LT6402 Linear Technology Corporation, LT6402 Datasheet - Page 12

• Manufacturer Part Number: LT6402
• Description: 300mhz Low Distortion, Low Noise Differential Ampli?er/ Adc Driver Av = 20db
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
LT6402-20
Filtered Applications
(Using the +OUTFILTERED and –OUTFILTERED Pins)
Filtering at the output of the LT6402-20 is often desired
to provide either anti-aliasing or improved signal to noise
ratio. To simplify this fi ltering, the LT6402-20 includes an
additional pair of differential outputs (+OUTFILTERED and
–OUTFILTERED) which incorporate an internal lowpass
fi lter network with a –3dB bandwidth of 75MHz (Figure 5).
These pins each have an output impedance of 50Ω. Inter-
nal capacitances are 14pF to V
plus an additional 14pF 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 42pF capacitor shunting each
fi ltered output to AC ground, giving a –3dB bandwidth of
75MHz.
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
+OUTFILTERED and the other between –OUT and –OUTFIL-
12
Figure 5. LT6402-20 Internal Filter Topology –3dB BW ≈75MHz
Figure 6. LT6402-20 Internal Filter Topology Modifi ed
for 2x Filter Bandwidth (2 External Resistors)
LT6402-20
LT6402-20
50Ω
50Ω
50Ω
50Ω
14pF
14pF
14pF
14pF
U
V
V
V
V
EE
EE
EE
EE
14pF
14pF
U
8
7
6
5
8
7
6
5
–OUT
+OUT
–OUT
+OUT
–OUTFILTERED
+OUTFILTERED
–OUTFILTERED
+OUTFILTERED
EE
50Ω
50Ω
on each fi ltered output,
W
6402 F05
6402 F06
FILTERED OUTPUT
(75MHz)
FILTERED OUTPUT
(150MHz)
U
TERED (Figure 6). These resistors are in parallel with the
internal 50Ω resistor, lowering the overall resistance and
increasing fi lter bandwidth. To double the fi lter bandwidth,
for example, add two external 50Ω resistors to lower the
series resistance to 25Ω. The 42pF of capacitance remains
unchanged, so fi lter bandwidth doubles.
To decrease fi lter bandwidth, add two external capacitors,
one from +OUTFILTERED to ground, and the other from
–OUTFILTERED to ground. A single differential capacitor
connected between +OUTFILTERED and –OUTFILTERED
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 band-
width, for example, two 42pF capacitors could be added
(one from each fi ltered output to ground). Alternatively
one 21pF 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
Figure 8. LT6402-20 Output Filter Modifi ed for Bandpass
Filtering (1 External Inductor, 1 External Capacitor)
LT6402-20
LT6402-20
Figure 7. LT6402-20 Internal Filter Topology Modifi ed
for 1/2x Filter Bandwidth (3 External Capacitors)
50Ω
50Ω
50Ω
50Ω
14pF
14pF
14pF
14pF
V
V
V
V
EE
EE
14pF
EE
EE
14pF
8
7
6
5
8
7
6
5
–OUT
–OUTFILTERED
+OUTFILTERED
+OUT
–OUT
+OUT
–OUTFILTERED
+OUTFILTERED
6402 F08
6402 F07
14pF
14pF
14pF
FILTERED OUTPUT
FILTERED OUTPUT
(37.5MHz)
640220fa