Datasheet

LT6604-15
12
660415fb
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
MID
sets the output common mode volt age of
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 536 feedback resistor and the external
133 input resistor. The resulting 3.7mA common mode
DC current in each input path, must be absorbed by the
sources V
IN
+
and V
IN
–
. V
OCM
sets the common mode
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
OCM
differs from V
MID
by 0.5V, an additional 2.5mA
(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
MID
is shorted to V
OCM
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.
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 25 resistors
grounded, Figure 6, measure the total integrated noise
(e
S
) of the spectrum analyzer from 10kHz to 15MHz. With
the IC inserted, the signal source (V
IN
) disconnected, and
the input resistors grounded, measure the total integrated
noise out of the fi lter (e
O
) . W i t h t h e s i g n a l s o u r c e c o n n e c t e d ,
set the frequency to 1 MHz and adjust the amplitude until
V
IN
measures 100mV
P-P
. Measure the output amplitude,
V
OUT
, and compute the passband gain A = V
OUT
/V
IN
. Now
compute the input referred integrated noise (e
IN
) as:
e
IN
=
(e
O
)
2
–(e
S
)
2
A
Table 1 lists the typical input referred integrated noise for
various values of R
IN
.
Table 1. Noise Performance
PASSBAND
GAIN
R
IN
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 15MHz
INPUT REFERRED
INTEGRATED NOISE
10kHz TO 30MHz
4133 36µV
RMS
51µV
RMS
2267 62µV
RMS
92µV
RMS
1536109µV
RMS
169µV
RMS
APPLICATIONS INFORMATION