Section 1
OM6530-C1-00
19 November, 2019
1-5
1.1.4.
CIRCUIT DISCUSSION
The Model 6530 TeraOhm Bridge-Meter measures high values of resistance by charging
a small capacitor through the resistance to be measured. An operational integrator is
shown in Figure 1-4. The equations for this integrator are as follows:
delta V
out
(t) V
in
(The equality is not exact
______________
=
______
but is extremely close when
(1)
delta t R x C
the voltage gain is high)
or:
V
in
x delta t
R =
_________________
(2)
C x delta V
out
Where delta t = a change in time and delta V
out
= a change in output voltage V
out
over time
delta t.
When current is being measured, V
in
can be replaced by iR which simplifies (1) to the
form:
C x delta V
out
i =
________________
delta t
(3)
In the Model 6530:
*
V
in
is the test voltage for resistance measurement.
*
C is a stable capacitor selected from the nominal values of 27 pF, 270 pF or
2700 pF.
*
delta V
out
is the potential difference between two threshold voltages placed
symmetrically above and below ground (V
out
= 2V
thresh
where V
thresh
is
selectable from 0.1 volt, 1 volt or 10 volts).
In equations (2) and (3), all terms are constant except R, i and delta t. Therefore
delta t is proportional to R or inversely proportional to the current i. During
normal operation the 6530 calculates the unknown resistance R or current i by
taking measurements of the time delta t.
