LT6604-15 Linear Technology, LT6604-15 Datasheet - Page 12

• Manufacturer Part Number: LT6604-15
• Description: Dual Very Low Noise Differential Amplifier and 15MHz Lowpass Filter
• Manufacturer: Linear Technology

LT6604-15
APPLICATIONS INFORMATION
Common Mode DC Currents
In applications like Figure 1 and Figure 3 where the LT6604-15
not only provides lowpass fi ltering but also level shifts the
common mode voltage of the input signal, DC currents
will be generated through the DC path between input and
output terminals. Minimize these currents to decrease
power dissipation and distortion. Consider the application
in Figure 3. V
the 1st differential amplifi er inside the LT6604-15 channel
(see the Block Diagram section) at 2.5V. Since the input
common mode voltage is near 0V, there will be approxi-
mately a total of 2.5V drop across the series combination
of the internal 536Ω feedback resistor and the external
133Ω input resistor. The resulting 3.7mA common mode
DC current in each input path, must be absorbed by the
sources V
output voltage of the 2nd differential amplifi er inside the
LT6604-15 channel, and therefore sets the common mode
output voltage of the fi lter. Since, in the example of Figure
3, V
(1.25mA per side) of DC current will fl ow in the resistors
coupling the 1st differential amplifi er output stage to the
fi lter output. Thus, a total of 9.9mA per channel is used
to translate the common mode voltages.
A simple modifi cation to Figure 3 will reduce the DC com-
mon mode currents by 40%. If V
the common mode output voltage of both op amp stages
will be 2V and the resulting DC current will be 6mA per
channel. Of course, by AC coupling the inputs of Figure 3,
the common mode DC current can be reduced to 2.5mA
per channel.
12
OCM
differs from V
IN
+
MID
and V
sets the output common mode voltage of
IN
–
MID
. V
OCM
by 0.5V, an additional 2.5mA
sets the common mode
MID
is shorted to V
OCM
Noise
The noise performance of the LT6604-15 channel can
be evaluated with the circuit of Figure 6. Given the low
noise output of the LT6604-15 and the 6dB attenuation
of the transformer coupling network, it will be necessary
to measure the noise fl oor of the spectrum analyzer and
subtract the instrument noise from the fi lter noise mea-
surement.
Example: With the IC removed and the 25Ω resistors
grounded, Figure 6, 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 fi lter (e
set the frequency to 1 MHz and adjust the amplitude until
V
V
compute the input referred integrated noise (e
Table 1 lists the typical input referred integrated noise for
various values of R
Table 1. Noise Performance
PASSBAND
IN
OUT
S
) of the spectrum analyzer from 10kHz to 15MHz. With
e
GAIN
measures 100mV
IN
, and compute the passband gain A = V
4
2
1
=
(e
O
536Ω
)
133Ω
267Ω
2
R
A
IN
– (e
IN
S
.
O
P-P
)
). With the signal source connected,
2
INTEGRATED NOISE
INPUT REFERRED
. Measure the output amplitude,
10kHz TO 15MHz
109μV
36μV
62μV
RMS
RMS
RMS
www.DataSheet4U.com
IN
) disconnected, and
INTEGRATED NOISE
INPUT REFERRED
10kHz TO 30MHz
OUT
169μV
51μV
92μV
IN
/V
) as:
RMS
RMS
IN
RMS
. Now
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