4.3.8 Guarding High Value Resistance Measurements (SMX2044). Artison SMX2040, S-LCR-DM, SMX2042, SMX2044

4.3.8 Guarding High Value Resistance Measurements (SMX2044)

Measuring high value resistors using the 2-Wire function require special attention. Due to the high impedances involved during such measurements, noise pickup and leakage could be very significant. To improve this type of measurement it is important to use good quality shielded cables with a low leakage dielectric. Even with a good dielectric, if a significant length is involved, an error would result due to leakage. Figure 4.7 exemplifies this error source. It is important to emphasize that in addition to the finite leakage associated with the distributed resistance,

R

L

, there must also be a voltage present between the two conductors, the shield and the center lead, for leakage current to develop. Provided there was a way to eliminate this voltage, leakage would have been eliminated.

Figure 4-7. Error due to cable leakage.

The SM2044 provides an active guard signal that can be connected to the shield and prevent the leakage caused by the dielectric’s finite resistance. With the shield voltage guarded with Vx, as indicated in Figure 4-8, there is 0V between the shield and the high sense wire, and therefore no current flows through R

L

.

Figure 4-8. Guarding eliminates errors due to leakage associated with high resistance measurements.

4.4 RTD Temperature Measurement (SMX2044)

For temperature measurements, the SMX2044 measure and linearize RTDs. 4-wire RTD’s can be used by selecting the appropriate RTD type. Any ice temperature resistance between 25

Ω and 10 kΩ can be set for the platinum type RTDs. Copper RTDs can have ice temperature resistance values of 5

Ω to 200 Ω.

The highest accuracy is obtained from 4-wire devices, because the resistance of the test leads is nulled out. The connection configuration for RTDs is identical to 4-wire Ohms.

4.5 Internal Temperature (SMX2044)

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A special on board temperature sensor allows monitoring of the DMM’s internal temperature. This provides the means to determine when to run the self-calibration function (S-Cal) for the DMM, as well as predicting the performance of the DMM under different operating conditions. When used properly, this measurement can enha nce the accuracy and stability of the DMM. It also allows monitoring of the chassis internal temperature, which is important for checking other instruments.

4.6 Diode Characterization

The Diode measurement function i s used for characterizing semiconductor part types. This function is designed to display a semiconductor device’s forward or reverse voltage. The DMM measures diode voltage at a selected current. The available source currents for diode I/V characterization include five DC current values; 100

ηA, 1 µA,

10

µA, 100 µA and 1 mA. The SMX2044 have an additional 10 mA range. The SMX2044 also has a variable current source which can be used concurrently with DCV measurement (see “Source Current / Measure Voltage”).

This allows a variable current from 10

ηA to 12.5 mA. The maximum diode voltage compliance is approximately 4

V.

Applications include I/V characteristics of Diodes, LEDs, Low voltage Zener diodes, Band Gap devices, as well as

IC testing and polarity checking. Typical current level uncertainty for diode measurements is 1%, and typical voltage uncertainty is 0.02%.

4.7 Capacitance Measurement (SMX2044)

The SMX2044 measure capacitance using a differential charge slew method, where variable currents are utilized to produce a dv/dt across the capacitor.

Use short high quality shielded probe cables with no more than 500 pF. With the exception of the 10

ηF range, each of the ranges has a reading span from 5% of range to full scale. Capacitance values less than 5% of the selected range indicate zero. Since some large value electrolytic capacitors have significant inductance, as well as leakage and series resistance, the Autoranging function may not be practical.

Because Capacitance measurement is sensitive to noise, you should keep the measurement leads away from noise sources such as computer monitors. For best measurement accuracy at low capacitance values, zero the DMM using the ‘Relative’ while in the 10

ηF range. The effect of the cable quality and its total capacitance is significant particularly on low value caps. For testing surface mount parts, use the optional Signametrics SMT Tweezer probes.

See Figure 4-9 for connection.

Figure 4-9.

Measuring capacitors or inductors is best handled with shielded probe wires.

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