Fo rc e -S e n s e S w it c h e s
/MAX456
8-WIRE PRECISION SOURCE-MONITOR
FORCE-SENSE AMPLIFIER
V+
CURRENT SENSE
+FORCE
+SENSE
+DRIVEN GUARD
TWINAX CABLE
V
V-
V+
GUARD AMPLIFIER
V-
V+
LEAKAGE CURRENT
LEAKAGE CURRENT
VOLTAGE SOURCE
LOAD
V-
V+
V-
GUARD AMPLIFIER
V +
-DRIVEN GUARD
-SENSE
-FORCE
V
CURRENT SENSE
TWINAX CABLE
V-
FORCE-SENSE AMPLIFIER
Figure 3. 8-Wire Guarded Force-Sense Measurements
Note that although the positive and negative circuits are
identical, they are not redundant. Both are always used,
even when one side of the load is grounded, because
maintaining a precision output voltage requires losses in
the ground leads to be corrected by a force-sense
amplifier. If more than one power supply and load are
operated together, and they have a common connec-
tion, this re q uire me nt b e c ome s e ve n more c ritic a l.
Separate 8-wire connections prevent current changes in
one load from changing voltage in the other load.
cables and connectors are used. The extra wires are
used for two reasons: 1) They provide better shielding
by having separate chassis grounds on each cable,
rather than separate ground wires external to the signal
c a b le s ; 2) In te s t e q uip me nt, whe re c onne c tion
changes are frequent, it is very convenient to use four
triax connectors or two quadrax (dual triax) connectors
for each load.
In addition, this method is slightly better for power sup-
plies or measurements that switch between constant
voltage and constant current, since separate driven
guards reduce circuit capacitance. Also, when trou-
bleshooting, it is convenient to be able to interchange
force and sense leads.
1 2 -Wire Me a s u re m e n t s
Figure 4 shows a 12-wire circuit, which is an elabora-
tion of the 8-wire system using separate driven guards
for the forc e a nd s e ns e wire s . Four s e ts of tria xia l
______________________________________________________________________________________ 17
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