LT6600-20 Linear Technology, LT6600-20 Datasheet - Page 10

• Manufacturer Part Number: LT6600-20
• Description: Very Low Noise Differential Amplifier and 20MHz Lowpass Filter
• Manufacturer: Linear Technology

LT6600-20
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 floor of the spectrum
analyzer and subtract the instrument noise from the filter
noise measurement.
Example: With the IC removed and the 25
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 LT6600-20 with R
fixture of Figure 7 (the instrument noise has been sub-
tracted 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 20MHz. With
IN
OUT
4
2
1
IN
/V
) as:
IN
( ) – ( )
IN
e
. Now compute the input referred integrated
O
OUT
100
200
402
measures 100mV
R
2
A
IN
, and compute the passband gain
IN
U
e
S
.
INTEGRATED NOISE
INPUT REFERRED
10kHz TO 20MHz
2
O
118 V
). With the signal source con-
U
42 V
67 V
RMS
RMS
RMS
P-P
IN
. Measure the output
W
) disconnected, and
IN
= 402 using the
INPUT REFERRED
NOISE dBm/Hz
–148
–143
–139
U
resistors
noise power added together, the resulting calculated noise
level will be higher than the true differential noise.
Power Dissipation
The LT6600-20 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 LT6600-20 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,
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
50
40
30
20
10
Figure 8. Input Referred Noise, Gain = 1
0
0.1
R
R
V
IN
IN
S
= 5V
1
7
2
8
SPECTRAL DENSITY
LT6600-20
–
+
– 2.5V
2.5V
6
3
FREQUENCY (MHz)
1
+
–
0.1 F
0.1 F
Figure 7
4
5
25
10
25
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INTEGRATED
TTWB-1010
COILCRAFT
66002 F08
1:1
100
250
200
150
100
50
0
JA
SPECTRUM
ANALYZER
, of about
INPUT
66002 F07
50
66002f
–