LT6604-2.5 Linear Technology, LT6604-2.5 Datasheet - Page 12

• Manufacturer Part Number: LT6604-2.5
• Description: Dual Very Low Noise Differential Amplifier and 2.5MHz Lowpass Filter
• Manufacturer: Linear Technology

APPLICATIONS INFORMATION
LT6604-2.5
input resistors grounded, measure the total integrated noise
out of the fi lter (e
the frequency to 100kHz and adjust the amplitude until
V
V
compute the input referred integrated noise (e
Table 2 lists the typical input referred integrated noise for
various values of R
Table 2. Noise Performance
PASSBAND
GAIN
4
2
1
Figure 7 is plot of the noise spectral density as a function
of frequency for an LT6604-2.5 channel with R
using the fi xture of Figure 6 (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.
12
IN
OUT
e
measures 100mV
IN
, and compute the passband gain A = V
=
( ) – ( )
Figure 7. Input Referred Noise, Gain = 1
50
40
30
20
10
e
0
R
402Ω
806Ω
1580Ω
0.01
O
IN
2
A
O
). With the signal source connected, set
IN
e
S
.
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 2.5MHz
18μV
29μV
51μV
P-P
SPECTRAL DENSITY
FREQUENCY (MHz)
0.1
2
. Measure the output amplitude,
RMS
RMS
RMS
INTEGRATED
1
660425 F07
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 5MHz
23μV
39μV
73μV
10
RMS
RMS
RMS
OUT
100
80
60
40
20
0
IN
IN
/V
= 1580Ω
) as:
IN
. Now
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 LT6604-2.5 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 LT6604-2.5 has an exposed pad (pin 35) which
is connected to the negative supply (V
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
temperature, T
dissipation is the product of supply voltage, V
supply current, I
given by:
where the supply current, I
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 worst-case conditions, use
43°C/W as the package thermal resistance, then apply the
equation for T
with DC differential input voltage of 1V, a differential
output voltage of 4V, no load resistance and an ambient
temperature of 85°C, the supply current (current into V
measures 37.6mA per channel. The resulting junction
temperature is: T
• 43) = 101°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.
T
J
= T
A
+ (P
J
D
A
. For example, using the circuit in Figure 3
, and power dissipation, P
S
• θ
J
. Therefore, the junction temperature is
= T
JA
) = T
A
+ (P
J
, is calculated from the ambient
A
S
D
+ (V
, is a function of signal level,
• θ
S
JA
www.DataSheet4U.com
• I
) = 85 + (5 • 2 • 0.0376
S
• θ
–
). Connecting the
JA
)
D
. The power
S
, and total
660425f
+
–
)
.