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Operation
2-16
Using the coulombs function to measure current
Note that the coulombs function can also be used to measure
current. The advantage of doing so is that noise in the mea-
surement is substantially reduced because of the integrating
process. To measure current using the coulombs function,
proceed as follows:
1. Place the instrument in the coulombs function, and se-
lect the desired range, or use auto-ranging, if desired.
2. Enable zero check, and connect the current to be mea-
sured to the INPUT jack (see Figure 2-9).
3. Disable zero check, and note the charge measurement at
the end of a specific interval of time (for example, 10
seconds).
4. To determine the current, simply divide the measured
charge by the time in seconds. For example, if a charge
of 12nC is seen after a 10-second interval, the current is
12nC/10 = 1.2nA. (Using Data Store at a 10-second rate
can simplify the process.)
5. As an alternative to the above procedure, connect a chart
recorder to the 2V ANALOG OUTPUT (paragraph 2.9),
and graph the measured charge. Since the current is giv-
en by I=dQ/dt, the current at any point is equal to the
slope of the graph at that point, after applying the appro-
priate scaling factor (100pC/V, 200pC range; 1nC/V,
2nC range; 10nC/V, 20nC range).
CAUTION
Connecting PREAMP OUT, COM, or
2V ANALOG OUTPUT to earth while
floating input may damage the instru-
ment.
Charge measurement considerations
A primary consideration when making change measure-
ments is the input offset current of the integrating amplifier.
Any such current is integrated along with the input signal
and reflected in the final reading. The Model 6512 has a max-
imum input offset current of 5
×
10
-15
A at 23°C. This value
doubles every 10°C. This input offset current translates into
a charge of 5
×
10
-15
C per second at a temperature of 23°C.
This value must be subtracted from the final reading to ob-
tain the correct value.
When using an external voltage source, the input current
should be limited to less than 1mA by placing a resistor in se-
ries with the high input lead. The value of this resistor should
be at least: R=1000
×
V
S
(in ohms) where V is the voltage
source value, or the compliance of the current being integrated.
2.7.7 Resistance measurements
The Model 6512 makes resistance measurements using the
constant-current method. (A current is forced through the
DUT, and the voltage across the DUT is measured.). The in-
strument can resolve resistances as low as 0.1
Ω
and measure
as high as 200G
Ω
.
To measure resistance with the Model 6512, use the follow-
ing procedure:
1. Turn on the power, and allow a two-hour warm-up peri-
od for rated accuracy.
2. Press the OHMS button to select the ohms mode.
3. For maximum accuracy, place the instrument on the
2k
Ω
range, and zero the instrument by enabling zero
check and then pressing the ZERO CORRECT button.
4. Select the desired range, or use auto-ranging, if desired.
5. Connect the triax cable to the INPUT jack. Keep the cable
as short as possible to minimize the effects of cable ca-
pacitance. Connect the other end of the cable to the resis-
tance to be measured, as shown in Figure 2-12. For
measurements above 1G
Ω
, it is recommended that you
use guarded connections, as described in paragraph 2.7.4.
6. Disable zero check, and allow the reading to settle.
7. Take the reading from the display. The exponent may be
placed in either the alpha or numeric modes, as de-
scribed in paragraph 2.5.
Summary of Contents for 6512
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