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1693 RLC Digibridge
3.6.6 Accuracy Enhancement by
Special Attention to Short-
Circuit Inductance
The ratio display (paragraph 3.3.6) enables very
high-resolution measurements of low inductance and
high capacitance - even beyond the limits of normal
RLC displays. If such measurements are planned,
especially if the test frequency is high, the inductance
of the "short circuit" used in the normal zeroing pro-
cedure should be considered.
The short circuit provided by a wire inserted into
the Digibridge test fixture (paragraph 3.1.3) has an
effective inductance in series with its very low resis-
tance. This inductance typically has a magnitude of
several nanohenries. To enhance accuracy of mea-
surements in which a few nanohenries of inductance
are significant, use a properly chosen shape, size, and
orientation of wire for the short circuit. For greatest
accuracy, particularly for axial-lead DUTs, also cor-
rect the measured value by suitable calculation.
Accuracy Enhancement Procedures. Three methods
are described. See Figure 3-32.
If measurements are to be made without any adap-
tors (radial-lead DUT), use a piece of no. 18 (AWG)
wire, 2.2 cm long (7/8 in.), bent into a hair-pin shape
as shown in "A". Press this wire fully down into the
Digibridge test fixture , keeping the straight sides of
the wire vertical. Measurement results now depend
on the geometry of the DUT leads, but will typically
56
Figure 3-32: Recommended Wire Shapes for Zeroing
contain a related error less than 10 nH. For even
smaller error, correct inductance measurements by
adding 5 nH to the displayed value.
If measurements are to be made with adaptors (axial-
lead DUT), for most situations, make the short-circuit
calibration WITHOUT the adaptors. Use a piece of
no. 18 (AWG) wire, 5 cm long (2 in.), bent into a
rectangular shape as shown in "B", Press this wire
fully down into the Digibridge test fixture, keeping the
center of the wire above the center of the fixture and
the straight sides of the wire vertical. Measurement
results (with adaptors) will typically contain a related
error of less than 5 nH, which can be verified by mea-
suring a OUT consisting of a straight wire of known
inductance - refer to one of the accompanying tables
of inductances. (Wire length is measured between
points of contact in the adaptors.)
If measurements are to be made with adaptors (axial-
lead DUT), for greatest accuracy (requiring a manual
calculation for every measurement), make the short-
circuit calibration WITH adaptors spaced exactly as
they will be for the DUT. Use any piece of straight
wire having a known self inductance La - refer to
Table 3-12. Measure the DUT using the series equiva-
lent circuit. Then make the following calculation for
each measurements:
• For an inductor: Ls = Lm + Lo
• For a capacitor: Cs = Cm / (1- ω2 LoCm =
approx Cm (1 + ω2 LoCm)
where Ls and Cs are the corrected series values, Lm
and Cm are the measured series values, ω represents
omega = 2 pi times frequency, and Lo is defined
above. (Refer to the specification: MIL 0-39010.)
Operation
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