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How to optimize low-current and high-resistance measurements (Part 1)

Posted: 22 Aug 2012 ?? ?Print Version ?Bookmark and Share

Keywords:multimeter? picoammeter? amplifier? resistor?

When weighing the suitability of an instrument for low-current applications, it is important to take time to review its accuracy specifications. Instrument data sheets can use different formats to express accuracy, however, which can complicate making comparisons. Gain and offset errors may be combined into a specification that expresses accuracy in terms of a percentage and a number of counts of the least significant digit. Accuracy may also be expressed in parts per million.

High-resistance measurement methods
Although resistance can be measured via either the source current/measure voltage or the source voltage/measure current technique, the source voltage/measure current method is recommended for high resistance measurements. This can be performed using an electrometer or a picoammeter and a voltage source (figure 3a). Some electrometers and picoammeters include a built-in voltage source and will calculate the resistance value automatically.

Figure 3: The source voltage/measure current method of measuring high resistances with an electrometer or picoammeter and a voltage source (a) or with an SMU (b).

An SMU can also be used to measure high resistance. In an SMU, the ammeter and voltage source are internally connected in series (figure 3b). The unknown resistance is connected between the Force HI and Force LO terminals of the SMU.

When making high resistance measurements using the source voltage/measure current method, the user must take into account the accuracy of both the voltage source and the ammeter, as well as the integrity of the device connections. The complete signal path from connectors through the cables and into the test fixture must be optimized to keep signal degradation to a minimum.

Cables and connectors
The two most common types of connectors used for making low current and high resistance measurements are BNC connectors, which have two conductors, and triaxial connectors, which have three. BNC connectors are used on coaxial cables, which consist of a single conductor surrounded by a shield. Shielded cables are important for minimizing noise when making sensitive measurements. Triaxial cables are used with SMUs because the third conductor enables guarded measurements.

When measuring high resistances, choose cables that have high insulation resistance between the conductors. Good quality triaxial cables use polyethylene insulators and have a typical conductor to shield insulation resistance of about 1E12 /ft. Always check the manufacturers' specifications to make sure both cables and connectors are suitable for high impedance measurements. Both coaxial and triaxial cables are available in low noise versions, which should be used for low-level measurements. Low-noise cables have internal graphite coatings to minimize offsets. In some cases, ordinary coaxial cable such as RG-58 may be adequate, although both leakage and noise currents will be higher than with low noise cables.

Cable resistance, capacitance, and leakage currents change as cable length increases, so it's important to keep all connecting cables as short as possible. Use shielded cables and test fixtures to prevent electrostatic pick-up. When testing light- sensitive components, always use a light-tight test fixture.

To avoid measurement errors, it's critical to make proper connections from the electrometer, SMU, or picoammeter to the DUT. Always connect the high resistance terminal of the meter to the highest resistance point of the circuit under test.

About the author
Mary Anne Tupta is a senior staff applications engineer at Keithley Instruments, Cleveland, Ohio, which is part of the Tektronix test and measurement portfolio. She earned a B.S. in physics/electronic engineering and an M.S. in physics from John Carroll University. She has assisted Keithley customers with instrument applications since 1988.

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