低电压，低电阻检测（英）

A g r e A t e r m e A s u r e o f c o n f i d e n c e Making Precision Measurements Low Voltage and Low Resistance Introduction .......................................................................... 2 Low Voltage Measurements ............................................... 2 Offset Voltages .....................................................................2 Noise ......................................................................................6 Common-Mode Current and Reversal Errors ..................8 Low Resistance Measurements .......................................... 9 Lead Resistance and Four-Wire Method ...........................9 Thermoelectric EMFs & Offset Compensation Methods .....9 Non-Ohmic Contacts ......................................................... 12 Device Heating .................................................................... 12 Dry Circuit Testing .............................................................. 12 Testing Inductive Devices .................................................. 13 Applications Low-V: Hall Effect Measurements ....................................... 14 Low-R: Superconductor Resistance Measurements ...... 17 Selector Guide .................................................................... 18 Glossary .............................................................................. 19 Contact Us................................................................................... 22 Making Precision Low VoLtage and Low Resistance MeasuReMents A g r e A t e r m e A s u r e o f c o n f i d e n c e 2 Ask a Question or Get a Quote. Low Voltage Measurements Introduction Low voltage and low resistance measurements are often made on devices and materials with low source impedance. This e-handbook discusses several potential sources of error in low voltage measurements and how to minimize their impact on measurement accuracy, as well as potential error sources for low resistance measurements, including lead resistance, thermoelectric EMFs, non-ohmic contacts, device heating, dry circuit testing, and measuring inductive devices. Significant errors may be introduced into low voltage measurements by offset voltages and noise sources that can normally be ignored when measuring higher voltage levels. These factors can have a significant effect on low voltage measurement accuracy. Offset Voltages Ideally, when a voltmeter is connected to a relatively low impedance circuit in which no voltages are present, it should read zero. However, a number of error sources in the circuit may be seen as a non-zero voltage offset. These sources include thermoelectric EMFs, offsets generated by rectification of RFI (radio frequency interference), and offsets in the voltmeter input circuit. Figure 1: Effects of Offset Voltages on Voltage Measurement Accuracy As shown in Figure 1, any offset voltage (VOFFSET) will add to or subtract from the source voltage (VS) so that the voltage measured by the meter becomes: VM = VS ± VOFFSET The relative polarities of the two voltages will determine whether the offset voltage adds to or subtracts from the source voltage. Steady offsets can generally be nulled out by shorting the ends of the test leads together, then enabling the instrument’s zero (relative) feature. Note, however, that cancellation of offset drift may require frequent rezeroing, particularly in the case of thermoelectric EMFs. ThermoelecTric emFs Thermoelectric voltages (thermoelectric EMFs) are the most common source of errors in low voltage measurements. These voltages are generated when different parts of a circuit are at different temperatures and when conductors made of dissimilar materials are joined together, as shown in Figure 2. The Seebeck coefficients (QAB) of various materials with respect to copper are summarized in Table 1. Figure 2: Thermoelectric EMFs FeATured resources n Troubleshooting Low Voltage Measurement Problems n Accurate Low-Resistance Measurements Start with Identifying Sources of Error AddiTionAl resources n Understanding Low Voltage Measurements n Problem: Errors in Low Resistance Measurements intRoduction | Low VoLtage MeasuReMents | Low Resistance MeasuReMents | Low-V appLication: HaLL effect MeasuReMents | Low-R appLication: supeRconductoR Resistance MeasuReMents | seLectoR guide | gLossaRy | contact us Making Precision Low VoLtage and Low Resistance MeasuReMents A g r e A t e r m e A s u r e o f c o n f i d e n c e 3 Ask a Question or Get a Quote. Low Voltage Measurements Table 1: Seebeck Coefficients Paired materials* seebeck coefficient, QAB Cu - Cu ≤0.2 μV/°C Cu - Ag 0.3 μV/°C Cu - Au 0.3 μV/°C Cu - Pb/Sn 1–3 μV/°C Cu - Si 400 μV/°C Cu - Kovar ~40–75 μV/°C Cu - CuO ~1000 μV/°C * Ag = silver Au = gold Cu = copper CuO = copper oxide Pb = lead Si = silicon Sn = tin Constructing circuits using