Precision resistors and tolerance stackup in general

This must be the season for great graphics. After seeing the solar cell output over temperature graph a couple days ago, today I see this great article about the reality of using precision resistors. It is from the great folks at Vishay, by way of my former co-workers at ECN Magazine.


Vishay shows what can happen to their beautiful resistors once you and your customers get your grubby hands on them. TCR means temperature coefficient of resistance.

The same chart got used in an article in EDN, where I worked. The graph also saw use in an Electronic Design article about foil and thin-film resistors. The mother lode was from a Vishay app note by Yuval Hernik.

If you are using a resistor to measure current you should not trivialize the accuracy problems that come with the real world. You can see in the chart that the ±0.05% resistor you buy from Vishay can end up being a ±1% resistor after a few years in the field. It’s not Vishay’s fault. They did not stress the resistor soldering into the board. They didn’t expose it to humidity and temperature gradients that damage the device. They didn’t drop it and shock it and over-voltage it.

The point of this is that you can’t build a product that specs ±0.05% accuracy if you start with ±0.05% resistors. You customers don’t care what you buy from Vishay and they don’t care what you built. They care about they use, perhaps years later, at some horrible temperate in some inhospitable humidity over some astronomical altitude. When I was at Analog Devices they had a test for voltage references that was running for years. Years! This was to evaluate the long-term drift that the parts would exhibit. I am happy to say that the ADI parts seemed better than most.

And here is the thing— when it comes to these drift problems, no one can tell you what is going on. We simply don’t understand the physics of it. I contend we really don’t understand noise either, but that is an argument for another day. But drift, which you can think of as “dc noise” if you want mess with your head, is a universal problem. We older folks that used to wait for tube radios to “warm up” seem more comfortable with the concept. But op-amps and maybe even discrete components have to settle in as well. This is not the few microseconds it takes for the internal circuits to start working. It is the minutes or days it takes for the amplifier to come to its final dc offset error.

I have several pals that are trying to make their own test equipment to save money or just build things like a Maker movement. That is fine if you don’t really have to trust it. Believe me, Fluke and Agilent and Tektronix earns every penny they ask you for. This is why I am wary of cheap knock-off test equipment. I would rather buy used name-brand equipment that I can trust to keep accurate over their lifetime.

As to these resistor tolerance issues, one answer is that you calibrate the product every time it’s turned on, or even more often. When I did automotive test equipment at HP (before Agilent split off) my solution was to use the best voltage reference that money can buy. Back then it was Thaler. Since then (1998) I found out that the Thaler part I used was a National Semiconductor part that was hand-selected by Thaler. No matter where you get it, you have to have a low-drift and low TC (temperature coefficient) part. I also used very good initial accuracy parts, since I did not want to have to calibrate the board the first time in the factory.

This way, I had the acquisition system measure its own reference. That way I could calibrate any errors or drift in the attenuator resistors. The other aspect was using a very good crystal. This way you know voltage and time. Most everything else you can derive in firmware. I called it “a rock and a ref,” since rock was slang for the quartz crystals. I still remember Bob Shaw asking me what pots had to be adjusted on the board for manufacturing. I told him there were no trim pots or trim capacitors. He was astonished. I told him about a rock and a ref. I joked that if he really wanted pots I could add them back in. He told me no, and thanked me for designing something that did not need factory calibration, since it just calibrated itself. The other horrible thing about pots is that they are terribly unreliable components. Only electrolytic and tantalum capacitors are worse. If you have vibration, pots are a really bad idea.

OK, product pitch time, these accuracy problems are why you should think about using Atmel AFE (analog front ends). We make them for the smart power meters. And I don’t mean to imply that Atmel is the only outfit. All the semiconductor makers make AFEs for various tasks. If it can offload your accuracy problems with calibration or the precise accuracy that comes with semiconductor processes, it is always a good deal to pay for an integrated solution rather than build it yourself. For years I told National Semi that people would pay for precise ratiometric resistors. It took Linear Technology to actually make the parts.

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