Signametrics
28
up to 330 k
Ω
. 4-wire measurements are disabled above 330 k
Ω
since the extra set of leads can actually
degrade
the
accuracy, due to additional leakage and noise paths.
Figure 4-3. The
I- and I+
sense leads should be closest to the body of the resistor when making 4W
Ω
mea
s
e
SE. This function is implemented for the SMX2040, SMX2042 and SMX2044.
ue to it’s different hardware, the SMX2042 is different in that one of the DMM relays is being used to perform
this
ard voltage prevents
e shunting of the DMM Ohms source current from the resistor-under-test to other components. The Guard Source
respect to the chassis or
ny other DMM terminal. Do not apply any voltages greater than 35 V to the DIN connector pins. Violating
a
normal resistance measurement, the 510
Ω
and 220
Ω
would shunt most of the DMM Ohms source current, causing
surements.
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 variou
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. Th
default value of this function is FAL
D
operation, which can be heard.
4.3.4 6-wire Guarded Resistance Measurement (SMX2044)
The SMX2044 provides a guarded 6-wire resistance measurement method. It is used to make resistance
measurements when the resistor-under-test has other shunting paths that can cause inaccurate readings. This method
isolates the resistor-under-test by maintaining a guard voltage at a user-defined node. The gu
th
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
a
this limit may result in personal injury and/or permanent damage to the DMM.
Example: Assume a 30 k
Ω
resistor is in parallel with two resistors, a 510
Ω
and a 220
Ω
, connected in series. In
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