LT6600-20
10
66002fb
APPLICATIONS INFORMATION
Noise
The noise performance of the LT6600-20 can be evaluated
with the circuit of Figure 7.
Given the low noise output of the LT6600-20 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 25Ω resistors
grounded, Figure 7, measure the total integrated noise (e
S
)
of the spectrum analyzer from 10kHz to 20MHz. 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 integratednoise (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 function of frequency for an LT6600-20 with
R
IN
= 402Ω 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 20MHz
INPUT REFERRED
NOISE dBm/Hz
4 100 42V
RMS
–148
2 200 67V
RMS
–143
1 402 118V
RMS
–139
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 LT6600-20 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 LT6600-20 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 LT6600-20
(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
FREQUENCY (MHz)
0.1
0
30
40
50
1 10 100
66002 F08
20
10
0
150
200
250
100
50
NOISE SPECTRAL DENSITY (nV
RMS
/√Hz)
INTEGRATED NOISE (µV
RMS
)
SPECTRAL DENSITY
INTEGRATED
V
S
= 5V
–
+
0.1µF
0.1µF
2.5V
–2.5V
–
+
LT6600-20
3
4
1
7
2
8
5
6
R
IN
R
IN
25
25
66002 F07
SPECTRUM
ANALYZER
INPUT
50
V
IN
COILCRAFT
TTWB-1010
1:1
