Datasheet

11
LT1204
U
S
A
O
PP
L
IC
AT
I
WU
U
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
V
OUT
PIN 15
Transient at Input Buffer
V
IN0
AND V
IN1
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
V
IN0
AND V
IN1
CONNECTED TO 2MHz SINEWAVE
PIN 8 VOLTAGE = –6.8V, V
S
= ±15V
A0 PIN 9
V
IN0
PIN 1
1204AI05
SWITCHING BETWEEN V
IN0
AND V
IN1
R
S
= 50Ω, V
REF
= –6.8V, V
S
±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
R
F
V
OUT
FB
I
2
R
G
V
IN0
TO LOGIC
Q2
Q1
I
1
Q3
1204 F01
V
+
V
–
–V
Figure 1. Tee Switch