DG641 Datasheet, PDF (9/10 Pages) Vishay Siliconix – Low On-Resistance Wideband/Video Switches

DG641/642/643
Vishay Siliconix
APPLICATIONS
Power Supplies
+15 V
Power supply flexibility is a useful feature of the
DG641/642/643 series. It can be operated from a single
positive supply (V+) if required (V– connected to ground).
+
C₁
C₂
V+
S₁
D₁
Note that the analog signal must not exceed V– by more than
S₂
–0.3 V to prevent forward biasing the substrate p-n junction.
S₃
DG64X
D₂
D₃
The use of a V– supply has a number of advantages:
S₄
D₄
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.
V–
GNDs
C₁
C₂
2. The value of on capacitance [C_S(on)] may be reduced. A
property known as ‘the body-effect’ on the DMOS switch
devices causes various parametric effects to occur. One
of these effects is the reduction in C_S(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.). A reduction in V+ causes an increase in r_DS(on),
hence a compromise has to be achieved. It is also useful
to note that tests indicate 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
+
–3 V
FIGURE 7. Supply Decoupling
3. V– eliminates the need to bias the analog signal using
potential dividers and large coupling capacitors.
Board Layout
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 DG641/642/643 series 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. Suitable decoupling capacitors are 1- to 10-mF
tantalum bead, plus 10- to 100-nF ceramic or polyester.
Rules:
PCB layout rules for good high frequency performance must
also be observed to achieve the performance boasted by
these analog switches. 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.
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 be of a suitable type with good high
frequency characteristics – tantalum bead and/or ceramic
disc types are adequate.
Document Number: 70058
S-52433—Rev. E, 06-Sep-99
Figure 8 shows a 4-channel video multiplexer using a DG641.
In Figure 9, two coax cables terminated on 75 W bring two
video signals to the DG642 switch. The two drains tied together
lower the on-state capacitance. An Si582 video amplifier
drives a double terminated 75-W cable. The double terminated
coax cable eliminates line reflections.
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