LT6555

10

6555f

To maintain the LT6555’s channel isolation, it is beneficial

to shield parallel input and parallel output traces using a

ground plane or power supply traces. Vias between topside

and backside metal may be required to maintain a low

inductance ground near the part where numerous traces

converge. See Figures 6 and 7 for photos of an optimized

layout.

Input Expansion

In applications with more than two inputs per channel,

multiple LT6555s can be connected by several different

methods. The simplest method is to connect the outputs

after the 75

Ωseriestermination,asshowninFigure2. The

compromise of this approach is that the internal gain

setting resistors cause a 435

Ωshuntacrossthe75Ωcable

termination, resulting in increased gain error.

APPLICATIO S I FOR ATIO

Figure 3. Disabled Amplifiers Load the Cable

Termination with 435

Ω Each

Figure 3 illustrates the loading effect of expanding the

number of inputs. The resultant gain error can be calcu-

lated by the following formula using n as the number of

LT6555s:

Gain Error (dB) = 6dB + 20log

435

n–1

Ω

Ω

+

Ω

Ω

⎛

⎝

⎜

⎜

⎜

⎞

⎠

⎟

⎟

⎟

75

75

435

1

75

n

dB

–

For example, two LT6555s would result in a gain error of

–0.74dB per channel. Three LT6555s (i.e., six red inputs,

six green inputs and six blue inputs), would have a gain

error of –1.4dB.

Figure 2. Two LT6555s Build a 4-Input Router

IN1A

IN1B

IN1C

IN1D

CHIP

SELECT

6555 F02

EN

LT6555 #1

75

Ω

LT6555 #2

74HC04

EN

75

Ω

OUT

75

Ω

CABLE

75

Ω

R2

75

Ω

6555 F03

360

Ω

IN1A

IN1C

IN1B

IN1D

75

Ω

1/3 LT6555 #1

1/3 LT6555 #2

360

Ω

OFF

360

Ω

360

Ω

OFF

360

Ω

75

Ω

75

Ω

360

Ω

OFF

360

Ω

360

Ω

ON

⇒

435

n – 1

n = NUMBER OF LT6555s

IN PARALLEL

.

.

.

n