DG540/541/542

Vishay Siliconix

Document Number: 70055

S-00399—Rev. G, 13-Sep-99

www.vishay.com

S FaxBack 408-970-5600

4-9

APPLICATIONS

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.

property known as ‘the body-effect’ on the DMOS switch

devices causes various parametric effects to occur. One

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

normally requires V+ to be reduced (since V+ to V– = 21 V

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 tanta-

lum bead, plus 10- to 100-nF ceramic.

+

+

–3 V

GNDs

+15 V

DG540

V+

V–

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.