Datasheet

LT6604-15
9
660415fb
APPLICATIONS INFORMATION
Interfacing to the LT6604-15
Note: The LT6604-15 contains two identical fi lters. The
following applications information only refers to one fi lter.
The two fi lters are independent except that they share the
same negative supply voltage V
–
. The two fi lters can be
used simultaneously by replicating the example circuits.
The referenced pin numbers correspond to the A channel
fi lter
The LT6604-15 channel requires two equal external re-
sistors, R
IN
, to set the differential gain to 536/R
IN
. The
inputs to the fi lter are the voltages V
IN
+
and V
IN
–
presented
to these external components, Figure 1. The difference
between V
IN
+
and V
IN
–
is the differential input voltage. The
average of V
IN
+
and V
IN
–
i s t h e c o m m o n m o d e i n p u t v o l t a g e .
Similarly, the voltages V
OUT
+
and V
OUT
–
appearing at Pins
27 and 29 of the LT6604-15 are the fi lter outputs. The dif-
ference between V
OUT
+
and V
OUT
–
is the differential output
voltage. The average of V
OUT
+
and V
OUT
–
is the common
mode output voltage. Figure 1 illustrates the LT6604-15
operating with a single 3.3V supply and unity passband
gain; the input signal is DC coupled. The common mode
input voltage is 0.5V, and the differential input voltage is
2V
P-P
. The common mode output voltage is 1.65V, and the
differential output voltage is 2V
P-P
for frequencies below
15MHz. The common mode output voltage is determined
by the voltage at V
OCM
. Since V
OCM
is shorted to V
MID
,
the output common mode is the mid-supply voltage. In
addition, the common mode input voltage can be equal
to the mid-supply voltage of V
MID
.
Figure 2 shows how to AC couple signals into the LT6604-15.
In this instance, the input is a single-ended signal. AC cou-
pling allows the processing of single-ended or differential
signals with arbitrary common mode levels. The 0.1µF
coupling capacitor and the 536 gain setting resistor
form a high pass fi lter, attenuating signals below 3kHz.
Larger values of coupling capacitors will proportionally
reduce this highpass 3dB frequency.
–
+
536
536
0.01µF
0.1µF
3.3V
–
+
V
IN
–
V
IN
+
25
27
4
34
6
2
29
7
660415 F01
V
OUT
+
V
OUT
–
V
t
3
2
1
V
IN
+
V
IN
–
V
t
3
2
1
V
OUT
+
LT6604-15
V
OUT
–
0 0
1/2
–
+
536
536
0.01µF
0.1µF
0.1µF
0.1µF
3.3V
–
+V
IN
+
25
1/2
27
4
34
6
2
29
7
660415 F02
V
OUT
+
V
OUT
–
V
3
2
2
1
V
t
1
0
0
–1
V
IN
+
LT6604-15
V
OUT
+
V
OUT
–
Figure 1
Figure 2