LMH6583

OUTPUT

BUFFER

VOUT

+

-

LMH6703

RL

1 k:

560:

560:

1

10

100

1000

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

FREQUENCY (MHz)

OUTPUT CONNECTED DIRECTLY

NO EXPANSION

VS = ±3.3V

VOUT = 2 VPP

RL = 150:

1

10

100

1000

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

FREQUENCY (MHz)

OUTPUT CONNECTED DIRECTLY

NO EXPANSION

VS = ±5V

VOUT = 2 VPP

RL = 150:

LMH6583

SNOSAP5E – APRIL 2006 – REVISED MARCH 2013

www.ti.com

Figure 36. Input Expansion Frequency Response

Figure 37. Input Expansion Frequency Response

DRIVING CAPACITIVE LOADS

Capacitive output loading applications will benefit from the use of a series output resistor ROUT. Capacitive loads

of 5 pF to 120 pF are the most critical, causing ringing, frequency response peaking and possible oscillation. For

applications where maximum frequency response is needed and some peaking is tolerable, the value of ROUT

can be reduced slightly from the recommended values. When driving transmission lines the 50

Ω or 75Ω

matching resistor makes the series output resistor unnecessary.

USING OUTPUT BUFFERING TO ENHANCE BANDWIDTH AND INCREASE RELIABILITY

The LMH6583 crosspoint switch can offer enhanced bandwidth and reliability with the use of external buffers on

the outputs. The bandwidth is increased by unloading the outputs and driving a higher impedance. The 1 k

Ω load

resistor was chosen to provide the best performance on our evaluation board. See Figure 9 in Typical

Performance Characteristics for an example of bandwidth achieved with less loading on the outputs. For this

technique to provide maximum benefit a very high speed amplifier such as the LMH6703 should be used, as

shown in Figure 38.

Besides offering enhanced bandwidth performance using an external buffer provides for greater system

reliability. The first advantage is to reduce thermal loading on the crosspoint switch. This reduced die

temperature will increase the life of the crosspoint. The second advantage is enhanced ESD reliability. It is very

difficult to build high speed devices that can withstand all possible ESD events. With external buffers the

crosspoint switch is isolated from ESD events on the external system connectors.

Figure 38. Buffered Output

12

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