AN504 데이터 시트보기 (PDF) - Vishay Semiconductors

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AN504

Video Crosspoint Switch Simplifies Large Matrix Designs

Vishay Semiconductors 

AN504

Vishay Siliconix

(Super-VHS) signals. S-VHS is a much enhanced form of the

VHS system from JVC, and it virtually eliminates signal

degradation caused by multi-separation and combination of

the luminance (Y) and chrominance (C) components that

make up a color video signal. In S-VHS, separate Y and C

signals are fed to the recorder, and at playback, Y and C

signals are obtained for feeding directly to a suitable TV

monitor, i.e., one fitted with an S-connector (SCART).

The S-VHS frequency spectrum extends the peak white

portion of the FM signal from 4.8 MHz to 7 MHz. Thus there is

a need for wideband two input/output crosspoint systems such

as the DG884. Figure 8 outlines the DG884 as a 4 x 2 x 2

crosspoint. A key feature of this use is the reduced change of

capacitance at the inputs, compared to the normal 8 x 4

connection. The worst-case input capacitance for one input to

two outputs is 80 pF, and the worst case for one input to one

output remains the same at 40 pF. Substituting these values

into the HF model outlined above, bandwidth and source

impedance trade-offs can be evaluated.

Power Supplies

The DG884 requires three power supplies for

ground-referenced signal handling (V+, V–, and VL = 5 V). The

negative and positive supplies should always be established

first. The logic supply should not be allowed to rise above the

V+ pin during power supply on/off sequencing. This is because

a PN junction exists between the V+ and VL pins, and large

currents could flow if this junction becomes forward biased. If

both V+ and VL are derived from the same supply there will be

no problems. Figure 9 shows power supplies and decoupling

for bipolar signal operation.

The DG884 may be operated in single supply situations (with

V– connected to 0 V) with no loss of logic threshold accuracy.

However, the signal should be offset to +3 V to preserve signal

fidelity through the device.

IN1

YC

IN2

YC

IN3

YC

IN4

YC

Switch Matrix

Y

OUT1

C

Y

OUT2

C

Decode Logic, Switch Drivers

4 Disable Outputs

(Quad Package Only)

WR

Current Event Latches

CS

I/O Control

B1

Logic

B0

Next Event Latches

RS

SALVO

I/O A3 A2 A1 A0

Typical Path Set-Up Might Be:

IN1 (C, Y Pair) to OUT1 Pair

IN4 to OUT2 Pair

or

IN2 to OUT1 and OUT2

www.vishay.com S FaxBack 408-970-5600

6-6

FIGURE 8. 4 2 2 Crosspoint

Document Number: 70610

05-Aug-99

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