Power Dissipation Effects

When measuring resistors designed for temperature measurements (or other resistive devices with large temperature coefficients), be aware that the multimeter will dissipate some power in the device-under-test. If power dissipation is a problem, you should select the multimeter’s next higher measurement range to reduce the errors to acceptable levels. Table 2-3 shows several examples.

Table 2-3. DUT Power Dissipation

 

 

 

 

 

 

 

 

DUT

 

 

Range

Test Current

Power at Full Scale

 

 

 

 

 

 

 

 

 

 

 

 

100Ω

1mA

100μW

 

 

 

 

 

 

 

1kΩ

1mA

1mW

 

 

 

 

 

 

 

10kΩ

100μA

100μW

 

 

 

 

 

 

 

100kΩ

10μA

10μW

 

 

 

 

 

 

 

1MΩ

5μA

25μW

 

 

 

 

 

 

 

10MΩ

500nA

2.5μW

 

 

 

 

 

 

 

 

 

 

 

Settling Time Effects

Both the HP E1312A and HP E1412A have the ability to insert automatic measurement settling delays with the TRIG:DEL command. These delays are adequate for resistance measurements with less than 200pF of combined cable and device capacitance. This is particularly important if you are measuring resistances above 100kΩ. Settling due to RC time constant effects can be quite long. Some precision resistors and multi-function calibrators use large parallel capacitors (1000pF to 0.1 μF) with high resistor values to filter out noise currents injected by their internal circuitry. Non-ideal capacitances in cables and other devices may have much longer settling times than expected just by RC time constants due to dielectric absorption (soak) effects. Errors will be measured when settling after the initial connection and after a range change.

Errors in High

Resistance

Measurements

When you are measuring large resistances, significant errors can occur due to insulation resistance and surface cleanliness. You should take the necessary precautions to maintain a “clean” high-resistance system. Test cables and fixtures are susceptible to leakage due to moisture absorption in insulating materials and “dirty” surface films. Nylon and PVC are relatively poor insulators (109 ohms) when compared to PTFE Teflon insulators

(1013 ohms). Leakage from nylon or PVC insulators can easily contribute a 0.1% error when measuring a 1MΩ resistance in humid conditions.

Making High-Speed

DC and Resistance

Measurements

The multimeter incorporates an automatic zero measurement procedure (autozero) to eliminate internal thermal EMF and bias current errors. Each measurement actually consists of a measurement of the input terminals followed by a measurement of the internal offset voltage. The internal offset voltage error is subtracted from the measurement for improved accuracy. This compensates for offset voltage changes due to temperature. For maximum reading speed, turn autozero off. This will more than double your reading speeds for dc voltage, resistance, and dc current functions. Autozero does not apply to other measurement functions.

Chapter 2

HP E1312A/E1412A Multimeter Application Information 31

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HP WaterSkis E1412A, WaterSkis E1312A manual Power Dissipation Effects, Errors in High Resistance Measurements