13
640212fa
LT6402-12
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
solution of 14pF from each fi ltered output to ground plus
a 14pF capacitor between the fi ltered outputs would also
halve the fi lter bandwidth (Figure 7).
Bandpass fi ltering is also easily implemented with just a
few external components. An additional 560pF and 62nH,
each added differentially between +OUTFILTERED and
–OUTFILTERED creates a bandpass fi lter with a 26MHz
center frequency, –3dB points of 23MHz and 30MHz, and
1.6dB of insertion loss (Figure 8).
Output Common Mode Adjustment
The LT6402-12’s output common mode voltage is set by the
V
OCM
pin. It is a high-impedance input, capable of setting
the output common mode voltage anywhere in a range
from 1.1V to 3.6V. Bandwidth of the V
OCM
pin is typically
200MHz, so for applications where the V
OCM
pin is tied to
a DC bias voltage, a 0.1μF capacitor at this pin is recom-
mended. For best distortion performance, the voltage at
the V
OCM
pin should be between 1.2V and 2.6V.
When interfacing with most ADCs, there is generally a
V
OCM
output pin that is at about half of the supply voltage
of the ADC. For 5V ADCs such as the LTC17XX family, this
V
OCM
output pin should be connected directly (with the
addition of a 0.1μF capacitor) to the input V
OCM
pin of the
LT6402-12. For 3V ADCs such as the LTC22XX families,
the LT6402-12 will function properly using the 1.65V from
the ADC’s V
CM
reference pin, but improved Spurious Free
Dynamic Range (SFDR) and distortion performance can
be achieved by level-shifting the LTC22XX’s V
CM
reference
voltage up to at least 1.8V. This can be accomplished as
shown in Figure 9 by using a resistor divider between the
LTC22XX’s V
CM
output pin and V
CC
and then bypassing
the LT6402-12’s V
OCM
pin with a 0.1μF capacitor. For a
common mode voltage above 1.9V, AC coupling capacitors
are recommended between the LT6402-12 and LTC22XX
ADCs because of the input voltage range constraints of
the ADC.
Figure 7. LT6402-12 Internal Filter Topology Modifi ed
for 1/2x Filter Bandwidth (3 External Capacitors)
Figure 8. LT6402-12 Output Filter Modifi ed for Bandpass
Filtering (1 External Inductor, 1 External Capacitor)
Figure 9. Level Shifting 3V ADC V
CM
Voltage for
Improved SFDR
APPLICATIONS INFORMATION
V
EE
V
EE
640212 F07
50Ω
14pF
LT6402-12
–OUT
+OUT
50Ω
14pF
14pF
14pF
14pF
14pF
–OUTFILTERED
+OUTFILTERED
8
7
6
5
FILTERED OUTPUT
(37.5MHz)
V
EE
V
EE
640212 F08
50Ω
14pF
LT6402-12
–OUT
+OUT
50Ω
14pF
14pF
–OUTFILTERED
+OUTFILTERED
8
7
6
5
FILTERED OUTPUT
640212 F09
IF IN
LT6402-12
–INA
–INB
V
OCM
2
31
6
7
1
2
+INB
+INA
14
13
15
150Ω
16
10Ω
10Ω
LTC22xx
0.1μF
0.1μF
+OUTFILTERED
–OUTFILTERED
AIN
+
AIN
–
4.02k
11k
1.9V
1.5V
3V
V
CM
