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I-20 Applications Guide Model 6485 Picoammeter Instruction Manual

The basic conﬁguration for measuring high resistance using the 6485 Picoammeter is

shown in Figure I-16. The HI terminal of the 6485 is connected to one end of the unknown

resistance (R) and the HI of the voltage source to the other end of the resistance. The LO

terminal of the 6485 is connected to the LO terminal of the voltage source. Both LO termi-

nals are also connected to earth ground. This should be done via the ground link on the

rear of the 6485.

Figure I-16

Measuring High Resistance Using the 6485

To prevent generated current due to electrostatic interference, place the unknown resis-

tance in a shielded test ﬁxture. The metal shield is connected to the LO terminal of the

6485.

If the voltage source is greater than 220V, a current limiting resistor in series with the 6485

HI terminal as well as protection diodes (IN 3595) across the meter input should be used

to prevent damage to the 6485 in the event the unknown resistance breaks down and

becomes shorted. The diodes should be in a light-tight enclosure to prevent light induced

leakage, A diagram of the overload protection circuit is shown in Figure I-17.

Figure I-17

Overload Protection Circuit for 6485 Picoammeter

Programmable

V-Source

(V)

6485

Picoammeter

Unknown Resistance

(R)

Measured

Current

Metal

Shield

Equivalent Circuit

To 6485 Input

IN 3595

Keithley 6485 User Manual

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