Operator`s manual

Figure 4-3. The I- and I+ sense leads should be closest to the body of the resistor when making 4WΩ
measurements.
4.3.3 Using Offset Ohms function
Inadvertent parasitic leakage currents, Thermo voltaic voltages and other voltages in series can effect resistance
measurements with the measured resistance. This is common particularly when doing in-circuit measurements, and
it could manifest as having a significantly different readings when changing Ohms ranges. In addition to eliminating
such error, this function can also be used to measure internal resistance of low value voltage sources such as various
batteries and supplies. Use the normal 2-Wire or 4-Awire Ohms connection, and set the Offset Ohms to the enabled
or disabled state using the DMMSetOffsetOhms() function. When set TRUE, the measurement rate will be about
1/10
th
of the set DMM rate. Both negative and positive polarity voltages can be handled as long as the total voltage
including the Ohms source current times the measured resistance plus the parasitic voltage are less than 3.7V. To
calculate this voltage consult the specification part of this manual for the specific current for each Ohms range. The
default value of this function is FALSE. This function is implemented for the SM2040, SM2042 and SM2044. Due
to it’s different hardware, the SM2042 is different in that one of the DMM relays is being used to perform this
operation, which can be heard.
4.3.4 6-wire Guarded Resistance Measurement (SM2044)
The SM2044 provides a guarded 6-wire resistance measurement method. It is used to make resistance
measurements when the resistor-under-test has other shunting paths, which can cause inaccurate readings. This
method isolates the resistor-under-test by maintaining a guard voltage at a user-defined node. The guard voltage
prevents the shunting of the DMM Ohms source current from the resistor-under-test to other components. The
Guard Source and Guard Sense terminals are provided at pins 1 and 6 of the DIN connector respectively.
Warning! The DIN connector pins are only protected to a maximum of 35 V with respect to the PC chassis
or any other DMM terminal. Do not apply any voltages greater than 35 V to the DIN connector pins.
Violating this limit may result in personal injury and/or permanent damage to the DMM.
Example: Assume a 30 kΩ resistor is in parallel with two resistors, a 510 Ω and a 220 Ω, which are connected in
series with each other. In a normal resistance measurement, the 510 Ω and 220 Ω would “swamp” the measurement
shunting most of the DMM Ohms source current. By sensing the voltage at the top of the 30 kΩ, and then applying
this same voltage to the junction of the 510 Ω and 220 Ω, there is no current flow through the shunting path. With
this “guarding”, the SM2044 accurately measures the 30 kΩ resistor.
Signametrics 28