Fo rc e -S e n s e S w it c h e s
/MAX456
Figure 8 shows how to switch a single guarded voltage
or current source between two grounded loads using a
MAX4555. By interchanging loads and sources, the cir-
cuits can be reversed so that two sources are switched
to a single load.
Hig h -Fre q u e n c y P e rfo rm a n c e
Although switching speed is restricted, once a switch is
in a steady state it exhibits good RF performance. In
50Ω systems, signal response is reasonably flat up to
50MHz (see Typical Operating Characteristics). The
force switches have lower on-resistance, so their inser-
tion loss in 50Ω systems is lower. Above 20MHz, the
on-response has several minor peaks that are highly
layout dependent. The problem with high-frequency
operation is not turning the switches on, but turning
them off. The off-state switches act like capacitors and
p a s s hig he r fre q ue nc ie s with le s s a tte nua tion. At
10MHz, off-isolation between input or output signals is
a p p roxima te ly -30d B in 50Ω s ys te ms , d e g ra d ing
(a p p roxima te ly 20d B p e r d e c a d e ) a s fre q ue nc y
increases. Higher circuit impedances also degrade off-
isolation.
S w it c h in g 8 -Wire Gu a rd e d Circ u it s
Figure 9 shows how to switch a single 8-wire guarded
force-sense voltage or current source between two
loads using two MAX4556s or two MAX4554s. By inter-
c ha ng ing loa d s a nd s ourc e s , the c irc uits c a n b e
reversed so that they switch two sources to a single
loa d . The two loa d s a re s hown is ola te d from e a c h
another, but if they have a common connection then the
circuit must remain as shown in order to maintain accu-
rate load voltage.
V+
V-
VL
MAX4555
COM1
COM2
NC1
NC2
GUARD AMPLIFIER
LOAD2
COM3
NC3
NC4
COM4
IN1
IN2
GND
LOAD1
IN3
IN4
VOLTAGE OR
CURRENT SOURCE
IN
LOGIC
IN
0
1
LOAD
2
1
Figure 8. Using the MAX4555 to Switch 3-Wire Guarded Circuits from One Source to Two Loads
______________________________________________________________________________________ 23
MAXIM [ MAXIM INTEGRATED PRODUCTS ]