the same material for all conductors minimizes thermoelectric EMF generation. For example, crimping copper sleeves or lugs onto copper wires results in copper-to-copper junctions, which generate minimal thermoelectric EMFs. Also, connections must be kept clean and free of oxides. Crimped copper-to- copper connections, called “cold welded,” do not allow oxygen penetration and may have a Seebeck coefficient of ≤0.2μV/°C, while Cu-CuO connections may have a coefficient as high as 1mV/°C. Minimizing temperature gradients within the circuit also reduces thermoelectric EMFs. A technique for minimizing such gradients is to place corresponding pairs of junctions in close proximity to one another and to provide good thermal coupling to a common, massive heat sink. Electrical insulators having high thermal conductivity must be used, but, since most electrical insulators don’t conduct heat well, special insulators such as hard anodized aluminum, beryllium oxide, specially filled epoxy resins, sapphire, or diamond must be used to couple junctions to the heat sink. Allowing test equipment to warm up and reach thermal equilibrium in a constant ambient temperature also minimizes thermoelectric EMF effects. The instrument zero feature can compensate for any remaining thermoelectric EMF, provided it is relatively constant. To keep ambient temperatures constant, equipment should be kept away from direct sunlight, exhaust fans, and similar sources of heat flow or moving air. Wrapping connections in insulating foam (e.g., polyurethane) also minimizes ambient temperature fluctuations caused by air movement. connecTions To Avoid ThermoelecTric emFs Connections in a simple low voltage circuit, as shown in Figure 3, will usually include dissimilar materials at different temperatures. This produces a number of thermoelectric EMF sources, all connected in series with the voltage source and the meter. The meter reading will be the algebraic sum of all these sources. Therefore, it is important that the connection between the signal source and the measuring instrument doesn’t interfere with the reading. Figure 3: Connections from Voltage Source to Voltmeter If all the connections can be made of one metal, the amount of thermoelectric EMF added to the measurement will be negligible. However, this may not always be possible. Test fixtures often use spring contacts, which may be made of phosphor-bronze, beryllium- copper, or other materials with high Seebeck coefficients. In these cases, a small temperature difference may generate a large enough thermoelectric voltage to affect the accuracy of the measurement. If dissimilar metals cannot be avoided, an effort should be made to reduce the temperature gradients throughout the test circuit by use of a heat sink or by shielding the circuit from the source of heat. intRoduction | Low VoLtage MeasuReMents | Low Resistance MeasuReMents | Low-V appLication: HaLL effect MeasuReMents | Low-R appLication: supeRconductoR Resistance MeasuReMents | seLectoR guide | gLossaRy | contact us Featured and Additional Resources Making Precision Low VoLtage and Low Resistance MeasuReMents A g r e A t e r m e A s u r e o f c o n f i d e n c e 4 Ask a Question or Get a Quote. Low Voltage Measurements Measurements of sources at cryogenic temperatures pose special problems since the connections between the sample in the cryostat and the voltmeter are often made of metals with lower thermal conductivity than copper, such as iron, which introduces dissimilar metals into the circuit. In addition, since the source may be near zero Kelvin while the meter is at 300K, there is a very large temperature gradient. Matching the composition of the wires between the cryostat and the voltmeter and keeping all dissimilar metal junction pairs at the same temperature allows making very low voltage measurements with good accuracy. reversinG sources To cAncel ThermoelecTric emFs When measuring a small voltage, such as the difference between two standard cells or the difference between two thermocouples connected back-to-back, the error caused by stray thermoelectric EMFs can be canceled by taking one measurement, then carefully reversing the two sources and taking a second measurement. The average of the difference between these two readings is the desired voltage difference. In Figure 4, the voltage sources, Va and Vb, represent two standard cells (or two thermocouples). The voltage measured in Figure 4a is: V1 = Vemf + Va – Vb The two cells are reversed in Figure 4b and the measured voltage is: V2 = Vemf + Vb – Va The average of the dif ference between these two measurements is: V1 – V2 = Vemf + Va – Vb – Vemf – Vb + Va or Va – Vb 2 2 Figure 4: Reversing Sources to Cancel Thermoelectric EMFs Notice that this measurement technique effectively cancels out the thermoelectric EMF term (Vemf), which represents