Digital to Analog Converters
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Example Product: Signal Generator
2
— A Signal Generator should be programmable.
— A user can use the the LCD display and the keyboard
to change the:
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Frequency scale
Amplitude scale
Offset on/off etc.
Waveform shape (Square, Sinusoidal, Triangle..)
— Analog control given by potentiometer
— Analog signal output by using a Digital to Analog
Converter (DAC)
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3
Digital to
Analog
Conversion
Digital data
representing an analog
signal (for instance an
MP3 file) sampled at a
(usually) fixed time
interval is fed to a
Digital to Analog
Converter
The output is an analog
waveform which is an
approximation to the
original analog signal
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Input Digital
Data
Digital to
Analog
Conversion
(DAC)
Output
“Sampled”
Analog
Waveform
The 741 Op.Amp
4
— Summing Amplifier
Vout = - (V1+V2+V3)
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The 741 Op.Amp
5
— R-2R Network to convert digital to analog
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6
Operation II
The basic operational
idea of the DAC we will
be using is simply to
have a series of switches
which control a resistor
network
Depending on which
bits are set, the output
voltage changes
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7
The TLC7524
DAC
For your analog signal
generator you will be
using an 8-bit DAC to
convert an 8-bit data
stream , from the
ATmega128 ports, to
analog signals of given
frequency, amplitude
and offset
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8
Write Cycles of
the TLC7524
The DAC has internal
registers to store the
input Data (1 Byte) and
signals which control
the write operation
(CS*, WR*):
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9
DAC
Operation
The output signal will
need to be fed into an
external op-amp
Data should be put on
the data bus using one
of the ATMEL PORTs
CS* can be held low
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10
Schematic
8 bit input data
From ATMEL PORT
You can write numbers
to the DAC in a similar
way as with the 3-byte
memory module using
the ATmega128 Ports
4
5
6
7
8
9
10
11
Write Enable*
13
12
DB7
DB6
DB5
DB4
DB3
DB2
DB1
DB0
RF B
30pf
OUT1
OUT2
1
CS
REF
CS* : keep it enabled
741
2
+
15
100 ⌦
WR
TLC7524
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16
100 ⌦
VCC
Output Voltage
Task Plan
11
• Design and construct a Signal Generator:
• The signals should be produced using a DAC
and an Operational Amplifier driven by one of
the ATmega128 ports.
• Verify operation by copying the input voltage
read from the potentiometer to the DAC output
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