National Diploma In Engineering
Data Communications
Electronics B NIII
Assignment no. 2
A/D and D/A Convertors and Display Devices
Weighting 20%
Name: Malcolm Brown
Class: NDD2
Tutor: Ken Hughs
Contents Page
Task 3
A/D and D/A Convertors 4
Analogue and Digital Signals 4
Analogue / Digital Conversions 5
Analogue to Digital Convertors 6
Digital To Analogue Converter 8
Glossary Of Terms 10
Visual Display Devices 11
Seven-Segment Displays 14
Dot Matrix Displays 16
Bibliology 18
Task
A/D and D/A Convertors
Explain two methods of converting analog signals to digital signals and compare them. Explain one method of digital to analog conversion. Choose two A/D convertor devices from the catalogue and list their characteristics, performance, cost, applications etc.
Display Devices
Describe how LED and LCD display devices operate - ie explain the principle behind their operation. Describe the features of the 7-segment, star-burst and dot matrix displays. Choose some devices from the catalogues and describe them.
You are required to produce a written report on your work. The report should be in standard report format and comprise of a front page with title, contents page summary, introduction, main body of the report describing the task and how you met the requirements of the task, circuit diagrams etc. and conclusions. Appendices may be placed in the report if necessary.
The report should be word processed and presented in a plastic folder. Your name class and subject should be clearly visible.
A/D and D/A Convertors
Analogue and Digital Signals
Analogue Signals - Signals whose amplitude and/or frequency vary continuously eg. sound. Fig 1.1 illustrates an analogue signal:-
Fig 1.1 Illustration of an analogue signal
Digital Signals - Signals which are not continous in nature but consist of discrete pulses of voltage or current known as bits which represent the information to be processed. Digital voltages can vary only in discrete steps. Normally only two levels are used ( 0 and 1 ).Fig 1.2 illustrates a digital signal.
Fig 1.2 Illustration of a digital signal
Analogue / Digital Conversions
In todays electronic system it is often necessary that the overall system may not be entirely analogue or entirely digital in nature. Thus a digital system may be controlled by input signals which are the amplified analogue outputs, perhaps of some measuring transducer (termister, LDR). Similarly a digital system output may be required to control the measured analogue system via analogue control values.Interfacing is therefore required between the analogue and digital subsystems and it is necessary to be able to convert an analogue signal into a digital equivalent signal and visa versa. A/D and D/A convertors are therefore used.
An analogue signal cannot be represented exactly by a digital signal and must be sampled at sufficient intervals for all relevant information to be retained. Sampling theory states that at least two samples must be obtained per period of the highest frequency component. If the highest frequency component is fs then the period of the sampling signal is given by:-
T < 1/2 fs
Fig. 2 Sample and Hold
Fig.2 shows a basic sample and hold circuit. The capacitor C is used as a store or memory to hold the value of the sample. It is connected to the analogue signal input via the resistor R. The time constant CR is chosen to be sufficiently short so that the capacitor voltage can follow the required analogue signal variations. At the instant that the sample is to be taken switch S is changed into the hold position and the sample voltage is available to the succeeding analogue to digital convertor.
The main disadvantage with this simple circuit lies in the voltage drift which occurs in the capacitor during the hold period. This is mainly due to the load placed upon the capacitor by the following circuitry and can be minimized by using a larger capacitor or by the use of a high impedance buffer amplifier.
Analogue to Digital Convertors
The two A/D convertors described below are known as the Ramp and Successive Approximation types.
Ramp A/D Convertor-
analogue
input Output Control
sample 0 if Va > Vc Logic
Va 1 if Va < Vc
Count up if input = 0
Count down if input = 1
VRef
n-bit Counter
Cloc
n-bit D/A
Convertor
n bit parallel digital output
Fig 3.1 Block Diagram of Ramp A/D Convertor
Fig 3.1 shows the block diagram for a Staircase Ramp analogue to digital convertor. This diagram consists of a clock pulse generator which sends clock pulses into the n-bit counter. The counter produces a parallel digital output which is converted into its analogue equivalent by the D/A convertor. The output of the D/A convertor is compared with the analogue input sample by the comparator. The output of the comparator is then fed into the control logic which in turn controls the counter.
