MIKROE-957 mikroElektronika, MIKROE-957 Datasheet - Page 68

• Manufacturer Part Number: MIKROE-957
• Description: Other Development Tools ASLK PRO ANALOG DEVELOPMENT SYSTEM
• Manufacturer: mikroElektronika

Specifications of MIKROE-957

• Rohs: yes
• Product: Analog System Lab Kit PRO
• Tool Is For Evaluation Of: TL082, MPY634
• Operating Supply Voltage: 2.5 V to 5.5 V
• Description/function: Analog Lab Kit for Undergraduate Engineering
• Maximum Operating Temperature: + 125 C
• Minimum Operating Temperature: - 40 C

13.1 Brief theory and motivation
More and more, we see the trend of using Digital Signal Processors and/or
Microcontrollers to control the behavior of the front-end signal conditioning circuits
in an instrumentation or RF system. Examples of such systems are Automatic Gain
Control system and Automatic Voltage Control systems. In this experiment, we will
demonstrate the use of a multiplying DAC to control the gain of a programmable gain
amplifier; we include an exercise at the end of this chapter to illustrate the use of a
microcontroller for controlling the gain of a programmable gain amplifier.
See Figure 13.1 for the circuit of an inverting amplifier; the gain of this amplifier
can be digitally controlled by changing the bit pattern presented to the input of the
multiplying DAC, DAC7821.
Let the 12-bit input pattern to DAC be given by
output voltage of the negative feedback amplifier is given by
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The goal of the experiment is to design a negative feedback amplifier whose
gain is digitally controlled using a multiplying DAC.
V
IN
Figure 13.1: Circuit for Digital Controlled Gain Stage Amplifier
Goal of the experiment
R
1
_
V
V
R R
A A
out
in
TL082
2
R
11
2
=
1
10
V
in
...
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R
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TL082
0
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/
11
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4096
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A A
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2
11
A
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1
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2
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I
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V
OUT1
OUT2
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1
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. The expression for the
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/
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DAC7821
4096
R
A
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2
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GND
V
V
V
DD
DD
REF
13.2 Specifications
13.3 Measurements to be taken
voltage amplitude. Select
measure the amplitude of the output voltage.
13.4 What should you submit?
To study the variation in gain when the bit pattern applied to the input of the DAC is
changed.
Apply a 100 Hz sine wave of 100mV peak amplitude at
1
2
3
The circuit of Figure 13.1 cannot be directly simulated, since the macro-model for
DAC7821 is not available at the time of writing. For the purpose of simulation,
we will use the macro model of a different 12-bit DAC, the MV95308. Simulate
the circuit schematic shown in Figure 13.2, which is equivalent to the circuit
of Figure 13.1. Observe the output waveforms for different bit patterns. The
typical simulation waveforms are of the form shown in Figure 13.3.
Use the circuit shown in Figure 13.1 for practical implementation of the Digital
programmable gain stage amplifier.
Apply the sine wave of fixed amplitude and vary the bit pattern, as shown
in Table 13.1. Note the Peak to Peak amplitude of the output. Compare the
simulation results with the practical results.
S.No.
1
2
3
4
Table 13.1: Variation in output amplitude with bit pattern
100000000000
010000000000
001000000000
000100000000
BIT Pattern
_
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V
R R
A A
out
in
2
11
=
1
10
V
to be 2.2. Vary the input bit pattern
in
...
$
A
R
R
0
2
1
i
$
/
11
0
4096
Peak to Peak Amplitude of the output
A
n
2
Analog System Lab Kit PRO
n
_
V
V
R R
A A
out
in
2
11
and measure the output
=
1
10
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in
...
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2
1
_
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V
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i
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11
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0
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=
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10
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n
in
...
2
$
n
A
R
R
0
2
1
i
$
and
/
11
0
4096
A
n
2
n