29
Signametrics
flow through the shunting path. With this “guarding”, the
MX2044 accurately measures the 30 k
Ω
resistor.
an inaccurate reading. By sensing the voltage at the top of the 30 k
Ω
, and then applying this same voltage to the
junction of the 510
Ω
and 220
Ω
, there is no current
S
Figure 4-4. 6-wire guarded in-circuit ohms measurement configuration.
The current compliance of the Guard Force is limited to a maximum of 20 mA and is short-circuit protected. The
resistor connected between the low of the 4-wire terminals and the guard point is the burden resistor, or R
b
. Due to
the limited guard source current, this resistor can not be lower than R
mple, selecting the 330
and vise
versa. For cases where this limit is a problem, simply set the measurement polarity such that R
a
is the higher of the
two burden resistors.
To measure values greater than 330 k
Ω
using the 6-wire guarded method, it is necessary to select the 2-wire ohms
function, and maintain the 6-wire connection as in Figure 4-4 above.
4.3.5 Leakage Measurements (SMX2044)
The SMX2044 measures leakage currents by applying a DC voltage across the load and measuring the current
through it. An external shunt resistor, R
S
is used to sense the current. See Figures 4-5 for connection. The DC
voltage at which leakage is measured is set using
DMMSetDCVSource()
, and the value of R
S
is entered using
DMMSetExternalShunt()
(the default is 1 M
Ω
). Leakage current is read using
DMMRead(), DMMReadStr()
or
DMMReadNorm()
functions. Depending on the value of R
S
and the leakage current, the test voltage may be set
between –10V and +10 V. Specifically; the maximum voltage is limited to 10.2V – (R
S
* I
L
). The leakage current
(I
L
) measurement range and accuracy depends on the shunt resistor being used. The range is set as 3.3V/R
S
, with
resolution of 10uV/R
S
. For instance, with R
S
equal to 1M
Ω
, the maximum leakage current is 3.3uA with resolution
of 10pA, and at full scale the maximum test voltage is 6.7V. The test voltage is adjusted whenever a measurement is
made. Therefore, if the leakage current being measured varies significantly over time, it is best to repeatedly read
the DMM allowing the DMM to make corrections to the internal source voltage, compensating for drop across the
shunt. Performing open terminal calibration will imrove the accuracy of this function (use
DMMOpenTerminalCal()
with all terminals open). Refer to Figures 4-5. See section 2.6 for specifics. Setting
the source to closed loop mode,
DMMSetSourceMode()
, will improve the accuracy of the test voltage.
bmin
: R
bmin
= I
o
* R
x
/ 0.02, where I
o
is the
ohms source current for the selected range, and R
x
is the resistance being measured. For exa
Ω
range and measuring a 300
Ω
resistor imposes a limit on R
b
of at least 15
Ω
or greater. Since the top burden
resistor, R
a
, does not have this limit imposed on it, selecting the measurement polarity, R
a
can become R
b
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