ltc6404-2 Linear Technology Corporation, ltc6404-2 Datasheet - Page 15

• Manufacturer Part Number: ltc6404-2
• Description: 600mhz, Low Noise, Ac Precision Fully Differential Input/output Ampli?er/driver
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
which incorporate an internal lowpass fi lter network with
a –3dB bandwidth of 88.5MHz (Figure 6).
These pins each have a DC output impedance of 50Ω. In-
ternal capacitances are 12pF to V
plus an additional 12pF capacitor connected differentially
between the two fi ltered outputs. This resistor/capaci-
tor combination creates fi ltered outputs that look like a
series 50Ω resistor with a 36pF capacitor shunting each
fi ltered output to AC ground, providing a –3dB bandwidth
of 88.5MHz, and a noise bandwidth of 139MHz. 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
OUTF
These resistors, in parallel with the internal 50Ω resistor,
lower the overall resistance and therefore increase fi lter
bandwidth. For example, to double the fi lter bandwidth,
add two external 50Ω resistors to lower the series fi lter
resistance to 25Ω. The 36pF of capacitance remains un-
changed, so fi lter bandwidth doubles. Keep in mind, the
series resistance also serves to decouple the outputs from
load capacitance including the onboard fi lter capacitors.
The unfi ltered outputs of the LTC6404-2 are designed to
drive 10pF to ground or 5pF differentially, so care should
be taken to not lower the effective impedance between
OUT
To decrease fi lter bandwidth, add two external capacitors,
one from OUTF
ground. A single differential capacitor connected between
+
+
and OUTF
and the other between OUT
+
–
Figure 6. LTC6404-2 Internal Filter Topology
14
7
+
+
to ground, and the other from OUTF
OUT
OUT
or OUT
50Ω
50Ω
12pF
–
+
13
8
12pF
12pF
OUTF
OUTF
–
LTC6404-2
and OUTF
–
+
V
V
–
–
64042 F06
V
V
–
–
12
–
9
–
on each fi ltered output,
and OUTF
–
below 25Ω.
FILTERED OUTPUT
(88.5MHz)
–
(Figure 7).
+
and
–
to
OUTF
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. LTC6404-2 Filter Topology Modifi ed for 1/2x
Filter Bandwidth (3 External Capacitors)
Figure 7. LTC6404-2 Filter Topology Modifi ed for 2x
Filter Bandwidth (2 External Resistors)
+
+
–
and OUTF
+
–
14
7
14
7
OUT
OUT
49.9Ω
49.9Ω
50Ω
50Ω
–
OUT
OUT
50Ω
50Ω
12pF
can also be used, but since it is being
–
+
12pF
–
+
13
8
13
8
12pF
12pF
OUTF
OUTF
12pF
12pF
OUTF
OUTF
LTC6404-2
LTC6404-2
–
+
V
V
–
+
–
–
64042 F08
V
V
–
–
64042 F07
V
V
–
–
V
V
12
9
–
–
12
9
12pF
LTC6404-2
12pF
12pF
FILTERED OUTPUT
(176MHz)
FILTERED OUTPUT
(44.25MHz)
15
64042f