Using The Ohms Function As A Current Source - Keithley 6512 Instruction Manual

OFF
V, Ω GUARD
R X
Figure 2-12
Resistance measurement connections
Resistance measurement considerations
When measuring high resistance values, there are two prima-
ry factors that can affect measurement accuracy and speed.
Any leakage resistance in the connecting cable or test fixture
can decrease the actual resistance seen by the instrument. Al-
so, 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 resistances above 1GΩ. Guard-
ing is further discussed in paragraph 2.7.4. Noise pickup can
also be a problem, in which case the resistor must be shield-
ed. Connect the shield to input low.
TRIAX CABLE
INPUT
ON
MODEL 6512
INPUT
AMPLIFIER
INPUT
R F
I RX
HI
LO
S
PREAMP
GND
OUT
COM
HI
LO
A. CONNECTIONS
RANGING AMPLIFIER
V R
S
100Ω
B. EQUIVALENT CIRCUIT
At low resistances, lead resistance can be a consideration.
Cancel the effects of lead resistance by shorting the ends of
the input leads and enabling suppress with zero check dis-
abled. Leave suppress enabled for subsequent measure-
ments.

2.7.8 Using the ohms function as a current source

The Model 6512 ohms function may also be used to generate
currents in decade values between 1nA and 100µA. To use
the instrument in this manner, simply connect the triax cable
to the INPUT jack, and connect the other end of the cable to
the circuit under test. Select the resistance range in accor-
dance with the desired current (see Table 2-5). Note that cur-
rent flows out from input high into input low. The test voltage
is less than 2V for all ranges 2GΩ and less, except when an
overload occurs, in which case the compliance voltage is
300V.
R X
MEASURED
RESISTANCE
SHIELD
(RECOMMENDED ABOVE 100MΩ)
TO A/D
CONVERTER
2V ANALOG
OUTPUT
S
Operation
2-17