Theory Of Operation; Overall Measurement Theory; Ohm's Law - Agilent Technologies 4349B Service Manual

Theory of Operation

This section describes the overall operation of the 4349B and the operation of each assembly.

Overall Measurement Theory

The 4349B measures the resistance of a Device Under Test (DUT) in the following manner.
In Figure 4-1, a DUT is connected in series with test voltage source V, and ammeter A. If the
ammeter measures I amperes when the source voltage is V, the DUT's resistance R is expressed
by (when ignoring r):
This equation means that if the DUT resistance R is extremely high, the current owing the
circuit is extremely low. For example, if R = 1 G , and V = 1 V, the current owing is 1 nA
(10 9 A). The 4-Channel High Resistance Meter 4349B is essentially four (or two) sets of very
sensitive current meters.
In practice, ammeter input resistance r, which is approximately 1 k , decreases
Note
the current owing through the DUT. The actual current, I, owing through the
DUT, R, is expressed by:
In the current measurement mode, the 4349B displays the actual current I
for the measurement result. For example, if R = 100 k , and V = 1 V, the
measurement result will be 9.9
In the resistance measurement mode, on the other hand, the 4349B displays the
\derived" DUT resistance which coincides with the DUT resistance. In other
words, the 4349B shows the resistance calculated by:
For example, if R = 100 k , and V = 1 V, the measurement result will be
100 k , even the actual current owing through the DUT, thus through the
ammeter with r is 9.9
Figure 4-1. Ohm's Law
V
=
R
I
V
=
I
+
R r
1
A (

100 +1
k
V
=
R r
I
A.

A ( 1 100
) rather than 10

k
).
k
T roubleshooting 4-3