LTC6401-20 Datasheet, PDF (12/16 Pages) Linear Technology – 1.3GHz Low Noise, Low Distortion Differential ADC Driver for 140MHz IF

LTC6401-20
APPLICATIONS INFORMATION
Output Common Mode Adjustment
The LTC6401-20’s output common mode voltage is set
by the V_OCM pin, which is a high impedance input. The
output common mode voltage is capable of tracking V_OCM
in a range from 1V to 1.6V. Bandwidth of V_OCM control is
typically 15MHz, which is dominated by a low pass filter
connected to the V_OCM pin and is aimed to reduce com-
mon 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 LTC6401-20. The V_OCM pin should be
tied to a DC bias voltage where a 0.1μF bypass capacitor
is recommended. When interfacing with A/D converters
such as the LT22xx families, the V_OCM can be normally
connected to the V_CM pin of the ADC.
Driving A/D Converters
The LTC6401-20 has been specifically designed to inter-
face directly with high speed A/D converters. In Figure 7,
an example schematic shows the LTC6401-20 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 wire inductance of either the ADC
input or the driver output. V_OCM of the LTC6401-20 is
connected to V_CM of the LTC2208 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 LTC6401-20.
The balun also converts input impedance to match 50Ω
source impedance.
0.1μF
1.25V
IF IN
66.5Ω
29Ω
0.1μF
V_OCM
10Ω
+IN
+OUT
+OUTF
LTC6401-20
–OUTF
–IN
–OUT
10Ω
ENABLE
20dB GAIN
AIN^{+ VCM}
LTC2208
AIN^–
LTC2208 130Msps
16-Bit ADC
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Figure 7. Single-Ended Input to LTC6401-20 and LTC2208
Test Circuits
Due to the fully-differential design of the LTC6401 and
its usefulness in applications with differing characteristic
specifications, two test circuits are used to generate the
information in this datasheet. Test Circuit A is DC987B,
a two-port demonstration circuit for the LTC6401 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
network analyzer. There are also series resistors at the
output to present the LTC6401 with a 375Ω differential
load, optimizing distortion performance. Due to the input
and output transformers, the –3dB bandwidth is reduced
from 1.3GHz to approximately 1.1GHz.
Test Circuit B uses a 4-port network analyzer to measure
S-parameters and gain/phase response. This removes the
effects of the wideband baluns and associated circuitry,
for a true picture of the >1GHz S-parameters and AC
characteristics.
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