Terminal Configuration.3-10 - HP 4262A Operating And Service Manual

3-30. TERMINAL CONFIGURATION.
IMPORTANT!
3-31. Connection of DUT. The 4262A Unknown
terminals consists of five binding post (type) con¬
nectors: Heim, H
pot , Lcur, Lpot
and GUARD.
By connecting the stationary shorting straps to
appropriate terminals, the UNKNOWN terminals
can be adopted for the desired measurement ter¬
minal configuration: the two, three, four or five
terminal method.
For measurements of samples having a medium
order of impedance (100ft to lOkft), the convenient
two terminal method is suited to measurement re¬
quirements for good accuracy as well as for ease
in connecting the sample. When converting to two
terminals, shorting straps are attached to the
UNKNOWN Hcur and
Hpot
terminals, and
Lcur
and
Lpot
terminals, respectively.
High impedance samples (greater than Ikft) --
which includes low capacitance, high inductance
and high resistance — should be measured by the
three terminal method to eliminate the effects of
stray capacitances on the measurements. For
this purpose, the guard conductor of the sample is
connected to the instrument GUARD terminal.
In the measurement of low impedance samples
(less than lkft), efforts should be made to elimi¬
nate the effects of contact resistance, lead resist¬
ance, residual inductance and other residual
parameters in the measuring apparatus. Four
terminal configuration measurements allow stable,
accurate measurement of high capacitance, low
inductance and low resistance samples at mini¬
mum incremental errors in the measurement of
low impedance samples. In the four terminal
method, the shorting straps are disconnected
to separate potential leads from current leads.
Thereby, the characteristics of the sample can be
precisely determined by the instrument irrespec¬
tive of the various residual parameters present in
the measuring signal current path. To ensure the
best accuracy, the potential leads should be con¬
nected near to the sample.
The five terminal method, which adds the guard
conductor to the four terminal configuration, ex¬
pands the applicable measurement range into the
higher impedance regions. Thus, this method
covers a broad range of measurements from low
to high impedance samples at the measuring fre¬
quency of the 4262A.
When test fixtures and test leads used have a
shielding conductor and are designed to consider
residual impedance, the measurement limitations
described above for the individual terminal con¬
figurations can vary to some extent depending on
the particular characteristics of the fixture and
connections. Three accessories, the 16061A Test
Fixture, the 16062A Test Leads, and thel6063A
Test Leads are available. The characteristics of
these accessories and applicable measurement
ranges are outlined in Figure 3-3. These acces¬
sories make it easy to construct the desired ter¬
minal configuration.
FOR CERTAIN TERMINAL MEAS¬
UREMENT CONFIGURATIONS, THE
Hcur
TERMINAL MUST BE CON¬
NECTED TO
Hpot
TERMINAL AND
THE
Lcur
TERMINAL CONNECTED
TO THE
Lpot
TERMINAL. OTHER¬
WISE, THE DISPLAYS WILL HAVE
NO MEANING AND THE LIFE OF
THE RELAYS USED IN THE INSTRU¬
MENT WILL SOMETIMES BE SHORT¬
ENED.
Note
The 4262A can not measure a sample
which has one lead connected to earth
(grounded).
3-32. OFFSET ADJUSTMENT.
3-33. Since test fixtures and test leads have
different inherent stray capacitances and residual
inductances, the measured value obtained with
respect to the same sample may possibly differ
depending on the test fixture (leads) used. These
residual factors can be read from the 4262A dis¬
play by properly terminating (short or open) the
measurement terminals of the test jig. The front
panel C ZERO ADJ and L ZERO ADJ controls
permit compensation for these residual factors
and can eliminate measurement errors due to the
test jig. The capacitance or inductance readout
can be set to zero for the particular test jig used
with the instrument. In capacitance and induct¬
ance measurements, an incomplete offset adjust¬
ment causes two types errors:
1) Deviation from zero counts.
When a small capacity or a small inductance
is measured, the measured capacitance
(inductance) value becomes the sum of the
capacitance (inductance) of sample and the
stray capacitance (residual inductance) of
test jig. The effects of the residual factors
are:
Cm = Cx + Cst
Lm = Lx + Lres
Where, subscripts are
m: measured value,
x: value of sample,
st: stray capacitance,
res: residual inductance.
Both Cst and Lres cause the same measure¬
ment error and are independent of sample
value.