LT6604 Linear Technology Corporation, LT6604 Datasheet - Page 9

• Manufacturer Part Number: LT6604
• Description: Dual Very Low Noise, Differential Ampli?er And 5mhz Lowpass Filter
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Interfacing to the LT6604-5
Note: The LT6604-5 contains two identical fi lters. The fol-
lowing applications information only refers to one fi lter.
The two fi lters are independent except that they share the
same negative supply voltage V
used simultaneously by replicating the example circuits.
The referenced pin numbers correspond to the A channel
fi lter.
Each LT6604-5 channel requires two equal external re-
sistors, R
inputs to the fi lter are the voltages V
to these external components, Figure 1. The difference
between V
The average of V
voltage. Similarly, the voltages V
at Pins 27 and 29 of the LT6604-5 are the fi lter outputs.
The difference between V
output voltage. The average of V
mon mode output voltage. Figure 1 illustrates the LT6604-5
operating with a single 3.3V supply and unity passband
IN
IN
, to set the differential gain to 806Ω/R
+
and V
IN
+
and V
IN
–
–1
2
1
0
3
2
1
0
is the differential input voltage.
OUT
3
2
1
0
V
V
IN
V
500mV
–
+
is the common mode input
and V
P-P
OUT
OUT
–
(DIFF)
V
. The two fi lters can be
V
IN
V
+
IN
V
IN
IN
+
V
+
OUT
+
IN
IN
–
and V
+
and V
–
+
and V
–
t
is the differential
t
t
OUT
OUT
V
V
IN
V
V
V
IN
IN
IN
IN
IN
–
–
+
–
–
–
+
+
is the com-
presented
appearing
0.1μF
0.1μF
806Ω
806Ω
62pF
200Ω
62pF
200Ω
IN
0.01μF
806Ω
806Ω
. The
0.01μF
0.01μF
Figure 1
Figure 2
+
–
Figure 3
34
4
6
2
34
4
6
2
2V
+
–
25
1/2
LT6604-5
+
–
7
34
1/2
LT6604-5
3.3V
25
4
6
2
7
5V
+
gain; the input signal is DC-coupled. The common mode
input voltage is 0.5V, and the differential input voltage is
2V
differential output voltage is 2V
5MHz. The common mode output voltage is determined
by the voltage at V
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
Figure 2 shows how to AC couple signals into the LT6604-5.
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 806Ω gain setting resistor form
a high pass fi lter, attenuating signals below 2kHz. Larger
values of coupling capacitors will proportionally reduce
this highpass 3dB frequency.
In Figure 3 the LT6604-5 is providing 12dB of gain. The
gain resistor has an optional 62pF in parallel to improve
the passband fl atness near 5MHz. The common mode
output voltage is set to 2V.
–
–
+
0.1μF
1/2
LT6604-5
25
+
–
0.1μF
7
3.3V
27
29
27
29
P-P
+
–
0.1μF
27
29
. The common mode output voltage is 1.65V, and the
V
V
OUT
OUT
V
V
OUT
OUT
+
–
V
V
+
–
OUT
OUT
+
–
3
2
1
0
3
2
1
0
V
3
2
1
0
V
V
OCM
. Since V
V
V
V
V
OUT
OUT
OUT
OUT
V
V
+
–
OUT
OUT
+
–
66045 F01
66045 F03
+
–
66045 F02
MID
t
t
P-P
OCM
t
.
for frequencies below
is shorted to V
LT6604-5
MID
66045f
9
,