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Troubleshooting Tips - Start From Nothing - Tom - 07-01-2021
Troubleshooting Tips - Start from Nothing
Back in the last century, I discovered an entertaining and informative series of columns in Electronic Design Magazine about troubleshooting electronics. The author was a fellow named Bob Pease at National Semiconductor. He had been at Philbrick for the design of the first practical operational amplifiers, and he had designed several important analog ICs for National. He was also quite the character. Sadly, he passed away suddenly in 2011. His magazine columns were turned into a book, called Troubleshooting Analog Circuits, which is worth seeking out. I do not presume to rise to that level, but I can offer some advice to beginners which may have some value for more experienced sorts, as well.
It is often best to start from nothing. By that, I mean nothing in, nothing out. A baseline may seem an uninteresting area for exploration, but noise or drift or jumps in your baseline will permeate your system, and baseline problems may be difficult to identify when looking at your output. For a data acquisition system, get as close to the Analog-to-Digital converter as you can, and ground the inputs. With zero volts in, you are looking for nothing out. Be patient. Problems can be intermittent, and drift occurs over a long time frame.
Drift tends to be thermal in nature, so you can often find a sensitive spot with a little applied heat. Fast heating is never uniform, so it may make a problem seem severe when it is barely there. Bringing a light bulb into the vicinity may be a good way to heat gradually when looking for thermal drift issues. Note that some electronic components, like diodes, can have a light sensitivity, so it is best to interpose an opaque layer if you are using light for thermal troubleshooting.
Another technique is to blow your warm breath selectively around data acquisition circuitry. A plastic or rubber tube won't short anything out and allows warm, humid air to be directed at will. For very high impedance circuits, the humidity, not the warmth, may have the larger influence by increasing surface leakage. To distinguish, a heat gun or hair dryer set at a low heat can provide the warmth without the moisture.
First, take all reasonable steps to minimize the drift. Then, you can consider improving your data with active calibration techniques. These methods can be transparent to the user, or can be done explicitly as a post-processing step. A zero voltage is measured periodically and is subtracted from the signal. That operation adds a little short-term noise, but can remove the large majority of slow thermal drift.
Other noise sources include popcorn noise, from the ICs themselves, and Johnson noise, which permeates literally everything. These noise sources are issues for circuit designers, and they establish the noise floor that you are going to have to live with. In a well-designed system the intrinsic noise should be limited to one or two bits. Two bits of noise on a 24 bit system leaves you with 22 bits, or one part in 4 million.
Once you have reduced drift problems to an acceptable minimum, look for other noise sources that you can do something about. Sometimes physical separation is a big help. Radiated noise follows the law of inverse squares, so doubling the separation reduces the noise pickup by a factor of four. Maybe just moving that power brick further away will help. Shielding and grounding are your other main tools. See the extensive shielding and grounding assistance found elsewhere on this website for guidance.
All of the above is in the interests of achieving nothing in, nothing out. Once you are satisfied with your baseline, you can inject a signal and look for the proper corresponding output. Whenever possible, examine the intermediate results first. By tracing a signal step by step through your system, you can be confident that each stage is behaving properly. If you look instead at the final result, it is easy to fool yourself with a speculative diagnosis.
To summarize, the temptation is to fire it up and see if it works. If it doesn't, the tendency is to start working backwards from the end. If you don't immediately identify the problem, I recommend working from the input side. When you start at the beginning, you can progress through a working system. Then, when you reach a problem, you know it right away. Working back from the output of a system that is not behaving properly makes it much harder, and it may be near impossible to pinpoint multiple, interacting problems.
Troubleshooting a simple problem is no big deal, no matter how you approach it. The hard part is troubleshooting when more than one thing is going wrong. For that, you will reach your destination sooner if you start at the beginning.
Tom Lawson
July, 2021