DG540APSLASH883 datasheet(9/10 Pages) TEMIC

9 / 10 page

DG540/541/542

Siliconix

S-53694—Rev. E, 28-May-97

Applications (Cont’d)

Power Supplies

A useful feature of the DG54X family is its power supply

flexibility. It can be operated from a single positive supply

(V+) if required (V– connected to ground).

Note that the analog signal must not exceed V– by more

than –0.3 V to prevent forward biasing the substrate p-n

junction.

The use of a V– supply has a number of

advantages:

1. It allows flexibility in analog signal handling, i.e.,

with V- = -5 V and V+ = 12 V; up to "5 V ac

signals can be controlled.

2. The value of on capacitance [CS(on)] may be

reduced. A property known as `the bodyeffect' on

the DMOS

switch devices causes various

parametric effects to occur. One of these effects is

the reduction in CS(on) for an increasing V

body-source. Note, however, that to increase V-

normally requires V+ to be reduced (since V+ to

V- = 21 V max.). Reduction in V+ causes an

increase in rDS(on), hence a compromise has to be

achieved. It is also useful to note that optimum

video linearity performance (e.g., differential phase

and gain) occurs when V- is around -3 V.

3. V- eliminates the need to bias the analog signal

using potential dividers and large coupling

capacitors.

Decoupling

It is an established RF design practice to incorporate

sufficient bypass capacitors in the circuit to decouple the

power supplies to all active devices in the circuit. The

dynamic performance of the DG54X is adversely affected

by poor decoupling of power supply pins. Also, of even

more significance, since the substrate of the device is

connected to the negative supply, adequate decoupling of

this pin is essential.

Rules:

1. Decoupling capacitors should be incorporated on

all power supply pins (V+, V-). (See Figure 7.)

2. They should be mounted as close as possible to the

device pins.

3. Capacitors should have good high frequency

characteristics - tantalum bead and/or monolithic

ceramic types are adequate.

Suitable decoupling capacitors are 1 to 10 mF

tantalum bead, plus 10 to 100nF ceramic.

–3 V

GNDs

+15 V

DG540

V+

V–

C1 = 10 mF Tantalum

C2 = 0.1 mF Ceramic

Figure 7.

Supply Decoupling

Board Layout

PCB layout rules for good high frequency performance

must be observed to achieve the performance boasted by the

DG540. Some tips for minimizing stray effects are:

1. Use extensive ground planes on double sided PCB,

separating adjacent signal paths. Multilayer PCB is

even better.

2. Keep signal paths as short as practically possible,

with all channel paths of near equal length.

3. Careful arrangement of ground connections is also

very important. Star connected system grounds

eliminate signal current flowing through ground

path parasitic resistance from coupling between

channels.

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