subject: Improving High Resistance Measurements With Voltmeter And Ammeter Instruments [print this page] High resistance measurements are integral to a variety of test applications, including surface insulation resistance (SIR) testing of printed circuit boards, resistivity measurements of insulating materials and semiconductors, and voltage coefficient testing of high ohmic value resistors. Ensuring the accuracy of high resistance measurements (i.e., resistances of 1G (109 ohms) or higher) requires the use of specific techniques and instruments, such as an electrometer, a Source-Measure Unit (SMU), or picoammeter/voltage source combination. Moreover, an electrometer can measure high resistance using either the constant-voltage or the constant-current method. Essentially, the use of all these instruments involves their voltmeter and ammeter functions, plus the application of Ohms Law, i.e., R = V/I.
Resistance Measurements with Constant-Voltage. Since high resistance is often a function of applied voltage, the constant-voltage method is generally preferable to the constant-current method. By testing at selected voltages, a resistance vs. voltage curve can be developed and the voltage coefficient of resistance can be determined.
This technique requires a constant voltage source and an instrument that can measure low currents accurately. The constant voltage source (V) is placed in series with the unknown resistance (R) and an ammeter (IM). Because the voltage drop across the ammeter is negligible, essentially all the test voltage appears across the resistance. Resistance is calculated in accordance with Ohms Law (R= V/IM).
While separate instruments could be used, some electrometers and picoammeters provide a built-in voltage source and can automatically calculate an unknown resistance from the resulting current through it. One example is the Keithley Model 6517B electrometer. A source-measure unit (SMU) can also apply a constant voltage and measure current through the resistance. Test voltages are usually less than 100VDC, so the resulting current through a 1G resistance, for example, will typically be less than 0.1A (10-7A). At lower voltages and higher resistances, its not uncommon to have resulting currents in the nanoamp (10-9A) range.
Clearly, the constant-voltage method requires measuring low current accurately, and error sources related to such measurements must be considered. These include improperly shielded connections to the ammeter, problems related to the ammeters voltage burden and input offset current, and the source resistance of the device under test. External sources of error can include leakage current from cables and fixtures, as well as currents generated by triboelectric or piezoelectric effects. With these precautions, using the constant-voltage method with an electrometer allows resistance measurements up to 10P (1016).
Determining Resistance with Constant-Current. Constant-voltage high resistance measurements are recommended if there is any chance the DUT resistance exhibits voltage dependency. However, the resistance level and type of instrument available may suggest the constant current method. Typical instrument configurations for high resistances measurements with the constant-current method are:
an electrometers voltmeter function and a current source
an electrometers ohmmeter function alone
an SMUs voltmeter with high input impedance and low current source ranges
Using the electrometer voltmeter with a separate current source or an SMU allows making a four-wire (Kelvin) measurement and controlling the amount of current through the sample. On the other hand, an electrometers ohmmeter function makes two-wire resistance measurements at a specific test current, depending on the measurement range.
Using an Electrometer Voltmeter and an External Current Source With a constant-current instrument configuration the current source, DUT, and electrometer are in a parallel circuit arrangement. Current from the source (I) flows through the unknown resistance (R) and the voltage drop is measured by the electrometer voltmeter (V). Although the basic procedure is relatively straightforward, some precautions are necessary. The input impedance of the electrometers voltmeter circuit must be high enough to keep the loading error within acceptable limits. Typically, the input impedance of an electrometer voltmeter is greater than 1014. Also, the output resistance of the current source must be much greater than the unknown resistance for the measurement to be linear. The voltage across the DUT depends on its resistance, which makes it difficult to account for voltage coefficient when using the constant-current method. If voltage coefficient is a concern, the constant-voltage method should be used instead. The constant-current method allows resistance measurements up to about 1012.
