MAX4554 Maxim, MAX4554 Datasheet - Page 14

• Manufacturer Part Number: MAX4554
• Description: The MAX4554/MAX4555/MAX4556 are CMOS analog ICs configured as force-sense switches for Kelvin sensing in automated test equipment (ATE)
• Manufacturer: Maxim

Force-Sense Switches
When a precise voltage must be applied to a load that
draws appreciable current, the resistance of the con-
ductors connecting the source and the load can
degrade the load voltage. The resistance of the con-
ductors forms a voltage divider with the load, so that
the load voltage is lower than the source voltage. The
greater the distance between the source and the load,
and the greater the current or conductor resistance, the
greater the degradation. The resulting signal reduction
can be overcome and the signal at the load guaranteed
by using a 4-wire technique known as Kelvin sensing,
or force-sense.
The basic idea behind the force-sense philosophy is to
use four wires, forcing a voltage or current through two
high-current wires to the load, and measuring (sensing)
the voltage with two separate wires that carry very low
or negligible current. One of two basic configurations is
used, depending on whether or not feedback is em-
ployed:
1) The sensed voltage can be completely independent
2) The sensed voltage can be part of a feedback cir-
In all cases, the resistance of the high-current conduc-
tors can be ignored and the sensed voltage is an accu-
rate measure of the load (or resistor’s) voltage, despite
appreciable voltage loss in the wires connecting the
source and load.
There are two limitations to this scheme. First, the maxi-
mum source voltage (compliance) must be able to
overcome the combined voltage loss of the load and
the connecting wires. In other words, the conductors in
the force circuit can have significant resistance, but
there is a limit. Second, the impedance of the sensing
circuit (typically a voltmeter, A/D converter, or feedback
amplifier) must be very high compared to the load
resistance and the sense wire resistance. These limita-
tions are usually simple to overcome. The source com-
pliance is usually required to be only a volt more than
the load voltage, and the sense circuit usually has a
multimegohm impedance. Typical 4-wire force-sense
configurations are shown in Figure 1.
14
______________Force-Sense Philosophy
of the forced voltage or current, as in the case of a
4-wire ohmmeter, where a constant current is forced
through one pair of wires and the voltage at the
resistor is measured by another pair.
cuit to force the load voltage to the desired value,
as in the case of a 4-wire power supply. (In rare
cases, this method is also used to measure resis-
tance; the source is forced to produce a desired
voltage in the resistor, and the source current
required to achieve this voltage is measured.)
______________________________________________________________________________________
Figure 1. 4-Wire Force-Sense Measurements
ARROWS INDICATE SIGNAL DIRECTION, NOT POLARITY
CURRENT SOURCE
VOLTAGE SOURCE
CURRENT SOURCE
4-WIRE RESISTANCE MEASUREMENT (CONSTANT VOLTAGE)
4-WIRE RESISTANCE MEASUREMENT (CONSTANT CURRENT)
VOLTAGE
VOLTAGE
MEASUREMENT
MEASUREMENT
VOLTAGE
MEASUREMENT
FEED-
BACK
FEED-
BACK
V
WIRE AND TERMINAL RESISTANCE
WIRE AND TERMINAL RESISTANCE
WIRE AND TERMINAL RESISTANCE
V
V
V
4-WIRE POWER SUPPLY
SENSE VOLTAGE
SENSE VOLTAGE
SENSE VOLTAGE
SENSE VOLTAGE
FORCE VOLTAGE
FORCE VOLTAGE
FORCE CURRENT
FORCE CURRENT
FORCE CURRENT
FORCE CURRENT
SENSE VOLTAGE
SENSE VOLTAGE
LOAD
MEASURED
RESISTANCE
MEASURED
RESISTANCE