LT6600-15
10
660015fb
APPLICATIONS INFORMATION
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 fl oor of the spectrum
analyzer and subtract the instrument noise from the fi lter
noise measurement.
Example: With the IC removed and the 25 resistors
grounded, Figure 7, measure the total integrated noise (e
S
)
of the spectrum analyzer from 10kHz to 15MHz. With the
IC inserted, the signal source (V
IN
) disconnected, and the
input resistors grounded, measure the total integrated noise
out of the fi lter (e
O
). With the signal source connected,
set the frequency to 1MHz and adjust the amplitude until
V
IN
measures 100mV
P-P
. Measure the output amplitude,
V
OUT
, and compute the passband gain A = V
OUT
/V
IN
. Now
compute the input referred integrated noise (e
IN
) as:
e
IN
=
(e
O
)
2
–(e
S
)
2
A
Table 1 lists the typical input referred integrated noise for
various values of R
IN
.
Figure 8 is plot of the noise spectral density as a func-
tion of frequency for an LT6600-15 using the fi xture of
Figure 7 (the instrument noise has been subtracted from
the results).
Table 1. Noise Performance
PASSBAND
GAIN (V/V) R
IN
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 15MHz
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 30MHz
4 133 36V
RMS
51V
RMS
2 267 62V
RMS
92V
RMS
1 536 109V
RMS
169V
RMS
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 LT6600-15 amplifi ers 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 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
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
JA
, of about
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.
Figure 7
Figure 8. Input Referred Noise, Gain = 1
–
+
0.1µF
0.1µF
2.5V
–2.5V
–
+
LT6600-15
3
4
1
7
2
8
5
6
R
IN
R
IN
25Ω
25Ω
660015 F07
SPECTRUM
ANALYZER
INPUT
50Ω
V
IN
COILCRAFT
TTWB-1010
1:1
FREQUENCY (MHz)
10
NOISE DENSITY (nV
RMS
/√Hz)
INTEGRATED NOISE (µV)
20
25
35
45
40
0.01 1 10 100
660015 F08
0
0.1
30
15
5
40
80
100
140
180
160
0
120
60
20
NOISE DENSITY,
GAIN = 1x
NOISE DENSITY,
GAIN = 4x
INTEGRATED NOISE,
GAIN = 1x
INTEGRATED NOISE,
GAIN = 4x
