User manual

page 68
Analog System Lab Kit PRO
experiment 13
The goal of the experiment is to design a negative feedback amplier whose
gain is digitally controlled using a multiplying DAC.
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
amplier;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amplier.
See Figure 13.1 for the circuitof an inverting amplier; the gain of this amplier
can be digitally controlled by changing the bit pattern presented to the input of the
multiplying DAC, DAC7821.
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
...AA A
VV
R
R
A
V
RR
2
4096
11 10 0
1
2
21
out in
n
n
in
0
11
$$
=
_i
/
and measure the output
voltage amplitude. Select
...AA A
VV
R
R
A
V
RR
2
4096
11 10 0
1
2
21
out in
n
n
in
0
11
$$
=
_i
/
to be 2.2. Vary the input bit pattern
...AA A
VV
R
R
A
V
RR
2
4096
11 10 0
1
2
21
out in
n
n
in
0
11
$$
=
_i
/
and
measure the amplitude of the output voltage.
The circuit of Figure 13.1 cannot be directly simulated, since the macro-model for
DAC7821 is not available at the time of writing. For the purpose of simulation,
wewillusethemacromodelofadierent12-bitDAC,theMV95308. Simulate
the circuit schematic shown in Figure 13.2, which is equivalent to the circuit
ofFigure13.1.Observetheoutputwaveformsfordierentbitpatterns.The
typical simulation waveforms are of the form shown in Figure 13.3.
Use the circuit shown in Figure 13.1 for practical implementation of the Digital
programmablegainstageamplier.
Applythesinewaveofxedamplitudeandvarythebitpattern,asshown
in Table 13.1. Note the Peak to Peak amplitude of the output. Compare the
simulation results with the practical results.
13.1 Brief theory and motivation
13.2Specications
13.3 Measurements to be taken
13.4 What should you submit?
Goal of the experiment
C1
R1
R2 DAC7821
R
FB VDD
GND
VREF
IOUT1
IOUT2
VDD
VOUT
VIN
TL082
TL082
Figure 13.1: Circuit for Digital Controlled Gain Stage Amplier
Table 13.1: Variation in output amplitude with bit pattern
Let the 12-bit input pattern to DAC be given by
...AA A
VV
R
R
A
V
RR
2
4096
11 10 0
1
2
21
out in
n
n
in
0
11
$$
=
_i
/
. The expression for the
outputvoltageofthenegativefeedbackamplierisgivenby
1
2
3
S.No. BIT Pattern Peak to Peak Amplitude of the output
1 100000000000
2 010000000000
3 001000000000
4 000100000000
...AA A
V V
R
R
A
V
R R
2
4096
11 10 0
1
2
2 1
out in
n
n
in
0
11
$ $
=
_ i
/