LT1204 LINER [Linear Technology], LT1204 Datasheet - Page 11

• Manufacturer Part Number: LT1204
• Description: 4-Input Video Multiplexer with 75MHz Current Feedback Amplifier
• Manufacturer: LINER [Linear Technology]

V
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.
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.
A
OUT
V
IN0
A0 PIN 9
A0 PIN 9
PPLICATI
PIN 15
PIN 1
V
PIN 8 VOLTAGE = –6.8V, V
SWITCHING BETWEEN V
R
IN0
S
= 50 , V
AND V
IN1
Channel-to-Channel Switching
REF
CONNECTED TO 2MHz SINEWAVE
O
Transient at Input Buffer
= – 6.8V, V
U
S
IN0
S
S
AND V
= 15V
15V
I FOR ATIO
U
IN1
W
1204 AI04
1204AI05
U
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.
CMOS MUX
TO LOGIC
BIPOLAR
V
IN0
MUX
V
IN0
AND V
Q1
I
1
IN1
V
V
+
–
CONNECTED TO 2MHz SINEWAVE
Q2
Figure 1. Tee Switch
Competitive MUXs
–V
Q3
I
2
FB
R
G
+
–
CFA
1204 AI06
LT1204
R
11
F
1204 F01
V
OUT