lt6600cs8-2.5-pbf Linear Technology Corporation, lt6600cs8-2.5-pbf Datasheet - Page 10

• Manufacturer Part Number: lt6600cs8-2.5-pbf
• Description: Very Low Noise, Differential Amplifier And 15mhz Lowpass Filter
• Manufacturer: Linear Technology Corporation

LT6600-15
Noise
The noise performance of the LT6600-15 can be evaluated
with the circuit of Figure 7.
Given the low noise output of the LT6600-15 and the 6dB
attenuation of the transformer coupling network, it is
necessary to measure the noise floor of the spectrum
analyzer and subtract the instrument noise from the filter
noise measurement.
Example: With the IC removed and the 25Ω resistors
grounded, Figure 7, measure the total integrated noise
(e
the IC inserted, the signal source (V
the input resistors grounded, measure the total integrated
noise out of the filter (e
nected, set the frequency to 1MHz and adjust the ampli-
tude until V
amplitude, V
A = V
noise (e
Table 1 lists the typical input referred integrated noise for
various values of R
Figure 8 is plot of the noise spectral density as a function
of frequency for an LT6600-15 using the fixture of Fig-
ure 7 (the instrument noise has been subtracted from the
results).
Table 1. Noise Performance
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
10
APPLICATIO S I FOR ATIO
PASSBAND
GAIN (V/V)
S
e
) of the spectrum analyzer from 10kHz to 15MHz. With
IN
OUT
4
2
1
=
IN
/V
) as:
IN
( ) – ( )
IN
e
. Now compute the input referred integrated
O
OUT
133Ω
267Ω
536Ω
measures 100mV
R
2
A
IN
, and compute the passband gain
IN
U
e
S
.
INTEGRATED NOISE
INPUT REFERRED
10kHz TO 15MHz
2
O
109µV
). With the signal source con-
U
36µV
62µV
RMS
RMS
RMS
P-P
IN
. Measure the output
W
) disconnected, and
INTEGRATED NOISE
INPUT REFERRED
10kHz to 30MHz
169µV
51µV
92µV
U
RMS
RMS
RMS
noise power added together, the resulting calculated noise
level will be higher than the true differential noise.
Power Dissipation
The LT6600-15 amplifiers combine high speed with large-
signal currents in a small package. There is a need to
ensure that the die junction temperature does not exceed
150°C. The LT6600-15 package has Pin 6 fused to the lead
frame to enhance thermal conduction when connecting 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 total
of 660 square millimeters connected to Pin 6 of the
LT6600-15 (330 square millimeters on each side of the PC
board) will result in a thermal resistance, θ
85°C/W. Without the extra metal trace connected to the V
pin to provide a heat sink, the thermal resistance will be
around 105°C/W. Table 2 can be used as a guide when
considering thermal resistance.
V
IN
45
40
35
30
25
20
15
10
Figure 8. Input Referred Noise, Gain = 1
5
0
0.01
R
R
IN
IN
NOISE DENSITY,
GAIN = 1x
NOISE DENSITY,
GAIN = 4x
INTEGRATED NOISE,
GAIN = 1x
INTEGRATED NOISE,
GAIN = 4x
1
7
2
8
0.1
LT6600-15
–
+
– 2.5V
2.5V
6
3
FREQUENCY (MHz)
+
–
0.1µF
0.1µF
Figure 7
4
5
1
25Ω
25Ω
10
TTWB-1010
COILCRAFT
660015 F08
1:1
100
180
160
140
120
100
80
60
40
20
0
JA
SPECTRUM
ANALYZER
, of about
INPUT
660015 F07
50Ω
660015f
–