Datasheet

LT6600-20
7
66002fb
APPLICATIONS INFORMATION
Figure 1
Interfacing to the LT6600-20
The LT6600-20 requires two equal external resistors, R
IN
,
to set the differential gain to 402/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 aver-
age of V
IN
+
and V
IN
–
is the common mode input voltage.
Similarly, the voltages V
OUT
+
and V
OUT
–
appearing at
Pins 4 and 5 of the LT6600-20 are the fi lter outputs. The
difference 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 LT6600-20 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 20MHz. The common mode
output voltage is determined by the voltage at Pin 2. Since
Pin 2 is shorted to Pin 7, 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 Pin 7
(see the Distortion vs Input Common Mode Level graphs
in the Typical Performance Characteristics section).
Figure 2 shows how to AC couple signals into the
LT6600-20. In this instance, the input is a single-ended
signal. AC coupling allows the processing of single-ended
or differential signals with arbitrary common mode levels.
The 0.1µF coupling capacitor and the 402Ω gain setting
resistor form a high pass fi lter, attenuating signals below
4kHz. Larger values of coupling capacitors will proportion-
ally reduce this highpass 3dB frequency.
Figure 2
Figure 3
–
+
402
402
0.01µF
0.1µF
3.3V
–
+
V
IN
–
V
IN
+
3
4
1
7
2
8
5
6
66002 F01
V
OUT
+
V
OUT
–
V
t
3
2
1
V
IN
+
V
IN
–
V
t
3
2
1
V
OUT
+
LT6600-20
V
OUT
–
0 0
–
+
402Ω
402Ω
0.01µF
0.1µF
0.1µF
0.1µF
3.3V
–
+V
IN
+
3
4
1
7
2
8
5
6
66002 F02
V
OUT
+
V
OUT
–
V
3
2
2
1
V
t
1
0
0
–1
V
IN
+
LT6600-20
V
OUT
+
V
OUT
–
–
+
100
100
0.1µF
0.01µF
5V
–
+
V
IN
–
V
IN
+
3
4
1
7
2
8
5
6
66002 F03
V
OUT
+
V
OUT
–
62pF
62pF
+
–
2V
V
t
3
2
1
0
V
IN
+
V
IN
–
V
t
3
2
1
0
V
OUT
+
V
OUT
–
LT6600-20
500mV
P-P
(DIFF)