LT6604-15
13
660415fb
Figure 8 is plot of the noise spectral density as a function
of frequency for an LT6604-15 with R
IN
= 536 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 LT6604-15 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 LT6604-15 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 voltages. For
a given supply voltage, the worst-case power dissipation
occurs when the differential input signal is maximum, the
common mode currents are maximum (see Applications
Information 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 worstcase conditions, use
34°C/W as the package thermal resistance, then apply the
equation for TJ. 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 50mA The resulting junction temperature is:
T
J
= T
A
+ (P
D
• θ
JA
) = 85 + (5 • 2 • 0.05 • 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
–
+
LT6604-15
25
27
4
34
6
2
29
7
R
IN
R
IN
25
25
660415 F07
SPECTRUM
ANALYZER
INPUT
50
V
IN
COILCRAFT
TTWB-1010
1:1
1/2
FREQUENCY (MHz)
10
NOISE DENSITY (nV
RMS
/√Hz)
INTEGRATED NOISE (µV)
20
25
35
45
40
0.01 1 10 100
660415 F08
0
0.1
30
15
5
40
80
100
140
180
160
0
120
60
20
NOISE DENSITY,
GAIN = 1x
NOISE DENSITY,
GAIN = 4x
INTEGRATED NOISE,
GAIN = 1x
INTEGRATED NOISE,
GAIN = 4x
Figure 7 Figure 8. Input Referred Noise, Gain = 1
