Datasheet

LT6600-2.5
11
660025fe
APPLICATIONS INFORMATION
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.
Figure 7 is plot of the noise spectral density as a function
of frequency for an LT6600-2.5 with R
IN
= 1580 using
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 the
LT6600-2.5 S8 (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
Figure 7. Input Referred Noise, Gain = 1
FREQUENCY (MHz)
0.01
0
30
40
50
0.1 1 10
660025 F07
20
10
0
60
80
100
40
20
NOISE SPECTRAL DENSITY (nV
RMS
/√Hz)
INTEGRATED NOISE (µV
RMS
)
SPECTRAL DENSITY
INTEGRATED
Figure 6. (S8 Pin Numbers)
–
+
0.1µF
0.1µF
2.5V
–2.5V
–
+
LT6600-2.5
3
4
1
7
2
8
5
6
R
IN
R
IN
25Ω
25Ω
660025 F06
SPECTRUM
ANALYZER
INPUT
50Ω
V
IN
COILCRAFT
TTWB-1010
1:1
Example: With the IC removed and the 25 resistors-
grounded, Figure 6, measure the total integrated noise (e
S
)
of the spectrum analyzer from 10kHz to 2.5MHz. 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 100kHz 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 2 lists the typical input referred integrated noise for
various values of R
IN
.
Table 2. Noise Performance
PASSBAND
GAIN (V/V) R
IN
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 2.5MHz
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 5MHz
4 402 18V
RMS
23V
RMS
2 806 29V
RMS
39V
RMS
1 1580 51V
RMS
73V
RMS