LT6402-12
12
640212fa
Figure 5. LT6402-12 Internal Filter Topology –3dB BW ≈75MHz
Figure 6. LT6402-12 Internal Filter Topology Modifi ed
for 2x Filter Bandwidth (2 External Resistors)
prior to the ADC to limit input noise at the ADC, thereby
improving system signal to noise ratio. Both the unfi ltered
and fi ltered outputs of the LT6402-12 can easily drive the
high impedance inputs of these differential ADCs. If the
fi ltered outputs are used, then cutoff frequency and the
type of fi lter can be tailored for the specifi c application if
needed.
Wideband Applications
(Using the +OUT and –OUT Pins)
In applications where the full bandwidth of the LT6402-12
is desired, the unfi ltered output pins (+OUT and –OUT)
should be used. They have a low output impedance;
therefore, gain is unaffected by output load. Capacitance
in excess of 5pF placed directly on the unfi ltered outputs
results in additional peaking and reduced performance.
When driving an ADC directly, a small series resistance
is recommended between the LT6402-12’s outputs and
the ADC inputs (Figure 4). This resistance helps eliminate
any resonances associated with bond wire inductances of
either the ADC inputs or the LT6402-12’s outputs. A value
between 10Ω and 25Ω gives excellent results.
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-
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
Figure 4. Adding Small Series R at LT6402-12 Output
Filtered Applications
(Using the +OUTFILTERED and –OUTFILTERED Pins)
Filtering at the output of the LT6402-12 is often desired
to provide either anti-aliasing or improved signal to noise
ratio. To simplify this fi ltering, the LT6402-12 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
EE
on each fi ltered output,
plus an additional 14pF capacitor connected differentially
APPLICATIONS INFORMATION
640212 F04
LT6402-12
–OUT
+OUT
8
5
10Ω TO 25Ω
10Ω TO 25Ω
ADC
640212 F05
50Ω
V
EE
V
EE
14pF
LT6402-12
–OUT
+OUT
50Ω
14pF
14pF
–OUTFILTERED
+OUTFILTERED
8
7
6
5
FILTERED OUTPUT
(75MHz)
V
EE
V
EE
640212 F06
50Ω
50Ω
14pF
LT6402-12
–OUT
+OUT
50Ω
50Ω
14pF
14pF
–OUTFILTERED
FILTERED OUTPUT
(150MHz)
+OUTFILTERED
8
7
6
5
