LTC6400-20
13
640020f
1.6V. The bandwidth of V
OCM
control is typically 15MHz,
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 300MHz,
allowing fast common mode rejection at the outputs of
the LTC6400-20. 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 LT22xx families, the V
OCM
pin can be connected to the V
CM
pin of the ADC.
Driving A/D Converters
The LTC6400-20 has been specifi cally designed to inter-
face directly with high speed A/D converters. In Figure 7,
an example schematic shows the LTC6400-20 with a
single-ended input driving the LTC2208, which is a 16-bit,
130Msps ADC. Two external 10Ω 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 LTC6400-20 is connected
to V
CM
of the LTC2208 V
CM
pin at 1.25V. Alternatively, a
single-ended input signal can be converted to differential
signal via a balun and fed to the input of the LTC6400-20.
The balun also converts input impedance to match 50Ω
source impedance.
Figure 8 summarizes the spurious free dynamic range
(SFDR) for IMD3 of the whole system in Figure 7.
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 LTC6400-20 and LTC2208
Figure 8. SFDR for the Combination of LTC6400-20 and LTC2208
APPLICATIONS INFORMATION
29Ω
66.5Ω
0.1μF
0.1μF
640020 F07
LTC6400-20
V
OCM
ENABLE
IF IN
LTC2208
10Ω
0.1μF
LTC2208 130Msps
16-Bit ADC
1.25V
10Ω
20dB GAIN
AIN
–
AIN
+
V
CM
–IN
+IN
+OUT
+OUTF
–OUTF
–OUT
FREQUENCY (MHz)
70
SFDR (dB)
94
92
90
88
86
84
82
120 170 220
640020 F08
270 300
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 imped-
ance transformation, allowing direct hook-up to a 2-port
