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lcamtuf's avatar

Since this made it to HN an Hackaday, I'd like to address several quips from folks who I think just skimmed the beginning of the article.

1) Supply voltage: if you have a circuit that uses higher voltages, the picks mentioned at the beginning obviously don't work! I am recommending these ICs because 95%+ of contemporary designs are single-supply 3.3 V or 5 V circuits. If you need more range, there's plenty of modern chips that fit the bill and work better than LM741 & co. For example, OPA1656 is great.

2) Cost: the ICs mentioned at the beginning are inexpensive, but they're not the cheapest op-amps out there. They're just balanced picks with good specs. If you need the lowest possible price, you're still better served by a 21st century design. This may include redesigned, drop-in replacements for the vintage chips - e.g., LMV324A.

3) If it works, it works: sure. I'm not saying you have to switch if you have a drawer full of LM324 and it happens to work for you. But in practice, the internet is full of posts from people trying to make sense of crossover distortion, phase reversal, or signal voltage limitations of these old chips. They're just not very good op-amps, and novices should probably stay away.

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Mike's avatar

Designing a quality and precise analog circuit is truly challenging. A few months ago, around September, I've been asked if I could make a controller for a fertilizer mixer. At a first glance - nothing fancy, just a pair of PID regulators for EC / pH each.

From the perspective of this project I can tell, that designing the right analog measuring circuits is a state of art. It is not only limited to selecting the right op-amps (most of the important factors You have nicely described above), but also many different aspects of the analog design.

As an example, the EC/pH probes are extremely high impedance devices, so using JFET op-amp is a must. But that's not all. I've learned the hard way, that EC/pH measuring circuits cannot be supplied with power from a same source. The electrodes of the probes not only have their high impedance [GigaOhms], but also own potential, and if they are put together into a conducting liquid, the currents start flowing in between probes rendering circuit readings unreliable.

Then, my second idea was to use a micro DC/DC inverters like TBA 1-0512E from Traco. It didn't work out either. The readings were unstable as before... despite measuring circuits were separated! Then, by using DSO I found a nice DC freewheeling diode ripple noise on the power line. By reading carefouly datasheets, I've learned Tracos have 100[mV p-p] ripple noise, where - i.e. the pH probe generates in between of -177..177[mV]. So yet another fail.

And I have mentioned, the circuits have to be separated. Then how to separate the analog circuits from the digital one? Either by converting Voltage-to-Frequency or by ADC converters that use isolated power lines, like AD7793.

Making long story short, after several months of hard work I am getting 0,7% of accuracy from the designs I have, and I believe I could do better in several aspects. Mainly that's the reason quality, industrial-grade analog products are pricey - and perhaps, in this very case I should go for the EC/pH transmitter device instead of reinventing the wheel.

Some could say that my words are sort of abstraction, because there are EC/pH meters on the market for a dozen of bucks to buy. Not gonna start my rant on them, let's say buying a six-pack would be a better use.

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