clc5523 National Semiconductor Corporation, clc5523 Datasheet - Page 10

• Manufacturer Part Number: clc5523
• Description: Low-power, Variable Gain Amplifier
• Manufacturer: National Semiconductor Corporation

Digital Gain Control
Digitally variable gain control can be easily realized
by driving the CLC5523’s gain control input with a
digital-to-analog converter (DAC). Figure 10 illustrates
such an application. This circuit employs National
Semiconductor’s eight-bit DAC0830, the LM351 JFET
input op-amp, and the CLC5523 VGA. With V
the circuit provides up to 80dB of gain control in
512 steps with up to 0.05% full scale resolution. The
maximum gain of this circuit is 20dB.
Automatic Gain Control (AGC) #1
Fast Response AGC Loop
The AGC circuit shown in Figure 11 will correct a 6dB
input amplitude step in 100ns. The circuit includes a two
op-amp precision rectifier amplitude detector (U1 and
U2), and an integrator (U3) to provide high loop gain at
low frequencies. The output amplitude is set by R9.
Some notes on building fast AGC loops:
Precision rectifiers work best with large output signals.
Accuracy is improved by blocking DC offsets, as shown in
Figure 11.
http://www.national.com
V
ref
Digital
Input
DAC0830
Figure 10: Digital Gain Control
R
fb
R10
500W
probe capacitance
I
I
o1
o2
Includes scope
CLC426
680pF
U3
C2
100W
-
+
LM351
R
C3
40pF
+
-
V
G
in
Figure 11: Automatic Gain Control Circuit #1
-5V
25W
2
3
R9
4.22k
CLC5523
500W
1
4
CLC5523 Applications
R8
100W
V
ref
R
7
in
g
6
R
1k
set to 2V,
f
2
3
CLC404
500W
CLC5523
R7
-
+
V
U2
1
4
o
10
+
-
25W
Signal frequencies must not reach the gain control port of
the CLC5523, or the output signal will be distorted
(modulated by itself).
additional filtering beyond the integrator stage to
block signal frequencies. This is provided in Figure 11
by a simple R-C filter (R10 and C3); better distortion
performance can be achieved with a more complex filter.
These filters should be scaled with the input signal
frequency.
integration time constants) may not need the R10 –
C3 filter.
Checking the loop stability can be done by monitoring the
V
amplitude. Changing the input signal amplitude can be
easily done with either an arbitrary waveform generator
or a fast multiplexer such as the CLC532.
Automatic Gain Control (AGC) #2
Figure 12 on the following page, illustrates an automatic
gain control circuit that employs two CLC5523’s. In this
circuit, U1 receives the input signal and produces an
output signal of constant amplitude. U2 is configured
to provide negative feedback. U2 generates a rectified
gain control signal that works against an adjustable
bias level which may be set by the potentiometer and
R
resultant gain control signal is applied to the U1 gain
control input V
output to be set at an arbitrary level less than the
maximum
Rectification is accomplished in U2 by driving both
the amplifier input and the gain control input with the
U1 output signal. The voltage divider that is formed
by R1, R2 and the V
rectifier gain.
7
R5
g
b
6
R
. C
voltage while applying a step change in input signal
Schottky
500W
f
1N5712
R6
i
integrates the bias and negative feedback. The
output
Loops with slower response time (longer
500W
g
. The bias adjustment allows the U1
R4
CLC404
500W
U1
R3
g
specification
input (pin 1) resistance, sets the
+
-
A fast settling AGC needs
R1
20W
R2
25W
C1
1.0mF
of
Output
20MHz,
0.1V
the
pp
amplifier.