Datasheet

LTC6404
24
6404f
Figure 8. LTC6404 Filter Topology Modifi ed for 1/2x Filter
Bandwidth (3 External Capacitors)
capacitor solution of 12pF from each fi ltered output to
ground plus a 12pF capacitor between the fi ltered outputs
would also halve the fi lter bandwidth (Figure 8).
Noise Considerations
The LTC6404’s input referred voltage noise is on the
order of 1.5nV/√Hz. Its input referred current noise is on
the order of 3pA/√Hz. In addition to the noise generated
by the amplifi er, the surrounding feedback resistors also
contribute noise. A noise model is shown in Figure 9.
The output noise generated by both the amplifi er and the
feedback components is governed by the equation:
e
e
R
R
IR
e
no
ni
F
I
nF
n
=
+
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
+
()
+•••
•
12
2
2
2
RRI
F
I
nRF
R
R
e••
⎛
⎝
⎜
⎞
⎠
⎟
⎛
⎝
⎜
⎞
⎠
⎟
+
2
2
2
A plot of this equation, and a plot of the noise generated
by the feedback components for the LTC6404 is shown
in Figure 10.
APPLICATIONS INFORMATION
Figure 9. Noise Model of the LTC6404
–
+
7
OUT
+
8
OUTF
+
14
OUT
–
13
OUTF
–
12
V
–
9
V
–
V
–
V
–
6404 F08
LTC6404
FILTERED OUTPUT
(44.25MHz)
50
12pF
12pF
12pF
12pF
12pF
50
12pF
–
+
1
SHDN
5 6
IN
–
7
OUT
+
8
OUTF
+
16 15
IN
+
NC
NC
14
OUT
–
13
OUTF
–
e
no
2
R
F2
2
V
+
3
V
–
V
+
V
+
V
–
V
+
V
–
V
OCM
V
OCM
12
V
–
11
V
+
10
V
+
9
V
–
V
–
V
–
6404 F09
LTC6404
e
nof
2
e
nRI2
2
SHDN
R
F1
R
I2
R
I1
V
–
V
+
V
–
4
e
nRF2
2
e
nRI1
2
e
ncm
2
e
ni
2
e
nRF1
2
i
n
+2
i
n
–2