LT6600 Linear Technology Corporation, LT6600 Datasheet - Page 12

• Manufacturer Part Number: LT6600
• Description: Very Low Noise, Differential Ampli?er And 2.5mhz Lowpass Filter
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT6600-2.5
Noise
The noise performance of the LT6600-2.5 can be evaluated
with the circuit of Figure 6.
Given the low noise output of the LT6600-2.5 and the 6dB
attenuation of the transformer coupling network, it will
be necessary to measure the noise fl oor of the spectrum
analyzer and subtract the instrument noise from the fi lter
noise measurement.
Example: With the IC removed and the 25Ω resistors-
grounded, Figure 6, measure the total integrated noise (e
of the spectrum analyzer from 10kHz to 2.5MHz. With the
IC inserted, the signal source (V
input resistors grounded, measure the total integrated noise
out of the fi lter (e
the frequency to 100kHz and adjust the amplitude until
V
V
compute the input referred integrated noise (e
Table 2 lists the typical input referred integrated noise for
various values of R
Table 2. Noise Performance
12
PASSBAND
GAIN (V/V)
IN
OUT
e
measures 100mV
IN
, and compute the passband gain A = V
4
2
1
=
V
IN
(e
1580Ω
O
402Ω
806Ω
R
R
R
)
2
IN
IN
IN
A
Figure 6. (S8 Pin Numbers)
O
– (e
). With the signal source connected, set
1
7
2
8
IN
LT6600-2.5
S
+
–
.
INTEGRATED NOISE
P-P
–2.5V
)
INPUT REFERRED
2.5V
6
10kHz TO 2.5MHz
3
2
+
–
. Measure the output amplitude,
0.1μF
0.1μF
18μV
29μV
51μV
4
5
RMS
RMS
RMS
IN
25Ω
25Ω
) disconnected, and the
COILCRAFT
TTWB-1010
1:1
INTEGRATED NOISE
INPUT REFERRED
10kHz TO 5MHz
OUT
23μV
39μV
73μV
SPECTRUM
ANALYZER
IN
/V
INPUT
RMS
RMS
RMS
) as:
660025 F06
IN
50Ω
. Now
S
)
Figure 7 is plot of the noise spectral density as a function
of frequency for an LT6600-2.5 with R
the fi xture of Figure 6 (the instrument noise has been
subtracted from the results).
The noise at each output is comprised of a differential
component and a common mode component. Using a
transformer or combiner to convert the differential outputs
to single-ended signal rejects the common mode noise and
gives a true measure of the S/N achievable in the system.
Conversely, if each output is measured individually and the
noise power added together, the resulting calculated noise
level will be higher than the true differential noise.
Power Dissipation
The LT6600-2.5 amplifi ers combine high speed with large-
signal currents in a small package. There is a need to
ensure that the die’s junction temperature does not exceed
150°C. The LT6600-2.5 S8 package has Pin 6 fused to the
lead frame to enhance thermal conduction when connect-
ing to a ground plane or a large metal trace. Metal trace
and plated through-holes can be used to spread the heat
generated by the device to the backside of the PC board.
For example, on a 3/32” FR-4 board with 2oz copper,
a totalof 660 square millimeters connected to Pin 6 of
theLT6600-2.5 S8 (330 square millimeters on each side
of the PC board) will result in a thermal resistance, θ
of about 85°C/W. Without the extra metal trace connected
50
40
30
20
10
Figure 7. Input Referred Noise, Gain = 1
0
0.01
SPECTRAL DENSITY
FREQUENCY (MHz)
0.1
INTEGRATED
1
660025 F07
IN
= 1580Ω using
10
100
80
60
40
20
0
660025fb
JA
,