the algebraic sum of all thermoelectric EMFs in the circuit except those in the connections between Va and Vb. If the measured voltage is the result of a current flowing through an unknown resistance, then either the current-reversal method or the offset-compensated ohms method may be used to cancel the thermoelectric EMFs. rFi/emi RFI (Radio Frequency Interference) and EMI (Electro- magnetic Interference) are general terms used to describe electromagnetic interference over a wide range of frequencies across the spectrum. RFI or EMI can be caused by sources such as TV or radio broadcast signals or it can be caused by impulse sources, as in the case of high voltage arcing. In either case, the effects on the measurement can be considerable if enough of the unwanted signal is present. RFI/EMI interference may manifest itself as a steady reading offset or it may result in noisy or erratic readings. A reading offset may be caused by input amplifier overload or DC rectification at the input. RFI and EMI can be minimized by taking several precautions when making sensitive measurements. The most obvious precaution is to keep all instruments, cables, and DUTs as far from the interference source as possible. Shielding the test leads and the DUT (Figure 5) will often reduce interference effects to an acceptable level. Noise shields should be connected to input LO. In extreme cases, a specially constructed screen room may be necessary to attenuate the troublesome signal sufficiently. If all else fails to prevent RF interference from being introduced into the input, external filtering of the device input paths may be required, as shown in Figure 6. In many cases, a simple one-pole filter may be sufficient; in more difficult cases, multiple-pole notch or band-stop filters may be required. In particular, multiple capacitors of different values may be connected in parallel to provide low impedance over a wide frequency range. Keep in mind, however, that such filtering may have other detrimental effects, such as increased response time on the measurement. intRoduction | Low VoLtage MeasuReMents | Low Resistance MeasuReMents | Low-V appLication: HaLL effect MeasuReMents | Low-R appLication: supeRconductoR Resistance MeasuReMents | seLectoR guide | gLossaRy | contact us Featured and Additional Resources Making Precision Low VoLtage and Low Resistance MeasuReMents A g r e A t e r m e A s u r e o f c o n f i d e n c e 5 Ask a Question or Get a Quote. Low Voltage Measurements Figure 5: Shielding to Attenuate RFI/EMI Interference Figure 6: Shielded Connections to Reduce Inducted RFI/EMI inTernAl oFFseTs Nanovoltmeters will rarely indicate zero when no voltage is applied to the input, since there are unavoidable voltage offsets present in the input of the instrument. A short circuit can be connected across the input terminals and the output can then be set to zero, either by front panel zero controls or by computer control. If the short circuit has a very low thermoelectric EMF, this can be used to verify input noise and zero drift with time. Clean, pure copper wire will usually be suitable. However, the zero established in this manner is useful only for verification purposes and is of no value in the end application of the instrument. If the instrument is being used to measure a small voltage drop resulting from the flow of current through a resistor, the following procedure will result in a proper zero. First, the instrument should be allowed to warm up for the specified time, usually one to two hours. During this time, the connections should be made between the device under test and the instrument. No current should be supplied to the device under test to allow the temperature gradients to settle to a minimum, stable level. Next, the zero adjustment should be made. In some instruments, this is done by pressing REL (for Relative) or ZERO button. The instrument will now read zero. When the test current is applied, the instrument will indicate the resulting voltage drop. In some applications, the voltage to be measured is always present and the preceding procedure cannot be used. For example, the voltage difference between two standard cells is best observed by reversing the instrument connections to the cells and averaging the two readings. This same technique is used to cancel offsets when measuring the output of differential thermocouples. This is the same method used to cancel thermoelectric EMFs. Zero driFT Zero drift is a change in the meter reading with no input signal (measured with the input shorted) over a period of time. The zero drift of an instrument is almost entirely determined by the input stage. Most nanovoltmeters use some form of chopping or modulation of the input signal to minimize the drift. The zero reading may also vary as the ambient temperature changes. This effect is