Datasheet

LT6604-10
12
660410fb
The LT6604-10 was designed to process a variety of
input signals including signals centered on the mid-sup-
ply voltage and signals that swing between ground and
a positive voltage in a single supply system (Figure 1).
The range of allowable input common mode voltage (the
average of V
IN
+
and V
IN
–
in Figure 1) is determined by
the power supply level and gain setting (see the Electrical
Characteristics section).
Common Mode DC Currents
In applications like Figures 1 and 3 where the LT6604-10
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 voltage of
the 1st differential amplifi er inside the LT6604-10 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 402 feedback resistor and the external
100 input resistor. The resulting 5mA 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 LT6604-10 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 12.5mA 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 36%. 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 8mA 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 LT6604-10 channel can be
evaluated with the circuit of Figure 6. Given the low noise
output of the LT6604-10 and the 6dB attenuation of the
transformer coupling network, it will be necessary to mea-
sure the noise fl oor of the spectrum analyzer and subtract
the instrument noise from the fi lter noise measurement.
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 10MHz. 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
). With the signal source connected,
set the frequency to 1MHz 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 10MHz
INPUT REFERRED
NOISE dBm/Hz
4 100 24µV
RMS
–149
2 200 34µV
RMS
–146
1 402 56µV
RMS
–142
APPLICATIONS INFORMATION