Monday, September 19, 2011

DMM Resistance Measurement Considerations

The digital multimeter or DMM offers two methods for measuring resistance: 2–wire and 4–wire ohms. For both methods, the test current flows from the input HI terminal and then through the resistor being measured. For 2–wire ohms, the voltage drop across the resistor being measured is sensed internal to the multimeter. Therefore, test lead resistance is also measured. For 4–wire ohms, separate "sense" connections are required. Since no current flows in the sense leads, the resistance in these leads does not give a measurement error. In this blog post I will discuss some general considerations and tips when making DMM resistance measurements.

4–Wire Ohms Measurements
4-wire ohm measurement use the HI and LO DMM leads as well as the HI-Sense leads (that is why they are called "4-wire"), the setup for a 4-wire ohms measurement is shown below. 

The sense leads essentially extend the DMM measurement to the DUT junctions instead of the HI and LO terminals . This eliminates the voltage drop across the HI and LO leads caused by the test current. Since the sense leads are high impedance there is essentially no current flow into the sense inputs. The 4–wire ohms method provides the most accurate way to measure small resistances. Test lead resistances and contact resistances are automatically reduced using this method. Four–wire ohms is often used in automated test applications where resistive and/or long cable lengths, numerous connections, or switches exist between the DMM and the DUT.

Removing Test Lead Resistance Errors
Modern DMMs offer a built-in function, often labeled as "Null" or "Math", for eliminating test lead error.To use the Math function on a DMM you short the test leads to together. The Math function will then make a resistance measurement of the test leads and store it. The DMM will then mathematically subtract the measured lead resistance for subsequent resistance measurements to cancel out the lead resistance error.

Minimizing Power Dissipation Effects
When measuring resistors designed for temperature measurements (or other resistive devices with large temperature coefficients), be aware that the DMM will dissipate some power in the device–under–test. If power dissipation is a problem, you should select the DMM's next higher measurement range to reduce the errors to acceptable levels. The following table shows examples of Agilent's 34410A and 34411A DMMs source current for various measurement ranges.


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 leads and fixtures are susceptible to leakage due to moisture absorption in insulating materials and "dirty" surface films. Nylon and PVC are relatively poor insulators (10^9 Ω) when compared to PTFE (Teflon) insulators (10^13 Ω). Leakage from nylon or PVC insulators can easily contribute a 0.1% error when measuring a 1 MΩ resistance in humid conditions.

Click here to check out the DMMs that Agilent offers



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