dg641 Vishay, dg641 Datasheet - Page 9

• Manufacturer Part Number: dg641
• Description: Low On-resistance Wideband/video Switches
• Manufacturer: Vishay

Power Supplies
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).
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.
2.
3.
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- F
tantalum bead, plus 10- to 100-nF ceramic or polyester.
Rules:
1.
2.
3.
Document Number: 70058
S-52433—Rev. E, 06-Sep-99
It allows flexibility in analog signal handling, i.e., with V– =
–5 V and V+ = 12 V; up to
controlled.
The value of on capacitance [C
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
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
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.
V– eliminates the need to bias the analog signal using
potential dividers and large coupling capacitors.
Decoupling capacitors should be incorporated on all
power supply pins (V+, V–). (See Figure 7).
They should be mounted as close as possible to the
device pins.
Capacitors should be of a suitable type with good high
frequency characteristics – tantalum bead and/or ceramic
disc types are adequate.
S(on)
5-V ac signals can be
S(on)
] may be reduced. A
for an increasing
DS(on)
,
Board Layout
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.
2.
3.
Figure 8 shows a 4-channel video multiplexer using a DG641.
In Figure 9, two coax cables terminated on 75
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- cable. The double terminated
coax cable eliminates line reflections.
C
C
1
2
= 10 F Tantalum
= 0.1 F Ceramic
Use extensive ground planes on double sided PCB,
separating adjacent signal paths. Multilayer PCB is even
better.
Keep signal paths as short as practically possible, with all
channel paths of near equal length.
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.
S
S
S
S
1
2
3
4
FIGURE 7. Supply Decoupling
–3 V
V+
V–
www.vishay.com FaxBack 408-970-5600
DG64X
C
C
+15 V
1
1
+
+
DG641/642/643
Vishay Siliconix
GNDs
C
C
2
2
bring two
D
D
D
D
1
2
3
4
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