
0
?10mv
?30mv
?50mv
?70mv
?20mv
?40mv
?60mv
?80mv
15
Quantization Error
Each input sample is assigned a quantization
interval that is closest to its amplitude height.
If an input sample is not assigned a quantization
interval that matches its actual height, then an
error is introduced into the conversion process.
This error is called quantization error/noise.
16
Reducing quantization error

• One way to reduce quantization noise is to
  increase the amount of quantization intervals.
• The difference between the input signal amplitude
  height and the quantization interval decreases as
  the quantization intervals are increased
  (increases in the intervals decrease the
  quantization noise).
• Solved by increasing the ADC resolution (number
  of bit) in proportion to the increase in
  quantization intervals.

17
Analogue Input Example

• Example - The LM35 series are precision
  integrated-circuit temperature sensors, whose
  output voltage is linearly proportional to the
  Celsius (Centigrade) temperature.

Calibrated directly in Celsius (Centigrade)
Linear 10.0 mV/C scale factor
0.5C accuracy guarantee able (at 25C)
Rated for full -55 to 150C range
Suitable for remote applications
Low cost due to wafer-level trimming
Operates from 4 to 30 volts
Less than 60 µA current drain
Low self-heating, 0.08C in still air
Nonlinearity only ¼C typical
Low impedance output, 0.1 Ohm for 1 mA load
18
LM35 LPC23xx Interface Example

• ADC range (LPC23xx) 0V to 3.3 V
• LM35 Linear 10.0 mV/C scale factor
• Use a basic Centigrade temperature sensor 2C to
  150C

• Max voltage from sensor 10mV150 1.5V

Scale the input voltage accordingly to match
input the range of the ADC
19
LM35 LPC23xx Interface Example

• step size reference voltage range / number of
  steps (3.3v 0v) / 1024 3.22265625 x10-3v
• Every 1C is now equivalent to 20.0 mV
• Temperature resolution gt 0.2C

AI0
20
ADC input

• Good practice to
• Limit the voltage range (input protection)
• Filter the signal

From sensor
To ADC input
21
ADC Block diagram
Interface to uP
Reference voltage
22
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)

23
Multiplexer and Sample/Hold

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

24
Summary

• Analogue inputs are often required in embedded
  applications and so ADCs are integrated into most
  microcontrollers (DACs less so)
• ADCs and DAC also exist as standalone IC devices
  - often specialist devices e.g. High speed or
  high resolution ADCs, fast DACs for video output.
• Main characteristics of interest is
• resolution - number of bits
• voltage range
• ADC - conversion time
• DAC - slew rate

25
ADC on the LPC23xx

• LPC23xx ADC Features
• 10 bit successive approximation analogue to
  digital converter.
• Input multiplexing among 6 pins or 8 pins.
• Power down mode.
• Measurement range 0 to 3.3 V.
• 10 bit conversion time 2.44 µs.
• Burst conversion mode for single or multiple
  inputs.
• Optional conversion on transition on input pin or
  Timer Match signal.
• Individual result registers for each A/D channel
  to reduce interrupt overhead.

26
Using the ADC

• The ADC like all other peripherals is accessed
  through a group of associated registers. (see
  pages 575ff of the user manual for a detailed
  description)
• As an example we will look at the A/D Control
  Register (AD0CR)

27
AD0CR
Selects which of the AD0.70 pins is (are) to be
sampled and converted. For AD0, bit 0 selects Pin
AD0.0, and bit 7 selects pin AD0.7. In
software-controlled mode, only one of these bits
should be 1. In hardware scan mode, any value
containing 1 to 8 ones. All zeroes is equivalent
to 0x01.
0- Conversions are software controlled and
require 11 clocks. 1- The AD converter does
repeated conversions at the rate selected by the
CLKS field, scanning (if necessary) through the
pins selected by 1s in the SEL field. The first
conversion after the start corresponds to the
least-significant 1 in the SEL field, then higher
numbered 1 bits (pins) if applicable. Repeated
conversions can be terminated by clearing this
bit, but the conversion thats in progress when
this bit is cleared will be completed.
This bit is significant only when the START field
contains 010-111. In these cases1- Start
conversion on a falling edge on the selected
CAP/MAT signal. 0 Start conversion on a rising
edge on the selected CAP/MAT signal
1 The A/D converter is operational. 0 - The A/D
converter is in power-down mode.
This field selects the number of clocks used for
each conversion in Burst mode, and the number of
bits of accuracy of the result in the LS bits of
ADDR, between 11 clocks (10 bits) and 4 clocks (3
bits). 000 11 clocks / 10 bits 001 10 clocks / 9
bits 010 9 clocks / 8 bits 011 8 clocks / 7
bits 100 7 clocks / 6 bits 101 6 clocks / 5
bits 110 5 clocks / 4 bits 111 4 clocks / 3
bits
The APB clock (PCLK) is divided by (this value
plus one) to produce the clock for the A/D
converter, which should be less than or equal to
4.5 MHz. Typically, software should program the
smallest value in this field that yields a clock
of 4.5 MHz or slightly less, but in certain cases
(such as a high-impedance analog source) a slower
clock may be desirable.
When the BURST bit is 0, these bits control
whether and when an A/D conversion is
started 000 No start (this value should be used
when clearing PDN to 0). 001 Start now. 010 Start
when the edge selected by bit 27 occurs on
P2.10/EINT0. 011 Start when the edge selected by
bit 27 occurs on P1.27/CAP0.1. 100 Start when the
edge selected by bit 27 occurs on MAT0.1. 101
Start when the edge selected by bit 27 occurs on
MAT0.31. 110 Start when the edge selected by
bit 27 occurs on MAT1.0. 111 Start when the edge
selected by bit 27 occurs on MAT1.1.
R Reserved, user software should not write or
read ones to reserved bits.
28
AD0CR
AD0CR ( 0x01 ltlt 0 ) / SEL1,select channel
07 on ADC0 / ( ( Fpclk / ADC_Clk - 1 ) ltlt 8 )
/ CLKDIV Fpclk / 1000000 - 1 / ( 0 ltlt 16
) / BURST 0, no BURST, software controlled
/ ( 0 ltlt 17 ) / CLKS 0, 11 clocks/10
bits / ( 1 ltlt 21 ) / PDN 1, normal
operation / ( 0 ltlt 24 ) / START 0 A/D
conversion stops / ( 0 ltlt 27 ) / EDGE 0
(CAP/MAT singal falling,trigger A/D conv /
29
Enable the ADC input pins

• Before using the ADC the appropriate pins must be
  set as ADC inputs. This is accomplished using the
  PINSEL register(s)

PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1 PINSEL1
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 4 4 4 4 0 0 0 0 0 0 0 0 0 0 0 0
PINSEL1 0x00004000 // Select only ADC
channel 0
30
Reading the ADC

• See example programs!

31
Example question

• A 5 bit ADC is used to encode an analogue signal
  in the range 0V to 5V for linear PCM encoding
  determine
• The step size
• Calculate the percentage resolution
• Calculate the dynamic range in dB
• Calculate the input voltage level corresponding
  10110.

32
Solution

• The step size

2. Calculate the percentage resolution
33
Solution
3 Calculate the dynamic range in dB
4 Calculate the input voltage level corresponding
10110
34
Voice signal
Bandpass Filtering
Bandwidth (3.1 kHz)
The human voice can produce sounds up to 20
kHz, but most sound is between 300 Hz and 3.4
kHz.The bandpass filter only passes this sound
to reduce bandwidth.