How to avoid measurement mistakes

Article By : Martin Rowe

EDN's large collection of basic measurement articles shows you how to get the most from your test equipment.

All analog measurements — voltage, current, temperature, humidity, etc. — include some amount of error. Your job is to minimize those errors enough to give your measurement sufficient credibility. EDN has a long history of providing engineers with articles that help you improve measurement quality. The technical articles you’ll find here cover such topics as

• Voltage measurement
• Current measurement
• Power measurement
• Resistance measurement
• Temperature measurement
• Connecting wires and probes
• Grounding and shielding
• EMI/ESD measurements

Even when you’re careful at setting up a measurement, things can still go wrong. Perhaps the most common measurement error comes from incorrect wiring. Start by making sure you’ve properly connected your sensors and circuits with the correct polarity.

When measuring voltage with an oscilloscope, your probes can often cause problems because of their input impedance, which drops as frequency increases. Ask yourself “Are you measuring your circuit or your scope probe?” That’s why it’s a good idea to understand and optimize your oscilloscope probes.

Just because you use the right probe and you’re gatting good measurements on your oscilloscope, things can still to wrong. Digital oscilloscopes: When things go wrong explains where you can still get errors.

• Aliasing: When your oscilloscope (or any other digitizer) samples to slowly.
• Synchronous sampling: If the sampling clock is synchronous or near synchronous with the signal, then samples are taken at or near the same phase every time. As the signal repeats, the same parts of the signal are sampled.
• Interpolation: If the signals have “fast” edges, with few sample on an edge, then the sin(x)/x interpolator can cause problems.

When measuring resistance with a digital multimeter (DMM), you can run into errors if you use just two wires. Many bench DMMs have our-wire resistance measurement capability. That lets you remove errors in current-carrying wires. Two-wire vs. four-wire resistance measurements shows you why you often need four-wires (Figure 1) to accurately measure resistance.

Figure 1. Four-wire measurements compensate for IR drops in current-carrying wires.

Four-wire configurations are often used for measuring temperature with a resistance-temperature detector (RTD). Minimize measurement errors in RTD circuits explains how to get better measurements when using RTDs.

While RTDs are popular temperature sensors, thermocouples are even more popular. Users of thermocouples and even designers of thermocouple circuits often miss the concept behind a thermocouple, thinking that a voltage occurs wherever two dissimilar metals touch. Even longtime EDN contributor Jim Williams, consider by many to be one of the world’s top analog engineers, got it wrong. Need some tips for better temperature measurements? We have them.

While thermocouples and RTDs measure temperature, there are other types of temperature sensors. Indeed, there are sensors for just about everything. For example, sensors measure pressure, humidity, motion, and a host of other parameters.

Remember that no measurement is perfect. All have some amount of error, called uncertainty by people in the calibration and metrology (measurement science) field. Fortunately, you can learn how to manage those measurement errors. Figure 2 shows the difference bertween accuracy and resolution, both of which cause errors.

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