1. Module 5: Digital Techniques and
Electronic Instrument Systems
5.3 Data Conversion

2. Analog and digital signals
Analog signal:
can take all possible
values.
-2
-1
0
1
2
0 5 10
Digital Signal: can take
specific values (discrete
values).
0
1
2
3
4
0 5 10
0
2
4
6
0 2 4 6 8 10
0
2
4
6
0 5 10 15
2

3. Analog and digital circuits
3
 Analog circuits: Input and output signals are
analog.
 Digital circuits: Input and output signals are digital.
-2
-1
0
1
2
0 5 10
-1
0
1
2
0 5 10
0
2
4
6
0 5 10 15
0
2
4
6
0 5 10 15

4. Digital circuits advantages
4
 High reliability
 Easy design
 High flexibility
 Easy storage

5. Digital signals on the Aircraft
 Digital signals on the aircraft:
 Discrete values, usually binary
values: on / off, high / low.
 Examples:
 Is the aircraft on the ground or on the
earth?
 The Air Data Computer calculates the
airspeed, the altitude, the temperature
etc. from data sensor inputs. These
inputs are send to a digital bus
(ARINC) in a series of binary
numbers. These output serves as an
input for cockpit instruments, such as
the PFD.

6. Types of Computers
 Interactive Computers:
 The plane crew interfaces with the
computer.
 Example: Control Display Unit
(CDU).
 Reference Computers:
 Provides information used by
other computers.
 Example: Inertial Reference
System. (IRS – calculates
airspeed, heading, attitude etc.).
Information is provides to other
systems, such as Auto-throttle
computer, FMC etc.

7. Types of Computers
 Storage Computers:
 Store information such as flight
roots, total fuel, overspeed
threshold, etc.
 Controlling Computers:
 Control a specific device.
 e.g. Flap Electronic Unit (FEU).
Takes input from the Flap Lever
and gives input to the mechanism
that extends or detracts the
flaps, when the airspeed is within
the allowable values.
 Informational
Computes:
 Provide information
to the crew.
 e.g. ECAM
(Electronic
Centralized
Aircraft
Monitoring).

8. A/D & D/A Converters
 Temperature is an analogue value. It must be converted to
discrete value, to be displayed in a digital computer.
 The user input signal to drive a flight control surface is digital.
However, the signal that will drive the control surface must be
analog.

9. D/A Converters
 The circuit is called “Binary Weighted Ladder”.
3-bit
Digital
Input
Analog
Output

10. A/D: Sampling
 Resolution of A/D
conversion depends on:
 Sampling rate
 Number of bits used in
conversion and output
voltage range.
 An mp3 of 44000HZ
and 192kbps.
 Sampling rate: 44000
samples per sec.
 192Kbps: Affects the size
of the discrete areas.

11. D/A & A/D Converters Specifications
 Accuracy:
 The actual output vs. the converted output.
 E.g. if the maximum output of the converter is 10V and the
error is ±1%, then the maximum error for any voltage output is
10mV.
 Resolution of A/D converters:
 The number of steps the input is divided into. Is expressed in
power of 2. A 12-bit A/D converter divides the analog signal in
212 steps.
 Sample and Hold A/D converters:
 Freezes the analog input at the moment a new sample is taken.
 Throughput:
 The maximum number of bits “produced” or “consumed” per
second (the bit rate).