CLC408AJE NSC [National Semiconductor], CLC408AJE Datasheet - Page 9

• Manufacturer Part Number: CLC408AJE
• Description: Comlinear CLC408 High-Speed, Low-Power Line Driver
• Manufacturer: NSC [National Semiconductor]

We selected the component values as follows:
These values give excellent isolation from the other input:
The CLC408 provides large output current drive, while
consuming little supply current, at the nominal bias
point. It also produces low distortion with large signal
swings and heavy loads. These features make the
CLC408 an excellent choice for driving transmission
lines. The CLC426 was used as the receiver because
it has good high frequency CMRR.
Precision, Low 1/f Noise Composite Amplifier
The circuit in Figure 7 has the DC precision and low-
frequency performance of U1, and the high-frequency
performance of U2. This means that the 1/f noise
performance is dominated by U1, not U2. Vin needs to
be a low impedance source to minimize the impact of
U2’s non-inverting bias current (I
(i
source. The potentiometer R
frequencies to be manually matched to the gain at high
frequencies.
U1 needs to be an op amp with the following features:
voltage-feedback, low bandwidth (compared to U2),
low DC offsets and low 1/f noise. National
Semiconductor’s OP-07 meets all of these requirements.
U2 is a high-frequency op amp that meets your high-
frequency requirements. This application circuit will
assume a current-feedback op amp (the CLC408) for
U2. This circuit also works well when U2 is a high-
frequency, voltage-feedback op amp (such as the
CLC425 or CLC428).
Figure 7: Precision, Low-Noise Composite Amplifier
bn
). R
V
in
R
R
R
R
of the transmission line
R
value for the CLC426 at A
1
1
f1
m1
f2
t2
R
is an optional resistor that terminates the
7
= 3.0k , for unity gain of the CLC408
= R
= Z
(R ||R ) –
V
V
R
inB(A)
oA(B)
g2
2
f2
o
= 50 , the characteristic impedance
= 100
-
+
OP-07
g2
U1
38dB, f 5.0MHz
≥ R
R
2
m1
R
m1
R
3
7
4
, the recommended
25
allows the gain at low
BN
v
= 2
+
CLC408
-
) and current noise
R
R
5
6
U2
R
V
L
o
9
The transfer function is:
where A
Z
The approximations hold when the bandwidth of U1 is
much less than the bandwidth of U2. Now the gain of
the composite amplifier can be selected:
A
Make R
thermal noise, but large enough to not overload the
output of U2. Minimize the input offset voltage by
making R
The potentiometer should have a maximum value
about double the value calculated for R
potentiometer with multiple turn capability, and low
parasitics. Replace R
and step response flatness are not a concern.
Set R
the CLC408 at a gain of A
Select R
correct, and so that any change in output impedance of
U1 has a minimal impact:
The selection of R
U2 starts to dominate the performance of the composite
amplifier. This frequency is approximately:
U2
v
must be within the stable gain range of U1.
(j ) is the open-loop transimpedance gain of U2.
5
V
V
to the recommended feedback resistor value for
in
1
o
1
2
1
U1
3
, R
2
(j ) is the open-loop voltage gain of U1, and
R
R
R
R
and R
= (R
Z
6
5
3
5
3
1
U2
f
R
, A (j )
and R
UG
R
R
A
5
(j )
6
6
7
Z
R
R
V
1
U1
U2
|| R
4
3
R
5
4
R
R
R
so that the high-frequency gain is
R
R
A R
and R
A
5
(j )
,
3
R
R
1
3
4
7
7
R
5
3
v 2
v
A A (j )
5
3
):
7
small so that they produce little
6
U1
R
R
R
A
R
with a resistor when AC gain
1
6
7
1
R
R
5
v
4
6
v
1
R
, the value for gain flatness
A (j )
affects the frequency where
.
5
4
7
U1
1
R
1
7
R
R
A (j )
1
R
R
5
3
GBWP
U1
3
4
R
R
R
R
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5
3
5
4
U1
R
R
R
6
5
3
R
7
7
. Use a
R
7