clc5523 National Semiconductor Corporation, clc5523 Datasheet - Page 6

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

The CLC425 is a low noise wideband voltage feedback
amplifier. Setting R2 at 909W and R1 at 100W produces
a gain of 20dB. Setting R
and R
The total gain of this circuit is therefore approximately
46dB. It is important to understand that when partitioning
to obtain high levels of gain, very small signal levels will
drive the amplifiers to full scale output. For example, with
46dB of gain, a 20mV signal at the input will drive the out-
put of the CLC425 to 200mV, the output of the CLC5523
to 4V. Accordingly, the designer must carefully consider
the contributions of each stage to the overall characteris-
tics. Through gain partitioning the designer is provided
with an opportunity to optimize the frequency response,
noise, distortion, settling time, and loading effects
of
performance.
CLC5523 Gain Control Range and Minimum Gain
Before discussing Gain Control Range, it is important to
understand the issues which limit it. The minimum gain of
the CLC5523, theoretically, is zero, but in practical circuits
is limited by the amount of feedthrough, here defined as
the difference in output levels when V
V
package as well as coupling through the supplies will
determine the amount of feedthrough. Even at DC, the
input signal will not be completely rejected. At high fre-
quencies feedthrough will get worse because of its capac-
itive nature. At low frequencies, the feedthrough will be
80dB below the maximum gain, and therefore it can be said
that the CLC5523 has an 80dB Gain Control Range.
CLC5523 Gain Control Function
In the two plots, Gain vs. V
function of the control voltage. The first plot, sometimes
referred to as the S-curve, is the linear (V/V ) gain. This
is a hyperbolic tangent relationship. The second gain
curve plots the gain in dB and is linear over a wide range
of gains. Because of this, the CLC5523 gain control is
referred to as “linear-in-dB.”
For applications where the CLC5523 will be used at the
heart of a closed loop AGC circuit, the S-curve control
characteristic provides a broad linear (in dB) control
range with soft limiting at the highest gains where large
changes in control voltage result in small changes in
gain. For applications, requiring a fully linear (in dB)
control characteristic, use the CLC5523 at half gain and
below (V
Avoiding Overdrive of the CLC5523
Gain Control Input
There is an additional requirement for the CLC5523 Gain
Control Input (V
control circuitry may saturate and the gain may actually
be reduced. In applications where V
from a DAC, this can easily be addressed in the software.
If there is a linear loop driving V
http://www.national.com
G
= 0V. Capacitive coupling through the board and
each
g
at 50W, produces a gain of 26dB in the CLC5523.
G
²
amplifier
1V).
G
): V
G
must not exceed +2.5V. The gain
to
f
achieve
G
at 1000W as recommended
, we can see the gain as a
G
, such as an AGC loop,
improved
G
G
= 2V and when
is being driven
overall
6
other methods of limiting the input voltage should be
implemented.
resistive divider on the V
divider is operating off of ±5V supplies as well, its output
will not exceed 5V and through the divider V
exceed 2.5V.
Improving the CLC5523 Large Signal Performance
Figure 2 illustrates an inverting gain scheme for the
CLC5523.
The input signal is applied through the R
V
maximum gain range of this configuration is given in the
following equation:
The inverting slew rate of the CLC5523 is much higher
than that of the non-inverting slew rate.
performance improvement comes about because in the
non-inverting configuration, the slew rate of the overall
amplifier is limited by the input buffer. In the inverting
circuit, the input buffer remains at a fixed voltage and
does not affect slew rate.
Transmission Line Matching
One method for matching the characteristic impedance of
a transmission line is to place the appropriate resistor at
the input or output of the amplifier. Figure 3 shows a typ-
ical circuit configuration for matching transmission lines.
The resistors R
characteristic impedance, Z
cable. Use C
a greater frequency range.
increase of the op amp’s output impedance with frequency.
Signal
Input
in
pin should be grounded through a 25W resistor. The
+
-
Figure 3: Transmission Line Matching
R
25W
s
Figure 2: Inverting the CLC5523
o
Z
o
to match the output transmission line over
V
One simple solution is to place a 2:1
s
in
, R
R
i
A
R
R
i
vmax
g
, R
g
25W
2
3
G
25W
o
CLC5523
V
2
3
input. If the device driving this
, and R
= -
G
1
4
o
CLC5523
V
, of the transmission line or
G
1
4
æ
ç
è
It compensates for the
R
R
7
7
g
f
6
6
R
R
ö
÷
ø
T
f
f
are equal to the
C
R
g
o
o
V
resistor. The
o
This 2.5X
G
Z
o
can not
Output
R
T