15

ICS2008B

ICS2008B Rev D 4/05/05

Applications

Crystal Oscillator

This oscillator will operate

properly with either a serial or

parallel resonant crystal. If fre-

quency accuracy is critical,

a parallel resonant crystal is

recommended.

Threshold Bypass Pins

These pins provide access to the internal references for clamp

level (CTHRESH), SYNC slicer (STHRESH), and data slicer

(DTHRESH). In general, these pins are left open, and the levels

are output. However, should the user want to set other levels,

these pins can be over-driven with the desired threshold

level(s).

CTHRESH is the threshold

to which the input video

sync tips are clamped. The

CTHRESH level is nomi-

nally

1.3V. With

the

incoming video riding on

this 1.3V DC level, the in-

ternal SYNC separator

sizes the video at 20 IRE up

from the SYNC tips. This

level, STHRESH, is nomi-

nally

0.14V

above

CTHRESH. The SYNC separator ignores short pulses which

fall below the STHRESH level such as these that come from the

chroma component of the video. DTHRESH is the data slicer

reference. It is nominally 0.57V above CTHRESH.

Video Inputs

Y1, Y2, C1 and C2 pins

must

be

capacitively

coupled to the terminated

video source(s). These in-

puts are clamped to the

CTHRESH level. A typi-

cal coupling capacitance

is 0.1

µF.

Video Outputs

YOUT and COUT are

outputs of analog multi-

plexers which select the

video source from Y1,

C1 or Y2, C2. These

outputs are not buffered,

so minimizes signal dis-

tortion. It is, therefore,

important to keep the ca-

pacitive and resistive

load on the YOUT and

COUT pins to a mini-

mum. If DC coupling is desired, the plus input of the opamp

should be high impedance with a low bias current, and its out-

put should be able to drive a 75 ohm load with an appropriate

video bandwidth. In general, composite NTSC and S-video

signals have a bandwidth of 4.2 MHz. A minimum output

buffer bandwidth of 10 MHz is recommended. Care should

be taken in board layout to minimize stray capacitance on the

YOUT and COUT pins. Otherwise, there could be high fre-

quency roll-off which could result in a loss of chrominance

amplitude.

Self Biased Inputs

The CLICK and FRAME inputs are biased to 1/2 VDD and

connected to plus inputs of two comparators. The minus

inputs are internally biased to 1/2 VDD. When CLICK or

FRAME sources are ana-

log,

they

should

be

capacitively coupled to the

input pin. However, if the

sources are digital, they

may be tied to the pins di-

rectly. It is important to

make sure that the digital

levels into these pins swing

above and below the 1/2

VDD threshold of the com-

parators. This is not a

problem with digital CMOS

sources, but it could be

with TTL sources.

LTCIN + and LTCIN – are

comparator inputs for the

LTC input. This differential

input is provided to maximize noise immunity. If the LTC

source is single ended, the LTCIN – should be capacitively

coupled to the ground reference of that source. If the LTC

source is digital, set the LTCIN – to the desired threshold, and

connect the digital source to LTCIN+.

Fig. 4 - Crystal Oscillator

Fig. 5 –Threshold Bias

Fig. 6 – S-Video Input

Fig. 7 -Video Output

Fig. 8 - Self Biased Inputs