Agilent Technologies 4338B, Руководство по эксплуатации

Страница: 172 / 228

Dry Circuit Loop 4338B
The measured impedance , Z
M
, contains the measurement errors . Figure 7-1 (a) shows the
existence of residual impedance between the UNKNOWN terminals and the DUT , which is due
to the test xture . If the DUT's impedance , Z
DUT
, is low , the test xture's residual impedance
has more eect on a measurement than does the xture's stray admittance . The 4338B has
a SHORT correction function to cancel the residual impedance of the test xture so low
impedance devices can be accurately measured.
Figure 7-1 (b) describes the SHORT measurement technique which measures the residual
impedance of the test xture , Zs . A SHORT measurement is performed by connecting the test
electrodes directly together or connecting a shorting device to the electrodes . The residual
impedance of the shorting device must be signicantly lower than the DUT's impedance ,
otherwise it will directly aect the measurement results .
After the SHORT measurement is performed, the DUT's impedance , Z
DUT
, is calculated using
the equation given in Figure 7-1 (c).
Measurement Range
F or any measurement range , the maximum accuracy is obtained when the measured impedance
is close to the full-scale value of the measurement range being used. Conversely , if the
measured impedance is much lower than the full-scale value of measurement range being used,
the measurement accuracy will be reduced. This sometimes cause a discontinuity occurs in the
measurement values at the measurement range boundaries . If measurement range is set to
A uto range , the impedance curve will skip when impedance range change occurs . T o prevent
this from occurring, the impedance range should be set to the Hold range mode .
Dry Circuit Loop
When the voltage applied between the contacts is high, the oxidation lm which is formed
between the contact is destroyed, and the resistance value measured may be smaller than
under actual use . Moreover , once the oxidation lm is destroyed, the contact will never
recover the its former condition, even if the voltage across the contact is cut o.
A ccording to the recommended standards of the International Electrotechnical Commission
(IEC), measurement of contact resistance using ac test signal should be conducted using
instruments whose test signal peak voltage values are 20 mV or less .
The 4338B 's test voltage level goes to 0 V if the peak voltage value across the DUT exceeds
20 mV. The 4338B's dry circuit loop prevents the test signal voltage applied across the DUT
from exceeding 20 mV. Thus , even when an unknown DUT is measured without special
preparation, the contact resistance is still accurately measured without disturbing the state of
the contacts .
7-2 Measurement Basics
Artisan Technology Group - Quality Instrumentation ... Guaranteed | (888) 88-SOURCE | www.artisantg.com