usually referred to as the temperature coefficient of the voltage offset. In addition, an instrument may display a transient temperature effect. After a step change in the ambient temperature, the voltage offset may change by a relatively large amount, possibly exceeding the published specifications. The offset will then gradually decrease and eventually settle to a value close to the original value. This is the result of dissimilar metal junctions in the instrument with different thermal time constants. While one junction will adjust to the new ambient temperature quickly, another changes slowly, resulting in a temporary change in voltage offset. To minimize voltage offsets due to ambient temperature changes in junctions, make measurements in a temperature controlled environment and/or slow down temperature changes by thermally shielding the circuit. intRoduction | Low VoLtage MeasuReMents | Low Resistance MeasuReMents | Low-V appLication: HaLL effect MeasuReMents | Low-R appLication: supeRconductoR Resistance MeasuReMents | seLectoR guide | gLossaRy | contact us Featured and Additional Resources Making Precision Low VoLtage and Low Resistance MeasuReMents A g r e A t e r m e A s u r e o f c o n f i d e n c e 6 Ask a Question or Get a Quote. Low Voltage Measurements Noise Significant errors can be generated by noise sources, which include Johnson noise, magnetic fields, and ground loops. An understanding of these noise sources and the methods available to minimize them is crucial to making meaningful low voltage measurements. Johnson noise The ultimate limit of resolution in an electrical measure- ment is defined by Johnson or thermal noise. This noise is the voltage associated with the motion of electrons due to their thermal energy at temperatures above absolute zero. All voltage sources have internal resistance, so all voltage sources develop Johnson noise. The noise voltage developed by a metallic resistance can be calculated from the following equation: where: V = rms noise voltage developed in source resistance k = Boltzmann’s constant, 1.38 × 10–23 joule/K T = absolute temperature of the source in Kelvin B = noise bandwidth in hertz R = resistance of the source in ohms For example, at room temperature (290K), a source resistance of 10kΩ with a measurement bandwidth of 5kHz will have almost 1μV rms of noise. Johnson noise may be reduced by lowering the temperature of the source resistance and by decreasing the bandwidth of the measurement. Cooling the sample from room temperature (290K) to liquid nitrogen temperature (77K) decreases the voltage noise by approximately a factor of two. If the voltmeter has adjustable filtering and integration, the bandwidth can be reduced by increasing the amount of filtering and/or by integrating over multiple power line cycles. Decreasing the bandwidth of the measurement is equivalent to increasing the response time of the instrument, and as a result, the measurement time is much longer. However, if the measurement response time is long, the thermoelectric EMFs associated with the temperature gradients in the circuit become more important. Sensitive measurements may not be achieved if the thermal time constants of the measurement circuit are of the same order as the response time. If this occurs, distinguishing between a change in signal voltage and a change in thermoelectric EMFs becomes impossible. mAGneTic Fields Magnetic fields generate error voltages in two circum- stances: 1) if the field is changing with time, and 2) if there is relative motion between the circuit and the field. Voltages in conductors can be generated from the motion of a conductor in a magnetic field, from local AC currents caused by components in the test system, or from the deliberate ramping of the magnetic field, such as for magneto- resistance measurements. Even the earth’s relatively weak magnetic field can generate nanovolts in dangling leads, so leads must be kept short and rigidly tied down. Basic physics shows that the amount of voltage a magnetic field induces in a circuit is proportional to the area the circuit leads enclose and the rate of change in magnetic flux density, as shown in Figure 7. The induced voltage is proportional both to the magnitude of A andBB , as well as to the rate of change in A and BB , so there are two ways to minimize the induced voltage: n Keep both A andBB to a minimum by reducing loop area and avoiding magnetic fields, if possible; and n Keep both A andBB constant by minimizing vibration and movement, and by keeping circuits away from AC and RF fields. To minimize induced magnetic voltages, leads must be run close together and magnetically shielded and they should be tied down to minimize movement. Mu-metal, a special alloy with high permeability at low magnetic flux densities and at low frequencies, is a commonly used ma