AD8141(Rev0) 데이터 시트보기 (PDF) - Analog Devices

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AD8141
(Rev.:Rev0)

Low Cost, Triple Differential Drivers for Wideband Video

Analog Devices 

SINGLE 5 V SUPPLY APPLICATION INFORMATION

The AD8141 and AD8142 require a nominal voltage of 5 V

across their VS+ and VS− power supply pins, and that their EPADs

be connected to system ground; the voltage between VS+ and the

local system ground must be greater than or equal to 2.5 V and

less than or equal to 5 V. These requirements can be met by a

single +5 V supply, or split supplies such as ±2.5 V, +3 V/−2 V,

and so on. Operating the AD8141 and AD8142 with ±2.5 V

supplies provides considerable power savings compared with

other drivers operating at ±5 V supplies, without any disadvantages

with regard to input and output ranges in most cases.

The receivers used with the AD8141 and AD8142, such as the

AD8145, AD8143, AD8123, and AD8128, generally operate

with split supplies, ranging from ±5 V to ±12 V. The split supply

arrangement results in a receiver input common-mode range

that is centered at 0 V relative to the local ground reference and

ranges to within a volt or two from each rail. Ground potential

differences normally exist between the driver end and the receiver

end, and these differences cause the relative common-mode

voltages between the driver and receiver to shift. See Figure 43

for an example.

AD8141/AD8142

In Figure 43, VR, CM = VO, CM + VSHIFT. If VO, CM = 0 V and VSHIFT =

2 V, VR, CM is 2 V. This is because the receiver ground is shifted

down by 2 V relative to the driver ground, and the common-

mode level on the cable stays constant. It can be seen from this

example that the most margin to absorb ground shifts exists

when the center of the receiver input common-mode voltage

range relative to its ground is the same as the output common-

mode voltage of the driver with respect to its ground.

Most receivers operate with their input common-mode ranges

centered at 0 V; therefore, the best case for the driver is to set its

output common-mode voltage to 0 V. This is not possible for the

AD8141 or AD8142on a single 5 V supply, but can be accomplished

using split supplies. If a single 5 V supply is required, the rail-

to-rail output allows the AD8141 output common-mode voltage

to be set to less than 1 V to be as close as possible to the ideal

setting of 0 V. Whereas the AD8141 has uncommitted VOCM inputs,

the AD8142 has internal sync-encoding circuitry that fixes the

nominal output common-mode voltage at 1.5 V above the negative

rail. Each part has a resistive divider on the VOCM input that sets

the nominal output common-mode voltage to 1.5 V above the

negative rail when no external voltage is applied. The divider

consists of a 8.75 kΩ resistor to VS+ and a 3.75 kΩ resistor to VS−,

forming a Thevenin equivalent load of 2.6 kΩ to 30% of the

voltage across the supplies. In the single 5 V supply case, the

Thevenin load voltage is 30% of 5 V above 0 V, or 1.5 V.

DRIVER

AD8141/AD8142

+

VOCM

–

49.9Ω

100Ω

49.9Ω UTP

–

100Ω

+

+

VO, CM

–

+

VR, CM

–

+–

VSHIFT

Figure 43. End-to-End Common-Mode Shifts due to Ground Shifts

Rev. 0 | Page 19 of 24

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