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AN504

Video Crosspoint Switch Simplifies Large Matrix Designs

Vishay Semiconductors 

AN504

Vishay Siliconix

Transmission Loss

Transmission loss is a function of the load connected to the

crosspoint output for a fixed channel on-resistance and source

impedance. Transmission loss is given by

Loss

(dB)

+

20

log10

Rsource

)

Rload

Rchannel

)

Rload

Where Rchannel = RDS1 + RDS2 + RDS4.

This equation is accurate at frequencies below 1 MHz, where

device capacitances (i.e., on-state input capacitance) can be

safely ignored.

Input Overvoltage

are taken. A study of the equation for transmission loss reveals

that distortion caused by changing on-resistance is inversely

proportional to load resistance. Thus, distortion is given by

DIST = 20 x log (DRon/Rload)

From the above, the decibel level below the fundamental may

be calculated. Figure 5 shows how the on-resistance varies

with signal swing. DRon is 5 W for a signal swing of "5 V. Thus,

for Rload = 5 kW, distortion is –60 dB. For low-frequency

operation of the DG884 (where isolation and crosstalk

performance may be critical in demanding applications), signal

swings are usually much greater than those found at video

frequencies and above. Thus, Rload should be increased

accordingly. In all cases, some form of high-performance

buffer/amplifier will be required at the output to translate the

crosspoint load resistance to a lower “interface” value, i.e., 50

to 75 W for high-frequency designs.

Overvoltage effects are the third important characteristic that

can be visualized from the models shown in Figures 3 and 4.

Unlike mechanical switches, solid-state analog switches have

electrical “end-stops” beyond which the switch ceases to

behave as a switch. These “end-stops” are defined by the

power supply rails. For an ”on” channel, a signal at the input

can swing to the negative rail before the shunt-switch body

diodes (Figure 3) begin to conduct. When these diodes

conduct, a large current from the negative rail can result. This

current flow should be limited to 20 mA dc as defined in the

absolute maximum ratings. During the period of negative

overvoltage, the crosspoint outputs will be corrupted whether

they are selected or not. A single diode in series with the

negative rail will prevent reverse current flow and output

corruption during negative overvoltage.

In the positive direction, if the input is allowed to continue rising

towards the positive supply rail, its amplitude should be limited

to the breakdown voltage of the body-to-source junction of the

switching FETs. This is specified in the data sheet maximum

ratings section, where a value of 14 V above the negative rail

is quoted. Where rails of +15 V and –5 V are used, the

body-source breakdown voltage is the maximum positive limit

of the signal at the source (input) or the drain (output) terminals.

In a power down situation, where the positive and negative

rails are at zero, the maximum positive input signal is also 14 V

above the negative rail. No output corruption will occur during

positive overvoltage.

rDS(on) vs. Drain Voltage

200

V+ = 15 V

180 V– = –3 V

VL = 5 V

160 IS = –10 mA

140

120

125_C

100

25_C

80

60

40

–55_C

20

–2

0

2

4

6

8

10

VD – (V)

FIGURE 5. Change of On-Resistance with Signal Amplitude

Decoupling

Distortion

The change of on-resistance with signal swing, explained

above, will give rise to distortion unless specific precautions

A high-frequency signal path exists between the analog

channel and the power rails via the gate-to-source

capacitances of the switching FETs, as shown in Figures 3 and

4. This is because the drive logic applies either rail voltage to

the gate via a low impedance path. Good decoupling shunts

this signal leakage harmlessly to ground.

Document Number: 70610

05-Aug-99

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

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