LT6600-2.5
APPLICATIO S I FOR ATIO
NOISE SPECTRAL DENSITY (nV
RMS
/√Hz)
A = V
OUT
/V
IN
. Now compute the input referred integrated
noise (e
IN
) as:
e
IN
=
(e
O
– (e
S
A
)
2
)
2
Table 1 lists the typical input referred integrated noise for
various values of R
IN
.
Figure 7 is plot of the noise spectral density as a function
of frequency for an LT6600-2.5 with R
IN
= 1580Ω using
the fixture of Figure 6 (the instrument noise has been
subtracted from the results).
Table 1. Noise Performance
PASSBAND
GAIN (V/V)
4
2
1
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 2.5MHz
18µV
RMS
29µV
RMS
51µV
RMS
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 5MHz
23µV
RMS
39µV
RMS
73µV
RMS
R
IN
402Ω
806Ω
1580Ω
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 amplifiers combine high speed with large-
signal currents in a small package. There is a need to
ensure that the dies’s junction temperature does not
exceed 150°C. The LT6600-2.5 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 LT6600-2.5 (330 square millimeters on each side
of the PC board) will result in a thermal resistance,
θ
JA
, of
U
50
100
40
SPECTRAL DENSITY
30
60
80
W
U
U
INTEGRATED NOISE (µV
RMS
)
20
40
10
INTEGRATED
0
0.01
20
0
0.1
1
10
66002 F07
FREQUENCY (MHz)
Figure 7. Input Referred Noise, Gain = 1
Table 2. LT6600-2.5 SO-8 Package Thermal Resistance
COPPER AREA
TOPSIDE
(mm
2
)
1100
330
35
35
0
BACKSIDE
(mm
2
)
1100
330
35
0
0
BOARD AREA
(mm
2
)
2500
2500
2500
2500
2500
THERMAL RESISTANCE
(JUNCTION-TO-AMBIENT)
65°C/W
85°C/W
95°C/W
100°C/W
105°C/W
about 85°C/W. Without 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.
Junction temperature, T
J
, is calculated from the ambient
temperature, T
A
, and power dissipation, P
D
. The power
dissipation is the product of supply voltage, V
S
, and
supply current, I
S
. Therefore, the junction temperature is
given by:
T
J
= T
A
+ (P
D
•
θ
JA
) = T
A
+ (V
S
• I
S
•
θ
JA
)
where the supply current, I
S
, is a function of signal level,
load impedance, temperature and common mode
voltages.
For a given supply voltage, the worst-case power dissi-
pation occurs when the differential input signal is maxi-
mum, the common mode currents are maximum (see
Applications Information regarding Common Mode DC
660025i
9
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