Keithley 617 User Manual

Note that the coulombs function can also be used to measure
current. The advantage of doing so is that noise in the
measurement is substantially reduced because of the in-
tegrating procw. To measure current using the coulombs
function, proceed as follows:
1. Place the instrument in the coulombs function and select
the desired range, or use autoranging, if desired.
2. Enable zero check and connect the current to be measured
to the INPUT jack (see Figure Z-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
l2nC is seen after a lo-second interval, the current is
121010 = 1.2nA. (Using Data Store at a 10 second rate
can ease data taking).
5. As an alternative Lo the above procedure, connect a chart
recorder to the 2V ANALOG OUTPUT (paragraph 2.9)
and graph the measured charge. Since the current is given
by I=dQ/dt, the current at any point is equal to the slope
of the graph at that point, after applying the appropriate
scaling factor (loOpC/V, 2CGpC range; l&/V, 2nC range;
lonC/V, 2Onc range).
CAUTION CAUTION
Connecting PREAMP OUT, COM, or 2V ANALOG Connecting PREAMP OUT, COM, or 2V ANALOG
OUTPUT to earth while floating input may OUTPUT to earth while floating input may
damage the instrument. damage the instrument.
Charge Measurement Considerations: A primary considera-
tion when making charge measurements is the input offset
current of the integrating amplifier. Any such current is in-
tegrated along with the input signal and reflected in the final
reading. The Model 617 has a maximum input offset current
of 5 x lo-ISA at 23°C. This value double every 10°C. This
input offset current translates into a charge of 5 X lo-15C
per second at a temperature of 23OC. This value must be sub-
tracted from the fiil reading to obtain the correct value.
When using an external voltage source, the input current
should be limited to less than 1mA by placing a resistor in
series with the high input lead. The value of this resistor
should be at least: R=lOOOV (in ohms) where V is the
voltage across the capacitor, or the compliance of the cur-
rent being integrated.
2.7.7 Resistance Measurements
discussed in paragraph 2.8, uses the built in voltage source.
With the constant current method discussed here, the instru-
ment can resolve resistances as low as O.la and measure as
high as 2COGR.
To measure resistance with the Model 617, use the following
procedure:
1. Turn on the power and allow a two-hour warm-up period
for rated accuracy.
2. Press the OHMS button to place the instrument in the COT-
rect mode.
3. For maximum accuracv, place the instrument on the 2k0
4
5
range and zero the ins&&ent by enabling zero check and
then pressing the ZERO CORRECT button.
Select the desired range, or use autoranging, if desired.
Connect the Model 6011 or similar cable to the INPUT
jack. Keep the cable as short as possible to minimize the ef-
fects of cable capacitance. Connect the other end of the
cable to the resistance to be measured, as shown in Figure
2-12. For measurements above lGn, it is recommended
that you use guarded connections, as described in
paragraph 2.7.4.
6. Disable zero check.
7. Take the reading from the display. The exponent may be
placed in either the alpha or numeric modes, as described
in paragraph 2.5.
Resistance Measurement Considerations: When measuring
high resistance values, there are two primary factors that can
affect measurement accuracy and speed. Any leakage r&s-
tance in the connecting cable or test fixture can decrease the
actual resistance seen by the instrument. Also, capacitance of
the cable or input circuit can slow down the response time
considerably.
These two problems can be minimized by using guarding,
especially when measuring r&stances above lG% Guarding
is further discussed in paragraph 2.7.4. Noise pickup can also
be a problem, in which case the resistor must be shielded.
Connecl the shield to input low.
At low resistances, lead resistance can be a consideration.
Cancel the effects of lead resistance by shorting the input
leads and enabling suppress with zero check disabled. Leave
suppress enabled for subsequent measurements.
The Model 617 can make resistance measurements using two
different methods: the constant current method and the cons-
tant voltage method. The constant voltage method, which is
2-15

Keithley 617 Specifications

General

Brand	Keithley
Model	617
Category	Measuring Instruments
Language	English