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http://www.national.com
SPICE Models
SPICE models provide a means to evaluate op amp
designs. Free SPICE models are available that:
I
Support Berkeley SPICE 2G and its many
derivatives
I
Reproduce typical DC, AC, Transient, and
Noise performance
I
Support room temperature simulations
The
readme
file that accompanies the models lists the
released models, and provides a list of modeled
parameters. The application note
Simulation
SPICE Models for Comlinear’s Op Amps
schematics and detailed information.
contains
Differential Line Driver With Load
Impedance Conversion
The circuit shown in the
Typical Application
schematic
on the front page operates as a differential line driver.
The transformer converts the load impedance to
a value that best matches the CLC418’s output
capabilities.
The
single-ended
converted to a differential signal by the CLC418. The
line’s characteristic impedance is matched at both the
input and the output. The schematic shows Unshielded
Twisted Pair for the transmission line; other types of lines
can also be driven.
input
signal
is
Set up the CLC418 as a difference amplifier:
Make the best use of the CLC418’s output drive
capability as follows:
where R
eq
is the transformed value of the load
impedance, V
max
is the Output Voltage Range, and I
max
is the maximum Output Current.
Match the line’s characteristic impedance:
Select the transformer so that it loads the line with a
value very near Z
o
over your frequency range. The out-
put impedance of the CLC418 also affects the match.
With an ideal transformer we obtain:
where Z
o(418)
(j
ω
) is the output impedance of the CLC418,
and |Z
o(418)
(j
ω
)| << R
m
.
The load voltage and current will fall in the ranges:
The CLC418’s high output drive current and low
distortion make it a good choice for this application.
Lowpass Anti-aliasing Filter
with Delay Equalization
The circuit shown in Figure 7 is a 5th-order Butterworth
lowpass filter with group delay equalization. V
in
needs to
be a voltage source with low output impedance. Section
A
is
a
simple
single-pole
provides a single-pole allpass function for group delay
equalization. Sections C and D are Sallen-Key lowpass
biquad sections.
filter.
Section
B
Figure 7: Lowpass Anti-aliasing Filter
The filter specifications we built to are:
f
c
= 10MHz
f
s
= 20MHz
A
p
= 3.01dB (maximum passband attenuation)
A
s
= 30dB
(minimum stopband attenuation)
H
o
= 0dB
(DC gain)
(passband corner frequency)
(stopband corner frequency)
The designed component values are in the table below.
The pre-distorted values compensate for the finite band-
width of the CLC418.
CLC418 Applications
V
V
=
R
R
2
R
R
d
in
f1
g1
f2
g2
+
=
R
R
2 V
I
m
eq
max
max
+
=
R
R
Z
R
n
R
R
L
o
m
eq
L
eq
=
=
=
Return Loss
-20 log
n
Z
(j )
Z
, dB
10
2
o(418)
o
≈
V
n V
I
I
n
o
max
o
max
≤
≤
418 Fig7
+
-
C1/2
R
fA
V
oA
V
in
R
1A
C
2A
+
-
C1/2
R
fB
V
oB
R
1B
C
2B
R
3B
+
-
C1/2
R
fC
V
oC
R
3C
R
1C
C
4C
+
-
C1/2
R
fD
V
o
R
3D
C
4D
C
5C
U1
A
U1
B
U2
C
U2
D
C
5D
R
gD
R
1D
1-
α
D
R
1D
α
D