LTC6403-1 Linear Technology, LTC6403-1 Datasheet - Page 15

• Manufacturer Part Number: LTC6403-1
• Description: Low Power Fully Differential Input/Output Amplifier/Driver
• Manufacturer: Linear Technology

www.DataSheet4U.com
APPLICATIONS INFORMATION
These pins each have an output impedance of 100Ω. In-
ternal capacitances are 12pF to V
plus an additional 12pF capacitor connected differentially
between the two fi ltered outputs. This resistor/capacitor
combination creates fi ltered outputs that look like a se-
ries 100Ω resistor with a 36pF capacitor shunting each
fi ltered output to AC ground, providing a –3dB bandwidth
of 44.2MHz, and a noise bandwidth of 69.4MHz. The
fi lter cutoff frequency is easily modifi ed with just a few
external components. To increase the cutoff frequency,
simply add 2 equal value resistors, one between +OUT
and +OUTF and the other between –OUT and –OUTF
(Figure 7). These resistors, in parallel with the internal
100Ω resistors, lower the overall resistance and therefore
increase fi lter bandwidth. For example, to double the fi lter
bandwidth, add two external 100Ω resistors to lower the
series fi lter resistance to 50Ω. The 36pF of capacitance
remains unchanged, so fi lter bandwidth doubles. Keep in
mind, the series resistance also serves to decouple the
outputs from load capacitance. The unfi ltered outputs of
the LTC6403-1 are designed to drive 10pF to ground or
5pF differentially, so care should be taken to not lower the
effective impedance between +OUT and +OUTF or –OUT
and –OUTF below 25Ω.
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
+
–
Figure 6. LTC6403-1 Internal Filter Topology
14
7
–OUT
+OUT
100Ω
100Ω
12pF
13
8
12pF
12pF
–OUTF
+OUTF
LTC6403-1
V
V
–
–
64031 F06
V
V
–
–
12
–
9
on each fi ltered output,
FILTERED OUTPUT
(44.2MHz)
+OUTF and –OUTF can also be used, and 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 36pF capacitors
could be added (one from each fi ltered output to ground).
Alternatively, one 18pF capacitor could be added between
the fi ltered outputs, again halving the fi lter bandwidth.
Combinations of capacitors could be used as well; a three
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).
Figure 8. LTC6403-1 Filter Topology Modifi ed for 1/2x
Filter Bandwidth (3 External Capacitors)
Figure 7. LTC6403-1 Filter Topology Modifi ed for 2x
Filter Bandwidth (2 External Resistors)
+
–
+
–
14
7
14
7
–OUT
+OUT
100Ω
100Ω
100Ω
100Ω
–OUT
+OUT
100Ω
100Ω
12pF
12pF
13
8
13
8
12pF
12pF
–OUTF
+OUTF
12pF
12pF
LTC6403-1
–OUTF
+OUTF
LTC6403-1
V
V
–
–
64031 F08
V
V
–
–
64031 F07
V
V
–
–
V
V
12
9
–
–
12
9
12pF
LTC6403-1
12pF
12pF
FILTERED OUTPUT
(88.4MHz)
FILTERED OUTPUT
(22.1MHz)
15
64031f