10

LT1675/LT1675-1

Switching Transients

This MUX includes fast current steering break-before-

make SPDT switches that minimize switching glitches.

The switching transients of Figure 3 are input-referred

(measured between 75

Ω back termination and the 75Ω

5ns. This transient is small and fast enough to not be

visible on quality graphics terminals. Additionally, the

break-before-make SPDT switch is open before the alter-

nate channel is connected. This means there is no input

feedthrough during switching. Figure 4 shows the amount

of alternate channel that is coupled at the input.

Expanding Inputs

In video routing applications where the ultimate speed is

not mandatory, as it is in pixel switching, it is possible to

expand the number of MUX inputs by shorting the

LT1675 outputs together and switching with the

ENABLE pins. The internal gain set resistors have a nomi-

nal value of 750

Ω and cause a 1500Ω shunt across the

75

Ω cable termination. Figure 5 shows schematically the

effect of expanding the number of inputs. The effect of this

loading is to cause a gain error that can be calculated by

the following formula:

Gain

n

n

dB

Error (dB) = 6dB + 20log

–1

75 +

1575

–1

75

1575

75

Ω

Ω

Ω

Ω





















where n is total number of LT1675s. For example, using

ten LT1675s (20 Red, 20 Green and 20 Blue) the Gain Error

is only – 1.7dB per channel.

Figure 6 shows a 4-input RGB router. The response from

RED 1 Input to Red Output is shown in Figure 7 for a

25MHz square wave with Chip Select = 0V. In this case the

Gain Error is – 0.23dB. Toggling with Chip Select between

IC #1 and IC #2 is shown in Figure 8. In this case RED 1

Input is connected to 0V and RED 3 Input is connected to

an uncorrelated sinewave.

0V

3V

1V/DIV

0V

SELECT

PIN 10

50mV/DIV

RED OUT

PIN 15

1675 F03

Figure 3. Input-Referred Switching Transient

1675 F04

3V

0V

0V

1V/DIV

20mV/DIV

RED 1 IN

PIN 1

SELECT

PIN 10

Figure 4. Switching Transient at RED 1 (Pin 1)