data conversion

1. DATA CONVERSION
Digital signal
• A digital signal is a signal that is being used to represent data
as a sequence of discrete values; at any given time it can only
take on one of a finite number of values.[1][2][3] This contrasts
with an analog signal, which represents continuousvalues; at
any given time it represents a real number within a continuous
range of values.

2. ADC Process
• The analog to digital conversion process changes changes a
sampled analog voltage into digital form. This process,
conceptually involves the following two steps:
1. Quantizing: It is defined as the transformation of a
continuous analog input a set of discrete output states.
2. Coding: It is assignment of digital code word or number to
each output state.

3. Components used in A/D conversion
1. Buffer amplifier: provides a signal in a range close to but
not exceeding the full input range of the A/D converter.
2. Low- pass filter: is necessary to remove any undesirable
high frequency components in the signal could produce
aliasing. The cut-off frequency of the low pass filter
should not be greater than half the sampling rate.
3. Sample and hold amplifier : maintains a fixed input value
from an instantaneous sample during the short conversion
time of the A/D converter.
4. A/D converter: should have a resolution and analog
quantization size appropriate to the system and signal.
5. Computer: must be properly interfaced to A/D converter
to store and process the data.

4. Sensor
Buffer amplifier
Low-pass Filter
A/D converter
Computer/Memory
S/H

5. A/D converter
• A/D converter is an electronic devices that converts an analog
voltage to a digital code. The output of the A/D converter can
be interfaced to digital devices such as microcontroller and
computers
• The resolution of an A/D converter is the number of bits used
to digitally approximate the analog value of the input. The
number of possible states N is equal to the number of bit
combinations that can be output
output from the converter: N = 2"
N = is the number of bits.
The number of “analog decision points" that occur in the
process of quantising is (N-1). The
“analog quantisation size” Q is defined as the full scale range
of A/D converter by the number of output states.

6. A/D converters are designed based on a number of different
principles; these are:
Successive approximations.
Flash or parallel encoding.
Single-slope and dual-slope integration. Switched capacitor.
Delta sigma.

7. • The "multiplexer" is essentially an electronic switching
device which enables each inputs to be sampled in turn.
A "multiplexer" is a circuit that is able to have inputs of
data from a number of sources and then, by selecting an
input channel, give an output from just one of them
In applications where there is a need for measurement to
be made at a number of different locations, rather than use
a separate ADC and microprocessor of each
measurement, a "multiplexer" can be used to select each
input in turn and switch it through a single ADC and
microprocessor
• Demultiplexer" is similar to multiplexer but with reversed
action. It accepts a digital signal through its one input
and then channelizes it to a particular output selected by
binary value at the control port.

8. • Digital-to-Analog (D/A) Conversion
Invariably we have to reverse the process of analog-to-
digital (A/D) conversion by changing a digital value to an
analog value. This is called digital-to-analog (D/A)
• A D/A converter (DAC) allows a computer or other
digital device to interface with external analog circuits
and devices.
The input to a digital-to-analog converter (DAC) is a
binary word; the output is an analog signal that represents
the weighted sum of the non-zero bits represented
by the word.
Example: An output of 0010 must give an analog output
which is twice that given by an input of 0001.

9. D/A Converter designs
• Weighed resistor digital to analog converter
• R-2R ladder digital to analog converter

10. Data /Signal transmission
• The terms "Data transmission" and "Telemetry" refer to the
process by which the measurand is transferred to a remote
location for the purpose of being processed, recorded and
displayed.
A measuring device converts a primary indication into some
form of energy that can be easily displayed on a scale; some
transmitters also do the same things. In the Sense
"transmitters" could be considered as devices which transmit
the value of the primary variable at a considerable distance
from the primary element. If transmission is to be carried out
over very long distances, then devices are known as
"telemeters".

11. • For transmission purposes, the measured variable is converted into a
transmittable signal (either pneumatic or electrical), so that it can be
received by a remote indicating, recording, or controlling device. The
selection of transmission device depends upon nature of the variable
and the distance the signal is required to be sent.
For data transmission various methods have been developed; the
choice of a particular method depends upon
(1) The physical variable; (ii) The distance involved.
The hydraulic and pneumatic methods are employed for
transmission over a short distance
The pneumatic type transmission devices are generally suitable for
transmission upto maximum distance of 200 m.
• The electrical/electronic methods are suitable equally for short as
well as long distance
Generally short transmission is carried out on own communication
connections between sending and receiving devices.
The telemeters which are designed for long-distance transmission may
be designed to transmission over their own wire or over phone wires or
by microwave

12. • Mechanical Transmission
The rack and the pinion arrangement and the
"gear trains" as used in Bourdon-tube
pressure gauge and dial indicator gauge
constitute mechanical transmission. They
amplify the displacement and also transmit
the signal to a pointer which moves across a
calibrated dial.

13. • Hydraulic Transmission
The hydraulic method of transmission, which is commonly used
is shown in figure.
Here four bellows are employed, two at the transmission end and
two at the receiving end. The four bellows are connected by an
impulse pipeline and the whole system is filled with liquid. When
the actuating link, on the transmission end, is operated by the
measurand , then one below is expanded and other is contracted.
This expansion and contraction is communicated to receiving
end, which moves the receiving pointer an equal amount.
The purpose of using two bellows on either side is to compensate
for changes in ambient temperature.

15. • Pneumatic Transmission
•
Figure shows the one of the pneumatic methods of transmission
(Flapper nozzle mechanism).
It consists of an open nozzle which is supplied with air through a
restriction/orifice (its diameter being smaller than nozzle diameter for
proper functioning). In front of the nozzle there is a flapper which is
positioned by the measuring element. The force on the
flapper is produced by a transducer which converts the measurand into
linear displacement. The flapper is pivoted about a point and the other
end, it contains some balancing counter weight. When the flapper is
moved against the nozzle, the air cannot escape and maximum air
passes to the amplifier, and when flapper is moved away from the
nozzle minimum air passes to the amplifier as most of the air escapes to
atmosphere. Thus the movement of flapper from one extreme position
to another serves to control the
amplifier. Which produces an air pressure proportional to the
measurand of adequate strength for transmission over the required
distance.

17. • Magnetic Transmission
Figure shows the schematics of magnetic transmission. In this
arrangement an armature is attached at the end of the mechanical
moving part whose movement is to be transmitted outside the
armature moving inside a non-magnetic tube. A magnet is
placed around the armature outside the tube. The magnet follows
the movement of the armature and repositions a pneumatic
transmitter. The magnet movement could also be utilised to
operate an electronic transmitter.

19. • Electric Type of Transmitters
Most of the electric type of transmitters employ A.C. bridge
circuits in which degree of coupling between inductances is
varied by changing the amount of iron core within
coil. The common examples are:
1. Wheatstone bridge transmitter. 2. Inductance bridge.
3. Impedance bridge. 4. Differential transformer.
5. Self synchronous motor 6. Resistance manometers.
Converters
The converters are series of transducers which play an
important role in the modern instrumentation, linking electrical
(voltage and current based) and pneumatic together.
Following are the most commonly used converters
1. Current-to-pneumatic converters.
2. Pneumatic-to-current converters
3. Voltage-to-current converters.