High Resistance Measurements - Keithley 6430 Instruction Manual

High resistance measurements

Ohms measurement methods
The SourceMeter can make ohms measurements by either sourcing current, measuring volt-
age (constant-current method), or sourcing voltage, measuring current (constant-voltage
method). After the appropriate voltage and current readings are acquired, the resistance reading
is calculated using Ohms Law (R = V/I).
When using the constant-current method, the SourceMeter outputs a precise current and
then measures the voltage across the DUT. For high-impedance DUT, the considerations for
"High-impedance voltage measurements" apply.
When using the constant-voltage method, the SourceMeter outputs a precise voltage and
then measures the current through the DUT. For high-impedance DUT, the considerations and
techniques for "Low current measurements" apply.
Characteristics of high-valued resistors
Resistors with values of 1GΩ or more are often referred to as megohm resistors. Because of
their high resistances, these components are very unusual devices; accordingly, there are a
number considerations to take into account when measuring these devices: voltage and temper-
ature coefficients, the effects of mechanical shock, and contamination.
Two types of high-megohm resistors are widely used: carbon-film and metal-oxide.
Although other types are available, experience has shown that these two are the most useful.
Compared to conventional resistors, carbon-film high-megohm resistors are noisy, unstable,
have high temperature coefficients, display high voltage coefficients, and are very fragile.
Recent developments in metal-oxide types have resulted in resistors with much lower voltage
coefficients, as well as improved temperature and time stability. Modern devices exhibit volt-
age coefficients less than 5ppm/V and no significant drift after five years of tests. Temperature
coefficients are on the order of 0.01%/˚C at 100MΩ, 0.025%/˚C at 100GΩ.
Such delicate devices require extreme care in handling. Mechanical shock may significantly
alter the resistance by dislodging particles of the conductive material. It is also important that
the resistance element or the glass envelope that surrounds it not be touched; doing so could
change its resistance due to the creation of new current paths or small electrochemically gener-
ated currents.
The resistors are coated to prevent water films from forming on the surface. Therefore, if it is
suspected that the resistor has acquired surface films from careless handling or deposits from
air contaminants, it should be cleaned with a cotton swab and methanol. After cleaning, the
resistor should be dried in a low-humidity atmosphere for several hours to allow any static
charges to dissipate.
Measurement Considerations F-13