Tuesday, April 12, 2011

Understanding Resistance Measurement Errors

This post covers resistance measurement error external to the DMM and how to prevent it. Quality bench-top DMMs offer 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 resistance being measured and finally into the LO terminal. For 2-wire ohms, the voltage drop across the resistor being measured is sensed internal to the DMM. Therefore, test lead resistance is also measured. For 4-wire ohms, separate “sense” connections are required. Since no current (or very little) flows in the sense leads, the resistance in these leads does not give a measurement error. Errors for dc voltage measurements (see Agilent Application Note 1389-1) also apply to resistance measurements. However, there are additional error sources that are unique to resistance measurements.

Power Dissipation Effects — When measuring resistors designed for temperature measurements, such as thermistors and RTDs, be aware that the DMM’s stimulus current will dissipate some power in the DUT. This dissipated power will raise the temperature of the DUT and therefore change its resistance such that its resistance value no longer represents the ambient temperature of the environment it is in. If power dissipation is a problem, select the DMM’s next higher measurement range to reduce the amount of stimulus current used to make the measurement and therefore reduce the temperature error to an acceptable level. The figure below shows the stimulus current for Agilent’s 34401A at each resistance measurement range.

Settling Time Effects — Capacitive and inductive elements are a part of every measurement setup. The amount of setting time that is needed to make an accurate measurement depends on the values of the reactive elements and the time constants associated with their current paths. Quality DMMs like Agilent’s 34401A, have the ability to insert automatic measurement settling delays. These delays are adequate for resistance measurements with less than 200 pF of combined cable and device capacitance, which is particularly important when measuring resistances above 100 k. Settling time errors are most pronounced after a connection change, such as a switch closure in a test system, or after a range change in the DMM. In these types of scenarios allow a settle time that matches your accuracy needs. To check for setting time errors, make resistance measurements (on a known resistance) immediately after and at set time intervals after a range or connection change. Repeat this a number times and calculate the average and standard deviation for each set of measurements in a particular time interval. Once the average and standard deviation measurements of each time interval begin to match you can determine what your minimum setting time is.

High-Resistance Measurement Errors — When you measure 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 ohms) compared to PTFE Teflon insulators (10^13 ohms). Leakage from nylon or PVC insulators can easily contribute a 0.1% error when measuring a 1 M resistance in humid conditions.

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