Datasheet

MAX4554/MAX4555/MAX4556
Force-Sense Switches
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GUARD AMPLIFIER
VOLTAGE OR
CURRENT SOURCE
LOGIC
LOAD
2
1
0
1
IN
NC4
GND
LOAD2
LOAD1
NC2
COM1
V-
COM2
IN
COM3
COM4
IN1
IN2
IN3
IN4
NC1
NC3
MAX4555
V+
VL
Figure 8. Using the MAX4555 to Switch 3-Wire Guarded Circuits from One Source to Two Loads
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.
Switching 8-Wire Guarded Circuits
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-
changing loads and sources, the circuits can be
reversed so that they switch two sources to a single
load. The two loads are shown isolated from each
another, but if they have a common connection then the
circuit must remain as shown in order to maintain accu-
rate load voltage.
High-Frequency Performance
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
pass higher frequencies with less attenuation. At
10MHz, off-isolation between input or output signals is
approximately -30dB in 50Ω systems, degrading
(approximately 20dB per decade) as frequency
increases. Higher circuit impedances also degrade off-
isolation.