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3.6.7 Cable-Related Errors and How
to Correct for them
Test-fixture extension can introduce measurement
error so that specified accuracy may not be met.
In other words, some of the series impedances and
ground capacitances associated with connecting a
remote DUT can be large enough to introduce terms
that add significantly to the error permitted by the
accuracy specifications. In this paragraph, we discuss
the cable-related sources of error, how to estimate it,
and how to correct for it.
NOTE: We define the "normal DUT interface" here as
the 1689-9600 remote test fixture attached via 1689-
9602 BNC cable to the 1693 Digibridge.
The Digibridge automatically compensates for ca-
pacitance between "high" terminals and "low" in
the zero calibration. Also the 5-terminal "Kelvin"
circuitry is designed to minimize the effect of other
cable and test-fixture impedances on measurement
accuracy. However, the following terms can be
significant under some circumstances, particularly
if a long extender cable is used to reach beyond the
"normal DUT interface".
• Acm, common-mode accuracy term, most
significant on range 4.
• Ald, capacitive-loading accuracy term, most
significant on range 1, at high frequency.
Operation
Table 3-12: Inductance of Straight Round Wires
Formulas and typical constants are given below for
obtaining useful approximations to these terms.
Common-Mode Accuracy Tenn. (Applies to any exten-
sion beyond instrument.)
Acm = ±[(.05) (r + jx) / Z]% of measured impedance
where (r + jx) is the series impedance in the IL lead
including the cable, and Z is the DUT impedance.
However, if you have selected SERIES EQUIV CKT,
it is more useful to split Acm into the following 2
components, for treating Ls and Cs errors separately
from Rs error:
Acmx = ±[(.05) (x) / (DUT reactance)]% of measured Ls
or Cs
Acmr = ±[(.05) (r) / (Rs)] % of measured Rs
If either of these is significant, one can calculate and
use it to correct each corresponding measured value.
However, first make careful measurements with a
known low-impedance DUT, to determine whether
each correction should be positive or negative for
your particular test fixture.
Capacitive-Loading Error Term
Ald = [(.003) (Krange) (f2)(Csn / 1000 pf)] %of principal
measured value
1693 RLC Digibridge
57
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