5.3 data conversion Flashcards
digital computers are being used in new areas such as
thrust management and engine controls
the use of digital systems means the flight crews workload is
reduced
important considerations in using digital computers are
reducing system weight and size increasing data handling speed
improved system reliability
this contributes to an increase in overall aircraft efficiency
data is transmitted in analogue form e.g. hands on a clock
digital form e.g. numbers on a computer screen
.
aircraft systems typically convert data from analogue to digital and vice versa
.
Analogue and digital signals CAN BOTH contain the same information
.
digital data signals have two set signals in the
0 V represents OFF
5 V represents ON
Analogue singals vary continously with time and have many different values
.
Alternating current (AC) analogue singals can be bipolar
in that they have positive and negative limits
Direct Current (DC) analogue signals are unipolar
they fluctuate between 0 V AND either a positive or negative value but they never alternate between positive and negative voltages
by locating the transmitter of a synchro system remotely
an indicator can show fluid quantity or pressure, without the need to run tuning through the cockpit
Analogue signal can have an infinite number of possible values
as it varies from its minimum to its maximum extremes
digital data signals differ from analogue singals. why
they only have two (Binary) levels of voltage:
high for “ON” and low for “OFF”
a digital signal is a sequence of ones and zeros
.
fibre optic cable transmits data with light pulses on or off 1 or 0
.
digital signal must be what in some way before its understandable
decoded
a binary digit is also called a what
a bit
a bit or binary digit is what
smallest unit of information in a computer
analogue signals vary in magnitude uniformly e.g.
a dimmer switch
digital signals by contrast are simply ON/OFF e.g.
a simple light switch without dimming
digital computers process information faster and more efficiently than analogue computers but what is their disadvantage
(Digital) only process data in 1’s and 0’s
temperature varies and has to be converted to a digital representation of temp for digital computers to process temp information
.
the circuits used to interface digital computers to analogue computers are called
digital to analogue (D/A) converters DAC
Analogue to digital (A/D) converters ADC
D/A converters change
the digital data words of a digital computer to an equivalent analogue signal as either a voltage or current source
many different types of D/A converters but each one has same basic decription
A/D converters change
the analogue signals received from sensors to digital data understood by the digital computer
- many types of A/D converters exist
- type depends
aircraft with digital instruments use
pressure sensitive solid state sensors
output digital signals for collection and processing by aircraft computers
an ADC converts a continuous
input voltage into a digital data stream
an ADC can be used to convert analogue output from a microphone to digital signals for what
storage on a compact disc (CD)
three types of ADC are
Direct comparison converter
ramp generation A/D converter
successive approximation converter
fastest type of ADC is what and is often called what
Direct comparison converter
(DCC) AKA Flash ADC
DCC are fast and operate virtually without delay (LATENCY) before a transfer of data begins following an instruction
.
what is used when the highest possible sample rates are needed
DCC’s
- they convert analogue to digital
- comparing with known reference values. more reference values known the greater the accuracy
the greater resolution, the bigger and more power hungry the DCC becomes what happens to the sample rate
must be reduced
optimal resolution for DCC is
8-bit
applications for a DCC include
fibre optics
RADAR detection
wideband radio
fastest digital oscilloscopes
microwave measurements
ramp generation ADC is AKA
Counter-type ADC
uses a binary counter to feed a digital to analogue converter (DAC)
uses a counter for conversion from analogue to digital
advantages of ramp generation (Counter-Type) ADC
simple and easy to use
very accurate and the precision can be increased by increasing the number of bits
fair balance of accurate output and cheap hardware costs
disadvantages of ramp generation (counter-type) ADC
every time a new conversion starts, the counter is reset and has to start counting from all zeros
hence conversion time is significant
it cannot sustain high-frequency input
successive approximation ADC is an advanced version of the digital ramp type ADC
designed to reduce the conversion and increase speed of operation
successive aprroximation ADC the normal counter is replaced with a
(SAR) successive approximation register.
-the SAR counts by changing the bits from MSB to LSB according to input
Digital to analogue conversion is the reverse of the A/D process
with each binary number being converted back into nominal levels by reconstructing the original waveform
there are four main DAC circuits
Binary weighted resistor D/A converter circuit
Binary ladder or R-2R ladder D/A converter circuit
segmented DAC
Delta-sigma DAC
binary weighted ladder DAC is one of the simplest DAC each bit..
of a binary input controls a solid-state switch.
the switch connects either a reference voltage or grounds to a corresponding binary weighted resistor
advantages of R-2R ladder DAC
uses only two types of resistors
easily scalable to any number of bits
output impedance is always same despite the number of bits
disadvantages of weighted resistor DAC
Increasing the number of input bits requires a large value resistors(increases exponentially)
the value of large resistors are not accurate
reconstructing a signal from digital to analogue or from analogue to digital can degrade the signal by
background noise and loss of signal