The circuit operates as follows, the counter is emptied by resetting all bits to zero before a conversion is started. When the new analogue sample is present the control logic starts the count, ie clock pulses are fed into the counter. The counter digital output thus increases bit by bit at the clock frequency. The output from the digital to analogue convertor is a linear ramp made up of equal incremental steps. The count continues until the generated staircase ramp exceeds the value of the analogue sample voltage, when the capacitor output goes to logic 1 and stops the count.The counter output is at this time the digital equivalent of the analogue voltage.
Successive Approximation A/D Convertor
Shift Register
n-bit
digital
output
D/A Convertor
Fig 3.2 Block Diagram for a Successive Approximation A/D Convertor
Fig 3.2 shows the block diagram for a successive approximation A/D convertor. The diagram consists of a shift register to store the digital output connected to a D/A convertor whose output is compared with the analogue input sample by use of a comparator. The output of the comparator is then fed is then fed into the shift register.
The circuit operates by repeatedly comparing the analogue signal voltage with a number of approximate voltages which are generated at the D/A convertor.
Initially the shift register is cleared and then the D/A convertor output is zero. The first clock pulse applies the MSB to the register to the D/A convertor. The output of the D/A convertor is then one-half of its full scale voltage range (FSR). If the analogue voltage is greater than FSR/2 the MSB is retained (stored by a latch), if it is less than the FSR/2 the MSB is lost. The next clock pulse applies the next lower MSB to the D/A convertor producing a D/A convertor output of FSR/4 . If the MSB has been retained the total D/A convertor output voltage is now 3FSR/4. If the MSB has been lost the output of the D/A convertor is now FSR/4. In either case the analogue and D/A convertor voltages are again compared. If the analogue voltage is the larger of the two the second MSB is retained (latched), if not it is not the MSB is lost.
A succession of similar triats are carried out and after each the shift register output bit is either retained by a latch or is not. Once n+1 clock pulses have been supplied to the register the conversion has been completed and the register output gives the digital word that represents the analogue input sample voltage.
The characterics of two A/D convertors are shown in Appendices 1 +2
Digital To Analogue Converter
A typical 4-bit D/A converter is shown in fig 4.1. The circuit uses precision resistors that are weighted in digital progression ie 1,2,3,4. Vref is an accurate reference voltage. The circuit has 4 inputs (d0,d1,d2,d3) and 1 output Vout. When a bit is high it produces enough base current to saturate its transistor this acts as a closed switch. When a bit is low the transistor is cut off (open switch). By saturating and cutting off the transistor (opening and closing switch ) 16 different output currents from 0 to 1.875 Vref/R can be produced. If for example Vref =5V and R=5KW then the total output current varies from 0 to 1.875 mA as shown in table 1.
Fig 4.1 D/A converter using switching transistors
D3 D2 D1 D0 Output current mA Fraction of maximum
0 0 0 0 0 0
0 0 0 1 0.125 1/15
0 0 1 0 0.25 2/15
0 0 1 1 0.375 3/15
0 1 0 0 0.5 4/.15
0 1 0 1 0.625 5/15
0 1 1 0 0.75 6/15
0 1 1 1 0.875 7/15
1 0 0 0 1 8/15
1 0 0 1 1.125 9/15
1 0 1 0 1.25 10/15
1 0 1 1 1.375 11/15
1 1 0 0 1.5 12/15
1 1 0 1 1.625 13/15
1 1 1 0 1.75 14/15
1 1 1 1 1.875 15/15
Table 1 Output Current
By sending out a nibble to D3 - D0 in ascending levels ie. 0000 , 0001 , 0011 etc. the output current of the D/A converter is shown in fig 4.2. The output moves one step higher until reaching the maximum current. Then the cycle repeats. If all resistors are exact and all transistors matched all steps are identical in size.
Fig 4.2 Output current of D/A convertor
Glossary Of Terms
Resolution - One way to measure the quality of a D/A converter is by its resolution. The resolution is the ratio of the LSB increment to the maximum output. Resolution can be calculated by the formula.-
Resolution = 1 / 2n - 1 where n = number of bits
Percentage resolution = 1 / resolution * 100%
The greater the number of bits the better the resolution table 2 is a summary of the resolution for converters with 4 to 18 bits.
