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LTC6400-14

640014fb

which is dominated by a low pass ﬁlter connected to the

VOCM 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 VOCM 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 VOCM

pin can be connected to the VCM pin of the ADC.

Driving A/D Converters

The LTC6400-14 has been speciﬁcally 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 4.99

Ωresistorshelpeliminate

potential resonance associated with stray capacitance of

PCB traces and bond wires of either the ADC input or the

driver output. VOCM of the LTC6400-14 is connected to VCM

of the LTC2208 VCM 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

ﬁlter would be desirable to limit the wideband noise of

the ampliﬁer. 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

640014 F07

LTC6400-14

VOCM

ENABLE

IF IN

LTC2208

4.99Ω

0.1μF

LTC2208 130Msps

16-Bit ADC

1.25V

4.99Ω

14dB GAIN

AIN–

AIN+

VCM

–IN

+IN

+OUT

+OUTF

–OUTF

–OUT

FREQUENCY (MHz)

–40

–110

–100

–90

–80

–70

–60

–50

100

150

200

640014 F08

250

300

SINGLE-ENDED INPUT

fS = 122.8Msps

DRIVER VOUT = 2VP-P COMPOSITE

Top Silkscreen

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 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

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