Using an SMU in the Source I, Measure V Mode An SMU can measure high resistance in the source-current/measure-voltage mode by using either a two-wire (local sense) or four-wire (remote sense) method. The latter eliminates contact and lead resistance, which is especially important when measuring the resistivity of semiconductor materials. These measurements usually involve measuring low voltages, and the resistance of the metal probe to semiconductor contact can be quite high, dictating a remote sense arrangement. In this arrangement two force leads supply constant current to the DUT, while the remote sense leads measure the voltage. The voltage difference between HI Force and HI Sense and between LO Force and LO Sense terminals is usually limited to a specified value. Exceeding this voltage difference can produce erratic measurements.
In addition to the voltage drop limitation, some SMUs have automatic remote sensing resistors located between the HI Force and HI Sense terminals and between the LO Force and LO Sense terminals. This may further limit the use of a single SMU in remote mode for certain applications, such as semiconductor resistivity. If this is the case, the SMU can be used as a current source in the two-wire mode, and a separate voltmeter(s) used to measure the voltage difference.
Using the Electrometer Ohmmeter Using this electrometer function, the current source is internally connected to the voltmeter and the two cannot be used separately. The current source and voltmeter form a parallel circuit across the DUT. The electrometer automatically calculates and displays the measured resistance. Various factors, including electrostatic interference and leakage currents, can affect measurement accuracy. In addition, this is a two-wire resistance measurement, in contrast with using the electrometer voltmeter with an external current source, which can make a more accurate four-wire measurement.
Guarding and Shielding. In any high resistance measurement system, guarding and shielding are important, because the two most common sources of measurement error are electrostatic interference and leakage current. Shielding the high impedance circuitry can help minimize the effects of electrostatic interference (the shield is connected to circuit LO). Guarding offers a very effective way to reduce leakage currents and improve measurement accuracy. A guard circuit uses a unity gain amplifier to create a low impedance point in the circuit (i.e., on the shield) thats at nearly the same potential as the high impedance lead being guarded. Guarding is typically part of an electrometers input circuitry.
Circuit Settling Time. The measurement circuits settling time is particularly important when measuring high resistances. The settling time is affected by the shunt capacitance, which is due to the connecting cable, test fixturing, and the DUT. The shunt capacitance (CSHUNT) must be charged to the test voltage by the current (IS). The amount of time required to charge the capacitor is determined by the RC time constant (one time constant, T = RS x CSHUNT, so its typically necessary to wait four or five time constants to achieve an accurate reading. When measuring very high resistance values, the settling time can range up to minutes, depending on the amount of shunt capacitance in the test system. To minimize settling times when measuring high resistance values, keep shunt capacitance in the system to an absolute minimum by keeping connecting cables as short as possible. The use of guarding can also decrease settling times substantially.
Characterizing High Ohmic Valued Resistors. Resistors with values of 1G (109ohms) and higher are often referred to as high megohm resistors, which come in two types: carbon-film and metal-oxide. Several factors must be considered when measuring them, including voltage and temperature coefficients, the effects of mechanical shock, and contamination. Compared to conventional resistors, carbon-film high megohm resistors are very fragile, and their measurements tend to be noisy, unstable, have high temperature coefficients, and display high voltage coefficients. Newer metal-oxide types have much lower voltage coefficients (less than 5ppm/V), as well as improved temperature and time stability.
High megohm resistors require extreme care in handling because mechanical shock may significantly alter their resistance by dislodging particles of the conductive material. Its also important to avoid touching the resistance element or the glass envelope that surrounds it; doing so could change its resistance due to the creation of new current paths or small electrochemically generated currents. In addition, these resistors are coated to prevent water films from forming on their surfaces. Therefore, if a resistor acquires surface films from careless handling or deposits from air contaminants, it should be cleaned with a foam-tipped 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.
References. More techniques and tips on high resistance measurements are contained in Keithleys Low Level Measurements Handbook, which is available for download at http://gw1.vtrenz.net/?SIP0AWDVFA.
Summary. A wide variety of applications require the measurement of resistances greater than one G, which can be challenging. Still, the right combination of instrumentation and measurement techniques can go a long way toward ensuring the accuracy of these high resistance measurements.
