Datasheet

LT6604-10
13
660410fb
Figure 8 is plot of the noise spectral density as a function
of frequency for an LT6604-10 channel with R
IN
= 402
using the fi xture of Figure 7 (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 LT6604-10 amplifi ers combine high speed with large
signal currents in a small package. There is a need to en-
sure that the die’s junction temperature does not exceed
150°C. The LT6604-10 has an exposed pad (pin 35) which
is connected to the lower supply (V
–
). Connecting the pad
to a ground plane helps to dissipate the heat generated
by the chip. Metal trace and plated through-holes can be
used to spread the heat generated by the device to the
backside of the PC board.
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 volt-
ages.
For a given supply voltage, the worst-case power dis-
sipation occurs when the differential input signal is
maximum, the common mode currents are maximum (see
the Applications Information section regarding Common
Mode DC Currents), the load impedance is small and
the ambient temperature is maximum. To compute the
junction temperature, measure the supply current under
these worst-case conditions, use 34°C/W as the package
thermal resistance, then apply the equation for T
J
. For
example, using the circuit in Figure 3 with DC differential
input voltage of 250mV, a differential output voltage of 1V,
no load resistance and an ambient temperature of 85°C,
the supply current (current into V
+
) measures 48.9mA per
channel. The resulting junction temperature is:
T
J
= T
A
+ (P
D
• θ
JA
) = 85 + (5 • 2 • 0.0489 • 34) = 102°C.
The thermal resistance can be affected by the amount of
copper on the PCB that is connected to V
–
. The thermal
resistance of the circuit can increase if the exposed pad
is not connected to a large ground plane with a number
of vias.
APPLICATIONS INFORMATION
–
+
0.1µF
0.1µF
2.5V
–2.5V
–
+
25
27
4
34
6
2
29
7
R
IN
R
IN
25
25
660410 F07
SPECTRUM
ANALYZER
INPUT
50
V
IN
COILCRAFT
TTWB-1010
1:1
1/2
LT6604-10
FREQUENCY (MHz)
0.1
SPECTRAL DENSITY (nV
RMS
/√Hz)
INTEGRATED NOISE (µV
RMS
)
35
30
25
20
15
10
5
0
140
120
100
80
60
40
20
0
1.0 10 100
660410 F08
SPECTRAL DENSITY
INTEGRATED
NOISE
Figure 7
Figure 8