Bit Resolution Percent
4 1 part in 15 6.67
6 1 part in 63 1.54
8 1 part in 255 0.392
10 1 part in 1,023 0.0978
12 1 part in 4095 0.0244
14 1 part in 16,383 0.0061
16 1 part in 65,535 0.00153
18 1 part in 262,143 0.000381
Table 2 Resolution table
Accuracy - The conformance of a measured value with its true value; the maximum error of a device such as a data converter from the true value.
Absolute Accuracy - The worst case input to output error of a data converter referred to the NDS (National Bureau Of Standards) , standard volt.
Relative Accuracy - The worst case input to output error of a data converter as a percent of full scale referred to the converter reference. The error consists of offset gain and linearity components.
Conversion Rate - The number of repetitive A/D or D/A conversions per second for a full scale change to specified resolution and linearity.
Visual Display Devices
Visual displays are often employed in electronic equipment to indicate the numerical value of some quantity eg. digital watches, electronic calculators and digital voltmeters. A variety of display devices are available but the most common are the Light Emitting Diode (LED) and the Liquid Crystal Display (LCD).
Light Emitting Diode (LED)- The majority of Light Emitting Diodes are either gallium phosphide (GaP) or gallium-arsenide-phosphide (GaAsP) devices. An LED radiates energy in the visible part of the electromagnetic spectrum when the forward bias voltage applied across the diode exceeds the voltage that turns it ON. This voltage depends upon the type of LED and the light it emits. Table 3 displays information on different LED types and fig.5.1 the electronic symbol for a LED.
Colour Material Wavelength (peak radiation) nm Forward voltage at 10mA current (V)
Red GaAsp 650 1.6
Green GaP 565 2.1
Yellow GaAsP 590 2.0
Orange GaAsP 625 1.8
Blue SiC 480 3.0
Table 3 LED Types
Blue LEDs are a fairly recent development and these devices use silicon carbide (SiC)
Fig 5.1 LED Symbol
The current flowing in a LED must not be allowed to exceed a safe figure, generally 20-60 mA, and if necessary a resistor of suitable value must be connected in series with the diode to limit the current.
Often a LED is connected between one of the outputs of a TTL device and either earth or +5V depending upon when the LED is required to glow visibly. If for example, a LED is expected to glow when the output to which it is connected is low, the device should be connected as in fig 5.2 . Suppose the low voltage to be 0.4V and the sink current to be 16mA. Then if the LED voltage drop is 1.6V and the value of the series resistor will be
( 5 - 1.6 - 0.4 ) / ( 16 * 10 -3 ) = 188 W
When the output of the device is high (@ 4V), no current flows and the LED remains dark. When the LED is to glow to indicate the high output condition, the circuit shown in fig.5.3 must be used.
R1 = ( 5 - 1.6 ) / (16 * 10-3 ) = 213 W
When a LED is reverse biased it acts very much like a zenar diode with a low breakdown voltage (@ 4 V ).
Light Emitting Diodes are commonly used because they are cheap, reliable, easy to interface and are readily available from a number of sources. Their main disadvantage is that their luminous efficiency is low, typically 1.5 lumens/watt.
Fig 5.2
Fig 5.3
The characteristics of a LED Display is displayed in Appendix 3
Liquid Crystal Displays (LDR)-
A solid crystal is a material in which the molecules are arranged in a rigid lattice structure. If the temperture of the material is increased above it melting point, the liquid that is formed will tend to retain much of the orderly molecular structure. The material is then said to be in its liquid crystalline phase. There are two classes of liquid crystal known, respectively as nematic and smetic but only the former is used for display devices.
A nematic liquid crystal does not radiate light but instead it interferes with the passage of light whenever it is under the influence of an applied electric field. There are two ways in which the optical properties of a crystal can be influenced by an electric field. These are dynamic scattering and twisted nematic. The former was commonly employed in the past but now its application is mainly resisted to large-sized displays. The commonly met liquid crystal displays, eg. those in digital watches and hand calculators, ars all of the twisted nematic type.
