Datasheet

LT1993-10
13
199310fb
APPLICATIO S I FOR ATIO
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To decrease fi lter bandwidth, add two external capaci-
tors, one from +OUTFILTERED to ground, and the other
from –OUTFILTERED to ground. A single differential
capacitor connected between +OUTFILTERED and –OUT-
FILTERED 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 bandwidth, for example, two 36pF capacitors could
be added (one from each fi ltered output to ground). Al-
ternatively 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 7).
Bandpass fi ltering is also easily implemented with just a
few external components. An additional 120pF and 39nH,
each added differentially between +OUTFILTERED and
–OUTFILTERED creates a bandpass fi lter with a 71MHz
center frequency, –3dB points of 55MHz and 87MHz, and
1.6dB of insertion loss (Figure 8).
Figure 6. LT1993-10 Internal Filter Topology Modifi ed
for 2x Filter Bandwidth (2 External Resistors)
Output Common Mode Adjustment
The LT1993-10’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 300MHz, so for applications where the V
OCM
pin
is tied to a DC bias voltage, a 0.1µF capacitor at this pin is
recommended. For best distortion performance, the voltage
at the V
OCM
pin should be between 1.8V 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 LT1993-10.
For 3V ADCs such as the LTC22XX families, the LT1993-
10 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 LT1993-10’s
V
OCM
pin with a 0.1µF capacitor. For a common mode volt-
age above 1.9V, AC coupling capacitors are recommended
between the LT1993-10 and the LTC22XX ADC because of
the input voltage range constraints of the ADC.
Figure 7. LT1993-10 Internal Filter Topology Modifi ed
for 1/2x Filter Bandwidth (3 External Capacitors)
V
EE
V
EE
19932 F06
25
25
12pF
LT1993-10
–OUT
+OUT
25
25
12pF
12pF
–OUTFILTERED
FILTERED OUTPUT
(350MHz)
+OUTFILTERED
8
7
6
5
V
EE
V
EE
199310 F07
25
12pF
LT1993-10
–OUT
+OUT
25
12pF
12pF
12pF
12pF
12pF
–OUTFILTERED
+OUTFILTERED
8
7
6
5
FILTERED OUTPUT
(87.5MHz)
Figure 8. LT1993-10 Output Filter Topology Modifi ed for
Bandpass Filtering (1 External Inductor, 1 External Capacitor)
V
EE
V
EE
199310 F08
25
12pF
LT1993-10
–OUT
+OUT
25
12pF
12pF
120pF39nH
–OUTFILTERED
+OUTFILTERED
8
7
6
5
FILTERED OUTPUT
(71MHz BANDPASS,
–3dB @ 55MHz/87MHz)