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LTC6401-20

640120f

Output Common Mode Adjustment

The LTC6401-20’s output common mode voltage is set

by the VOCM pin, which is a high impedance input. The

output common mode voltage is capable of tracking VOCM

in a range from 1V to 1.6V. Bandwidth of VOCM control is

typically 15MHz, which is dominated by a low pass ﬁlter

connected to the VOCM 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 VOCM 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 VOCM can be normally

connected to the VCM 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. VOCM of the LTC6401-20 is

connected to VCM 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.

Figure 7. Single-Ended Input to LTC6401-20 and LTC2208

APPLICATIONS INFORMATION

66.5

0.1

μF

640120 F07

LTC6401-20

VOCM

ENABLE

IF IN

LTC2208

0.1

μF

LTC2208 130Msps

16-Bit ADC

1.25V

20dB GAIN

AIN+

AIN–

VCM

–IN

+IN

+OUT

+OUTF

–OUTF

–OUT

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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