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LT1675/LT1675-1

APPLICATIONS INFORMATION

Power Supplies

The LT1675 will function with supply voltages below

±2V

(4V total), however, to ensure a full 1VP-P video signal

(2VP-P at the output pins), the power supply voltage

should be between

±4V to ±6V. The LT1675 is designed

to operate on

±5V, and at no time should the supplies

exceed

±6V. The power supplies should be bypassed with

quality tantalum capacitors. It may be necessary to add

0.01

µF or 0.1µF in parallel with the tantalum capacitors if

there is excessive ringing on the output waveform. Even

though the LT1675 is well behaved, bypass capacitors

should be placed as close to the LT1675 as possible.

Smallest Package and PC Board Space

The LT1675 has the internal gain set for + 2V/V or 6dB,

because it is designed to drive a double-terminated 50

Ωor

Ω cable that has an inherent 6dB loss. There are several

advantages to setting the gain internally. This topology

eliminates six gain set resistors, reduces the pin count of

the package and eliminates stray capacitance on the

sensitivity feedback node. The LT1675 fits into the small

SSOP package, and these advantages lead to the smallest

PC board footprint with enhanced performance. The

LT1675-1 eliminates two gain set resistors and is available

in the tiny MSOP package and the cost-effective SO-8

package.

Fast Switching

The key to the LT1675 fast switching speed is Linear

Technology’s proprietary high speed bipolar process.

Internal switches can change state in less than 1ns, but the

output of the MUX switches in about 2.5ns, as shown in

Figure 1. The additional delay is due to the finite bandwidth

and the slew rate of the current feedback amplifier that

drives the cable.

For minimum ringing, it is important to minimize the load

capacitance on the output of the part. This is normally not

a problem in a controlled impedance environment, but

stray PC board capacitance and scope probe capacitance

can degrade the pulse fidelity. Figure 2 shows the

response of the output to various capacitive loads mea-

sured with a 10pF scope probe.

MEASURED AT PIN 15

RL = 150Ω, 10pF SCOPE PROBE

1675 F02

CL = 20pF

CL = 10pF

CL = 0pF

2V/DIV

Figure 2. Response to Capacitive Loads

1V/DIV

500mV/DIV

SELECT

PIN 10

RED OUT

PIN 15

RED 1 = 1V, RED 2 = 0V

MEASURED BETWEEN 75

Ω BACK TERMINATION

AND 75

Ω LOAD

1675 F01

Figure 1. Toggling at 25MHz

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