© 2004 Fairchild Semiconductor Corporation

www.fairchildsemi.com

FMS6501 Rev. 1.0.2

11

Thermal issues are significantly reduced with AC-cou-

pled outputs, alleviating special PC layout requirements.

Each of the outputs can be independently powered down

and placed in a high-impedance state with the ENABLE

bit. This function can be used to mute video signals, to

parallel multiple FMS6501 outputs, or to save power.

When the output amplifier is disabled, the high-imped-

ance output presents a 3k

Ω load to ground. The output

amplifier typically enters and recovers from the power-

down state in less than 300ns after being programmed.

When an output channel is not connected to an input, the

input to that channel’s amplifier is forced to approxi-

mately 150mV. The output amplifier is still active unless

levels depend on the programmed gain for that channel.

Crosstalk

Crosstalk is an important consideration when using the

FMS6501. Input and output crosstalk are defined to rep-

resent the two major coupling modes in a typical applica-

tion. Input crosstalk is crosstalk in the input pins and

switches when the interfering signal drives an open

switch. It is dominated by inductive coupling in the pack-

age lead frame between adjacent leads. It decreases

rapidly as the interfering signal moves farther away from

the pin adjacent to the input signal selected. Output

crosstalk is coupling from one driven output to another

active output. It decreases with increasing load imped-

ance, as it is caused mainly by ground and power cou-

pling between output amplifiers. If a signal is driving an

open switch, its crosstalk is mainly input crosstalk. If it is

driving a load through an active output, its crosstalk is

mainly output crosstalk.

Input and output crosstalk measurements are performed

with the test configuration shown in Figure 12.

Figure 12. Test Configuration for Crosstalk

For input crosstalk, the switch is open. All inputs are in

while all other inputs are AC terminated with 75

Ω. All out-

puts are enabled and crosstalk is measured from IN1 to

any output.

For output crosstalk, the switch is closed. Crosstalk from

OUT1 to any output is measured.

Crosstalk from multiple sources into a given channel was

measured with the setup shown in Figure 6. Input IN1 is

puts Out1 to Out8. Input In9 is driven with a secondary,

asynchronous, gray-field video signal, and is connected

to Out9. All other inputs are AC terminated with 75

Ω.

Crosstalk effects on the gray field are measured and cal-

at the load.

If not all inputs and outputs are needed, avoid using

adjacent channels, where possible, to reduce crosstalk.

Disable all unused channels to further reduce crosstalk

and power dissipation.

Figure 13. Test Configuration for Multi-Channel

Crosstalk

Bias

IN1

OUT1

Bias

IN12

OUT9

Gain = 6dB

OUT1 = 2.0Vpp

Input crosstalk from IN1

to OUTx

Output crosstalk from

OUT1 to OUTx

TERMINATION

IN2 - IN12 are

AC-Term to

ground with

IN1 = 1Vpp

Open switch

for input

crosstalk

Close switch

for output

crosstalk

75

Bias

IN1

OUT1

Bias

IN12

OUT9

Gain = 6dB

OUT1 = 2.0Vpp

Measure crosstalk from

Channels 1-8 into Channel 9

TERMINATION

Bias

IN9

IN1 driven with

SD videio 1Vpp

IN9 driven with

asynchronous

SD video 1Vpp

IN2-8 + IN10-12

driven with

AC term to GND

with 75