LTC6405 Linear Technology, LTC6405 Datasheet - Page 19

• Manufacturer Part Number: LTC6405
• Description: Rail-to-Rail Input Differential Amplifier/Driver
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
The outputs of the LTC6405 are designed to drive 5pF to
ground, so care should be taken to not lower the effec-
tive impedance between +OUT and +OUTF or –OUT and
–OUTF below 15Ω.
To decrease fi lter 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 fi ltered output
as a single-ended capacitance of twice the value. To halve
the fi lter bandwidth, for example, two 3.9pF capacitors
could be added (one from each fi ltered output to ground).
Alternatively, one 1.8pF capacitor could be added between
the fi ltered outputs, which also halves the fi lter bandwidth.
Combinations of capacitors could be used as well; a three
capacitor solution of 1.2pF from each fi ltered output to
ground plus a 1.2pF capacitor between the fi ltered outputs
would also halve the fi lter 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 amplifi er, the surrounding
feedback resistors also contribute noise. A noise model is
shown in Figure 10. The output noise generated by both
Figure 9. LTC6405 Filter Topology Modifi ed for 1/2x Filter
Bandwidth (Three External Capacitors)
6405 F09
LTC6405
+
–
14
7
–OUT
+OUT
50Ω
50Ω
13
8
1.25pF
1.25pF
–OUTF
+OUTF
1.25pF
V
V
–
–
V
V
–
–
12
9
1.2pF
–OUTF
+OUTF
1.2pF
1.2pF
FILTERED OUTPUT
(425MHz)
the amplifi er and the feedback components is governed
by the equation:
A plot of this equation, and a plot of the noise generated
by the feedback components for the LTC6405 is shown
in Figure 11.
e
no
Figure 11. LTC6405 Output Spot Noise vs Spot Noise
Contributed by Feedback Network Alone
=
100
⎛
⎜
⎝
2 • e
0.1
Figure 10. Noise Model of the LTC6405
10
e
1
10
ni
⎛
⎜
⎝
• 1+
nRI
e
e
e
⎛
⎝ ⎜
nRI
nRI
ncm
TOTAL (AMPLIFIER AND
FEEDBACK NETWORK)
OUTPUT NOISE
2
2
2
•
i
i
n
n
e
⎛
⎝ ⎜
R
R
+2
–2
ni
R
R
100
R
2
I
I
R
F
I
FEEDBACK NETWORK
NOISE ALONE
F
I
⎞
⎠ ⎟
R
⎞
⎠ ⎟
⎞
⎟
⎠
I
= R
⎞
⎟
⎠
2
R
+
–
R
V
2
F
+ 2 • I
F
OCM
F
(Ω)
+ 2 • e
e
e
1000
nRF
nRF
6405 F10
(
2
2
n
nRF
• R
LTC6405
2
F
e
no
)
6405 F11
10000
2
2
+
19
6405fa