Chuck D. Bones
Circuit Wizard
Design margin, simply stated, is the difference between a design working and a design not working. So why do we want design margin and how much is enough?
We can build a circuit, have it work fine and think that we're done. Then we build 10 more and surprise, surprise, half of them don't work. This actually happened to me early in my engineering career and I have seen it happen to others, many many times. The root cause was insufficient design margin. A circuit can have 1% design margin and work fine. Take that margin down to 0% and all Hell breaks loose. Component tolerances, temperature drift, aging and variations in the power supply voltage are some of the things that can erode the design margin.
So what do we do about it? The pros perform worst-case analysis (WCA) to see what the circuit will do under the worst-case combination of component and environmental variations. They perform worst-case testing where all of the external variables, including power supply voltage and temperature, are taken beyond the expected operating limits. Needless to say, all this is time-consuming and expensive, so it should come as no surprise that most pedal designs never see any worst-case analysis or testing. The customers end up doing the worst-case testing at gigs. If the manufacturer gets enough warranty returns, they might actually do something about it because it starts cutting into their profits. Things manufacturers do about it:
Many years ago, I was negotiating requirements of a product with a customer and the topic of design margin came up. I told him that the amount of margin he wanted was going to increase the size and weight. That's a big deal when the product is an electronics package that goes on a satellite. Archie's succinct response was "margin doesn't save size or weight, the only thing it saves is your ass!"
We can build a circuit, have it work fine and think that we're done. Then we build 10 more and surprise, surprise, half of them don't work. This actually happened to me early in my engineering career and I have seen it happen to others, many many times. The root cause was insufficient design margin. A circuit can have 1% design margin and work fine. Take that margin down to 0% and all Hell breaks loose. Component tolerances, temperature drift, aging and variations in the power supply voltage are some of the things that can erode the design margin.
So what do we do about it? The pros perform worst-case analysis (WCA) to see what the circuit will do under the worst-case combination of component and environmental variations. They perform worst-case testing where all of the external variables, including power supply voltage and temperature, are taken beyond the expected operating limits. Needless to say, all this is time-consuming and expensive, so it should come as no surprise that most pedal designs never see any worst-case analysis or testing. The customers end up doing the worst-case testing at gigs. If the manufacturer gets enough warranty returns, they might actually do something about it because it starts cutting into their profits. Things manufacturers do about it:
- Build the cost of warranty returns into the sale price
- Discontinue the product
- Revise the design
Many years ago, I was negotiating requirements of a product with a customer and the topic of design margin came up. I told him that the amount of margin he wanted was going to increase the size and weight. That's a big deal when the product is an electronics package that goes on a satellite. Archie's succinct response was "margin doesn't save size or weight, the only thing it saves is your ass!"
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