LT1204CSW datasheet(11/20 Pages) LINER

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LT1204

APPLICATI

I FOR ATIO

monitors. The reference pin has no effect when the LT1204

is operating on

±5V, and should be grounded. On supply

voltages above

±8V, the range of voltages for Pin 8 should

be between – 6.5V and – 7.5V. Reducing Pin 8 voltage

below – 7.5V turns “on” the “off” tee switch, and the

isolation between channels is lost.

1204 AI04

Channel-to-Channel Switching

VOUT PIN 15

Transient at Input Buffer

VIN0 AND VIN1 CONNECTED TO 2MHz SINEWAVE

Competitive MUXs

Crosstalk

The crosstalk, or more accurately all hostile crosstalk, is

measured by driving a signal into any three of the four

inputs and selecting the 4th input with the logic control.

This 4th input is either shorted to ground or terminated in

an impedance. All hostile crosstalk is defined as the ratio

in decibel of the signal at the output of the CFA to the signal

on the three driven inputs, and is input-referred. Disable

crosstalk is measured with all four inputs driven and the

part disabled. Crosstalk is critical in many applications

where video multiplexers are used. In professional video

systems, a crosstalk figure of – 72dB is a desirable

specification.

The key to the outstanding crosstalk performance of the

LT1204 is the use of tee switches (see Figure 1). When the

tee switch is on (Q2 off) Q1 and Q3 are a pair of emitter

followers with excellent AC response for driving the CFA.

A0 PIN 9

VIN0 AND VIN1 CONNECTED TO 2MHz SINEWAVE

PIN 8 VOLTAGE = –6.8V, VS = ±15V

A0 PIN 9

VIN0 PIN 1

1204AI05

SWITCHING BETWEEN VIN0 AND VIN1

RS = 50Ω, VREF = –6.8V, VS ±15V

Competitive video multiplexers built in CMOS are bidirec-

tional and suffer from poor output-to-input isolation and

cause transients to feed to the inputs. CMOS MUXs have

been built with “break-before-make” switches to eliminate

the talking between channels, but these suffer from output

glitches large enough to interfere with sync circuitry.

Multiplexers built on older bipolar processes that switch

lateral PNP transistors take several microseconds to settle

and blur the transition between pictures.

BIPOLAR

MUX

CMOS MUX

1204 AI06

–

CFA

VOUT

VIN0

TO LOGIC

1204 F01

V+

V –

–V

Figure 1. Tee Switch

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