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LTC6417

6417f

applicaTions inForMaTion

Circuit Operation

The LTC6417 is a low noise and low distortion fully dif-

ferential unity gain ADC driver with a –3dB bandwidth

spanning DC to 1.6GHz, a differential input impedance of

18.5kΩ, and a differential output impedance of 3Ω. The

LTC6417 is composed of a fully differential buffer with

output common mode voltage control circuitry and high

speed voltage-limiting clamps at the output. Lowpass or

bandpass filters are easily implemented with just a few

external components. The LTC6417 is very flexible in

terms of I/O coupling. It can be AC- or DC-coupled at the

inputs, the outputs or both. When using the LTC6417 with

DC-coupled inputs, best performance is obtained with an

input common mode voltage between 1V and 1.5V. For

AC-coupled operation, the LTC6417 will take the voltage

applied to the VCM pin and use it to bias the inputs so

that the output common mode voltage equals VCM, thus

no external circuitry is needed. The VCM pin has been

designed to directly interface with the VCM pin found on

Linear Technology’s high speed ADC families.

Input Impedance and Matching

The LTC6417 has a high differential input impedance of

18.5kΩ. The differential inputs may need to be terminated

to a lower value impedance, e.g. 50Ω, in order to provide

an impedance match for the source. Figure 1 shows input

matchingandsingle-endedtodifferentialconversionusing

a 1:1 balun, while Figure 2 shows a similar circuit using

a 1:4 balun to achieve an additional 6dB of voltage gain.

These circuits provide a wideband impedance match.

The balun and matching resistors must be placed close

to the input pins in order to minimize the rejection due to

input mismatch. In Figures 1 and 2, the capacitor center-

tapping the two input termination resistors improves high

frequency common mode rejection. As an alternative to

this wideband approach, a narrowband impedance match

can be used at the inputs of the LTC6417 for frequency

selection and/or noise reduction.

Figure 2. Input Termination for Differential 50Ω Input Impedance Using a 1:4 Balun

Figure 1. Input Termination for Differential 50Ω Input Impedance Using a 1:1 Balun

VIN

IN+

IN–

24.9

0.1µF

24.9

OUT–

6417 F01

OUT+

LTC6417

0.1µF

1:1

MABA-007159-000000

+–

VIN

TCM4-19+

IN+

IN–

OUT–

6417 F02

OUT+

LTC6417

0.1µF

100

+–

100

0.1µF

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