Incident Light
Transmitted Light
Fig 6 (B)
Incident light
Fig 6 (A)
V Fig 6 (A) A liquid crystal cell
(B) and (C) operation of a liquid crystal cell No transmitted light
Fig 6 (C)
The construction of a Liquid Crystal cell is shown in fig. 6 (A) . A layer of a liquid crystal is placed in between two glass plates that have transparent metal film electrodes deposited on to their interior faces. A reflective surface, or mirror, is situated on the outer side of the lower glass plate (it may be deposited on its surface) . The conductive material is generally either tin oxide or a tin oxide or a tin oxide/indium oxide mixture and it will transmit light with about 90% efficiency. The incident light upon the upper glass plate is polarized in such a way that, if there is zero electric field between the plates, the light is able to pass right through and arrive at the reflective surface. Here it is reflected back and the reflected light travels through the cell and emerges from the upper plate (fig.6 (B). If a voltage is applied across the plates (fig.6 (C) the polarization of the light entering the cell is altered and it is no longer able to propagate as far as the reflective surface. Therfore no light returns from the upper surface of the cell and the display appears to be dark. Because the LDR does not emit light, it dissipates little power.
Liquid Crystal Displays, unlike LEDs, are not available as signal units and are generally manufactured in the form of a 7-segment display. The metal oxide film electrode on the surface of the upper glass plate is formed into the shape of the required 7 segments, each of which is taken to a separate contact, and the lower glass plate has a common electrode or backplate deposited on it. The idea is shown by fig 7 With this arrangement a voltage can be applied between the backplate and any one, or more of the seven segments to make that, or those particular segment(s) appear to be dark and thereby display the required number.
Nematic liquid crystal displays posses a number of advantages which have led to their widespread use in battery operated equipment. First, their power consumation is very small, about 1 m W per segment (much less than the LED); secondly their visibility is not affected by bright incident light (such as sunlight ); and third, they are compatible with the low-power NMOS/CMOS circuitry.
Fig 7 LCD 7-segment Display
The charactics of LCD display are displayed in appendix 4
Seven Segment Displays
Seven Segment displays are generally used as numerical indicators and consist of a number of LEDS arranged in seven segments as shown in Fig 8 (A). Any number between 0 and 9 can be indicated by lighting the appropriate segments ass shown in Fig 8 (B). A typical 7-segment display is manufactured in a 14-pin dil package with the cathode of each LED being brought out to each terminal with the common anode.
Fig.8 (A)
Fig 8 (B)
Clearly, the 7-seqment display needs a 7-bit input signal and so a decoder is required to convert the digital signal to be displayed into the corresponding 7-segment signal. Decoder/driver circuits can be made using SSI devices but more usually a ROM or a custom-built IC would be used. Fig.9 (A) shows one arrangement, in which the BCD output of a decade counter is converted to a 7-segment signal by a decoder.
When a count in excess of 9 is required, a second counter must be used and be connected in the manner shown by fig 10 (B).The tens counter is connected to the output of the final flip-flop of the units counter in the same way as the flip-flops inside the counters are connected.
Decade BCD to 7-segment
Counter 7-segment decoder display
Fig 10 (A)
Fig 10 (B)
Decade Decoder 7-segment
counter display
Dot Matrix Displays
A dot matrix display allows each alphanumeric character to be indicated by illuminating a number of dots in a 5 * 7 dot matrix. To allow for lower case letters and for spaces in between adjacent rows and columns each character fount is allocated a 6 * 12 space. Fig.11.1 shows 6 * 12 dot matrix. Every location in the dot matrix has a LED connected, as shown by Fig 11.2 for the top two rows of the matrix only. All the cathodes of the LEDs in one row, and all the anodes in one column are connected together. By addressing the appropriate locations in the diode and making the LEDs at those points to glow visibly any number or character in the set can be illuminated. Some examples are given in Fig.???
The circuitry required to drive a dot matrix display is too complex to be implemented using SSI devices. One 3-chip LSI dot matrix display controller, the Rockwell 10939, 10942 and 10943, is a general-purpose controller which is able to interface with other kinds of dot matrix as well as LED type.The controller can drive up to 46 dots and up to 20 characters selected out of the full 96 character ASCII code.
`
Fig 11.1
Fig 11.2
Bibliology
Microelectronic Systems a practical approach W Ditch
Basic Electrical And Electronic Engineering Ec.Bell and R.W. Bolton
Electronic and Electronic Principles for Technicians D.C Green
Data Conversion Components Datel
R S Data Library R S Components
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