LTC6400-14
13
640014fb
which is dominated by a low pass fi lter connected to the
V
OCM
pin and is aimed to reduce common mode noise
generation at the outputs. The internal common mode
feedback loop has a –3dB bandwidth around 400MHz,
allowing fast common mode rejection at the outputs of
the LTC6400-14. The V
OCM
pin should be tied to a DC bias
voltage with a 0.1μF bypass capacitor. When interfacing
with A/D converters such as the LTC22xx families, the V
OCM
pin can be connected to the V
CM
pin of the ADC.
Driving A/D Converters
The LTC6400-14 has been specifi cally designed to inter-
face directly with high speed A/D converters. In Figure 7,
an example schematic shows the LTC6400-14 with a
single-ended input driving the LTC2208, which is a 16-bit,
130Msps ADC. Two external 4.99Ω resistors help eliminate
potential resonance associated with stray capacitance of
PCB traces and bond wires of either the ADC input or the
driver output. V
OCM
of the LTC6400-14 is connected to V
CM
of the LTC2208 V
CM
pin at 1.25V. Alternatively, a single-
ended input signal can be converted to a differential signal
via a balun and fed to the input of the LTC6400-14.
Figure 8 summarizes the IMD3 of the whole system in
Figure 7. Note that Figure 7 shows a direct connection
to the LTC2208, but in many applications an anti-alias
fi lter would be desirable to limit the wideband noise of
the amplifi er. This is especially true in high performance
16-bit designs.
Test Circuits
Due to the fully-differential design of the LTC6400 and
its usefulness in applications with differing characteristic
Figure 7. Single-Ended Input to LTC6400-14 and LTC2208
Figure 8. IMD3 for the Combination of LTC6400-14 and LTC2208
APPLICATIONS INFORMATION
29Ω
66.5Ω
0.1μF
0.1μF
640014 F07
LTC6400-14
V
OCM
ENABLE
IF IN
LTC2208
4.99Ω
0.1μF
LTC2208 130Msps
16-Bit ADC
1.25V
4.99Ω
14dB GAIN
AIN
–
AIN
+
V
CM
–IN
+IN
+OUT
+OUTF
–OUTF
–OUT
FREQUENCY (MHz)
0
IMD3 (dBc)
–40
–110
–100
–90
–80
–70
–60
–50
100 15050 200
640014 F08
250 300
SINGLE-ENDED INPUT
f
S
= 122.8Msps
DRIVER V
OUT
= 2V
P-P
COMPOSITE
Top Silkscreen
specifi cations, two test circuits are used to generate the
information in this datasheet. Test Circuit A is DC987B,
a two-port demonstration circuit for the LTC6400 family.
The schematic and silkscreen are shown below. This
circuit includes input and output transformers (baluns)
for single-ended-to-differential conversion and impedance
transformation, allowing direct hook-up to a 2-port
