DG540 Datasheet, PDF (9/11 Pages) TEMIC Semiconductors – Wideband/Video T Switches

DG540/541/542
Vishay Siliconix
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).
Suitable decoupling capacitors are 1- to 10-mF tanta-
lum bead, plus 10- to 100-nF ceramic.
+15 V
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:
+
C₁
C₂
1. It allows flexibility in analog signal handling, i.e., with V– =
V+
–5 V and V+ = 12 V; up to "5-V ac signals can be
controlled.
S₁
D₁
2. The value of on capacitance [C_S(on)] may be reduced. A
S₂
D₂
property known as ‘the body-effect’ on the DMOS switch
DG540
devices causes various parametric effects to occur. One
S₃
D₃
of these effects is the reduction in C_S(on) for an increasing
V body–source. Note, however, that to increase V–
S₄
D₄
normally requires V+ to be reduced (since V+ to V– = 21 V
max.). Reduction in V+ causes an increase in r_DS(on),
hence a compromise has to be achieved. It is also useful
GNDs
V–
to note that optimum video linearity performance (e.g.,
differential phase and gain) occurs when V– is around
–3 V.
C₁ = 10 mF Tantalum
C₂ = 0.1 mF Ceramic
C₁
C₂
+
3. V– eliminates the need to bias the analog signal using
–3 V
potential dividers and large coupling capacitors.
Decoupling
FIGURE 7. Supply 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.
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.
Document Number: 70055
S-00399—Rev. G, 13-Sep-99
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