Boonton Electronics 71A, 71AR - Differential Capacitance Measurements

from approximately 10 pF to 1000 pF. Because this correc-
tion factor is very nearly a direct function of cable length,
for small corrections (10% or less), the correction required
for pairs of other lengths can be readily computed.
3.5 Increasing the Capacitance Measuring Range
The upper capacitance limt of 1000 pF may be increased by a
factor of up to approximately 10 (to O.Oll-LF) by use of the
"DIFF" posts, as follows:
(a) Set the instrument to the 1000 pF range.
(b) Attach a high quality mica capacitor whose value
accurately known to the "DIFF" terminals.
is
(c) Attach the specimen to the "TEST" terminals.
(d) The sum of the meter reading and the value of the
capacitor at the "DIFF" terminals is the value of
the test.
NOTE
The value of the capacitor at the "DIFF" terminals
should be selected to provide at least 15% of full
scale deflection for most satisfactory results. It
should be noted that while significant measurements
can be made in this manner, accuracy will be degraded
somewhat from that specified for the normal operating
range of the instrument.
3.6 Differential Capacitance Measurements
Measurement of the capacitance differential between two speci-
mens may be made by attaching one sample at the "DIFF" terminals
and the other to the "TEST" terminals, using all but the 1 pF
range. The scale will read the capacitance differential
directly in pF. (NOTE: If a down-scale reading is obtained,
interchange the specimens connected to the "TEST" and "DIFF"
posts for an up-scale reading.) The measurements must be made
using the range on which one of the specimens alone would
normally be measured, or one range below this for increased
resolution. However, it is important not to switch down more
than I range or serious errors may be introduced. Remote
differential measurements may also be made using coaxial cables
3-10
7lA
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