Datasheet

LTC6405
19
6405fb
For more information www.linear.com/6405
applications inForMation
tance. The outputs of the LTC6405 are designed to drive
5pF to ground, so care should be taken to not lower the
effective impedance between +OUT and +OUTF or –OUT
and –OUTF below 15Ω.
To decrease filter bandwidth, add two external capacitors,
one from +OUTF to ground, and the other from –OUTF to
ground. A single differential capacitor connected between
+OUTF and –OUTF can also be used, but since it is being
driven differentially it will appear at each filtered output
as a single-ended capacitance of twice the value. To halve
the filter bandwidth, for example, two 3.9pF capacitors
could be added (one from each filtered output to ground).
Alternatively, one 1.8pF capacitor could be added between
the filtered outputs, which also halves the filter bandwidth.
Combinations of capacitors could be used as well; a three
capacitor solution of 1.2pF from each filtered output to
ground plus a 1.2pF capacitor between the filtered outputs
would also halve the filter bandwidth (Figure 9).
Noise Considerations
The LTC6405’s input referred voltage noise is 1.6nV/√Hz.
Its input referred current noise is 2.4pA/√Hz. In addition
to the noise generated by the amplifier, the surrounding
feedback resistors
also contribute noise. A noise model is
shown
in Figure 10. The output noise generated by both
the amplifier and the feedback components is governed
by the equation:
e
no
=
e
ni
• 1+
R
F
R
I












2
+ 2 • I
n
• R
F
( )
2
+
2 • e
nRI
•
R
F
R
I












2
+ 2 • e
nRF
2
A plot of this equation, and a plot of the noise generated
by the feedback components for the LTC6405 is shown
in Figure 11.
Figure 9. LTC6405 Filter Topology Modified for 1/2x Filter
Bandwidth (Three External Capacitors)
–
+
7 8
14 13
12
V
–
9
V
–
V
–
V
–
6405 F09
LTC6405
FILTERED OUTPUT
(425MHz)
1.25pF
1.25pF
50Ω
50Ω
1.25pF
1.2pF
1.2pF
1.2pF
+OUTF
–OUTF
+OUT +OUTF
–OUT –OUTF
Figure 10. Noise Model of the LTC6405
Figure 11. LTC6405 Output Spot Noise vs Spot Noise
Contributed by Feedback Network Alone
–
+
e
no
2
R
F
V
OCM
e
nRI
2
R
F
R
I
R
I
e
nRF
2
e
nRI
2
e
ncm
2
e
ni
2
e
nRF
2
i
n
+2
i
n
–2
6405 F10
R
I
= R
F
(Ω)
10
0.1
1
10
100
100 1000 10000
6405 F11
TOTAL (AMPLIFIER AND
FEEDBACK NETWORK)
OUTPUT NOISE
FEEDBACK NETWORK
NOISE ALONE
nV/ Hz