DG540 Datasheet, PDF (9/10 Pages) TEMIC Semiconductors – Wideband/Video T Switches

DG540/541/542
Applications (Cont’d)
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).
3. Capacitors should have good high frequency
characteristics - tantalum bead and/or monolithic
ceramic types are adequate.
Suitable decoupling capacitors are 1Ć to 10ĆmF
tantalum bead, plus 10Ć to 100ĆnF ceramic.
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:
+15 V
+
1. It allows flexibility in analog signal handling, i.e.,
with V- = -5 V and V+ = 12 V; up to "5ĆV ac
signals can be controlled.
C₁
C₂
V+
2. The value of on capacitance [C_S(on)] may be
S₁
D₁
reduced. A property known as `the bodyĆeffect' on
S₂
D₂
the DMOS switch devices causes various
parametric effects to occur. One of these effects is
S₃
DG540
D₃
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
GNDs
V–
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.
C₁ = 10 mF Tantalum
C₂ = 0.1 mF Ceramic
C₁
+
C₂
–3 V
3. V- eliminates the need to bias the analog signal
using potential dividers and large coupling
capacitors.
Figure 7. Supply Decoupling
Decoupling
Board Layout
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.
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.
Siliconix
9
S-53694—Rev. E, 28-May-97