With power supplies and DMMs, the most common mistake is failing to take an organized approach to the selection process. Although parameters such as accuracy, resolution, and sensitivity are critical to these product selections, aspects like reading rates, specialized functions, and productivity enhancements must also be considered, especially if the product is intended for production testing applications. Also, certain features can help overcome external noise problems that would otherwise undercut the instrument’s accuracy and sensitivity specifications. Major selection factors include measurement variables, range, accuracy, resolution, sensitivity, speed, filters, test current limiting in resistance measurements, two-wire vs. four-wire measurements, triggering, number of measurement channels, and cost.
The meeting of minds.
How to avoid those errors
Selecting the most appropriate DMM for a specific application requires careful analysis of both the device under test (DUT) and the test environment. Answering the following ten questions is a good way to simplify the DMM selection process:
- What signals are being tested/measured?
- What is the range of expected signal values?
- What are the accuracy, resolution, and sensitivity requirements?
- What special DUT characteristics must be considered?
- What are the hardware and software interface issues?
- What are the noise sources?
- What are the throughput requirements?
- How many DUTs are to be tested?
- Would additional DMM features be helpful?
- How cost effective is the solution?
For power supplies, there are a several common errors related to specifying and using these instruments:
- Failing to use (or specify) remote sensing and not applying the desired voltage at the load due to the voltage drop in the test leads.
- Not specifying (or trying to use) a multi-channel supply with all channels isolated when testing optically isolated or transformer-isolated circuits.
- Failing to wait long enough for the supply to settle when making measurements after voltage step changes (usage error).
- Not using a power supply with sufficient resolution and accuracy for load current measurements (usage and specification error).
- Over or under specifying total power requirements. Standby, steady state and peak loads need to be considered.
- Not fully considering environmental aspects such as dust, humidity, altitude, ambient temperatures, etc.
- Not allowing sufficient time to procure the required power supplies. This is a common problem and is referred to as “the tail pipe syndrome” since many system designers tend to think about the power supply near the end of the project.
- Buying a low priced power supply with the expectation of saving money. You get what you pay for and typically low priced supplies tend to fail much sooner than higher quality supplies. The cost of a field failure far exceeds the “initial savings” of buying a cheap power supply.
Special thanks to Mel Berman, TDK-Lambda, and Jerry Janesh, Keithley Instruments.
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