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).
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.
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.). Reduction in V+ causes an increase in r
DS(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.
V– eliminates the need to bias the analog signal using
potential dividers and large coupling capacitors.
V+
S
1
S
2
S
3
S
4
GNDs
V–
D
1
D
2
Suitable decoupling capacitors are 1- to 10-mF tanta-
lum bead, plus 10- to 100-nF ceramic.
+15 V
+
C
1
C
2
2.
DG540
D
3
D
4
C
1
= 10
mF
Tantalum
C
2
= 0.1
mF
Ceramic
C
1
+
–3 V
C
2
3.
FIGURE 7.
Supply Decoupling
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.
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.
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.
Rules:
1.
2.
3.
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 have good high frequency
characteristics – tantalum bead and/or monolithic ceramic
types are adequate.
2.
3.
Document Number: 70055
S-00399—Rev. G, 13-Sep-99
www.vishay.com
S
FaxBack 408-970-5600
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