Comment by kragen | Hacker News Reader

Comment by kragen | Hacker News Reader

I found this post extremely interesting and informative, well above my expectations even given the eminence of its author. I'm not sure that everything in it is covered in The Art of Electronics — it's the kind of stuff the book covers, but I learned things from this post I didn't learn from the book. (But possibly it's information that's in the book that I just failed to absorb the last few times.)

I was recently looking at opamps as alternatives to the LM324 and found some interesting-sounding parts, in particular for a poor man's SMU application (precision, low current, and voltage requirements, but not much bandwidth). Haven't tried any of them yet. Comments would be welcome.

- LM324B: TI's improved LM324, with half the input offset voltage and otherwise improved ratings, and just as cheap, but still bipolar.

- OPA4197 and family: three dollars but it's a quad RRIO 36V 10MHz opamp in a 14-SOIC with ±15nA input bias current, ±100μV max input offset voltage, and 120dB min CMRR. The datasheet makes it sound amazing for the price. The OPA177 seems like it would be better but pricier.

- OP4177ARUZ: a 16-dollar quad 36V 1.3MHz opamp with ±2nA input bias current, 75μV max input offset voltage (at ±15V power supply), and min 120dB CMRR

Then I decided I'd screwed up my design sketch by requiring one of the opamps to sink significant current very close to the negative rail, which is something even "rail-to-rail" opamps can't do; I was planning to use millivolts from ground to represent measured nanoamps. If you want to look at a simulation with idealized opamps, it's at https://tinyurl.com/2aomvpn5, but don't take it as exemplary in any sense; it's a novice design with novice mistakes (and I would be grateful in the unlikely case that someone took the trouble to point some of them out). I think I need to redesign the circuit as a bipolarity-supply circuit or something, or use a differential output for the current measurement, or rethink it entirely.

Nowadays, it is hard to recommend a general-purpose opamp. Just plug the desired parameters into the search function and sort what's left by price.

(Distributors like DigiKey and Mouser have somewhat adequate search functions; I usually have to go to manufacturers' web sites like https://www.ti.com/amplifier-circuit/op-amps/general-purpose... to be able to filter by all important parameters. I'm mentioning TI because they have a large selection and a good search; even when you do not end up selecting on of theirs, you see what is possible.)

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If you need only a small negative supply and have nothing else, the LM7705 charge pump can generate −0.23 V. (This is designed to fit into the typically allowed 5.5 V range of a nominal 5 V opamp.)

I do not know what a "significant current" is for you, but there are opamps with strong outputs. (When comparing opamps, you usually have to estimate the drive strength from the short-circuit current.)

A general purpose OpAmp is just that, your general purpose first choice.

If you know more specific information about your circuit or it's application, the. You can specialize. But general purpose OpAmps are jack of all trades with specific known weaknesses to avoid.

In most cases, you calculate the error bars and none of the errors matter, so sticking with a cheap general purpose amp is best engineering.

Thanks! This is very useful advice!

What I meant by "requiring one of the opamps to sink significant current very close to the negative rail" is that, if you look at the schematic, the differential-to-single-ended op-amp that measures the voltage across the current-sense shunt resistor is using 10kΩ resistors in its feedback path, and the inverting input to that feedback network might be close to the positive voltage rail, say 12V, while the single-ended output is ideally millivolts from ground. So you have 12 volts across 20kΩ, which works out to 600μA, which has to be sunk into that op-amp's output.

600μA doesn't sound like a lot, and it certainly isn't going to strain the drive strength of any op-amp IC, but in this context we're hoping for millivolt precision down near the negative rail. The OPA4197 datasheet https://www.ti.com/lit/ds/symlink/opa4197.pdf figure 14, "Output Voltage Swing from Negative Power Supply vs Output Current (Maximum Supply)", shows what you might call a gently nonlinear output impedance roughly in the 40–80Ω range depending on temperature (2–4V at 50mA), which means 0.6mA of output current works out to tens of millivolts (24–48mV using those nominal impedances). Worse, even under no-load conditions, it's rated to swing only down to as much as 25mV from the negative rail (§6.7, "Electrical Characteristics: VS = ±4 V to ±18 V (VS = 8 V to 36 V) (continued)", p. 8, "Vₒ: Voltage output swing from rail, Negative rail").

In retrospect, it seems obvious that the op-amp's output isn't going to be able to reach beyond the input rails (unless it integrates a charge pump like the LM7705 internally) and is going to have trouble getting too close to them when it's sinking any current (for the negative rail, or sourcing for the positive). Because where is that current being sunk to? You need some voltage drop to get the electrons and holes to move in the desired direction through the silicon. A small negative supply might be the right solution. Or a differential output, which would be easy.

>Nowadays, it is hard to recommend a general-purpose opamp. Just plug the desired parameters into the search function and sort what's left by price.

This 100%. If you need a comparator get a comparator not an op amp. Current measuring? There are specialized chips for that as well, etc.

In this strong form, this is excellent advice for someone who is not me and is not doing what I am doing.

I live in a third-world country where importing chips from abroad is expensive, unreliable, slow, and sometimes dangerous. There are circuits I cannot build because I cannot get the very specialized parts they need. Obviously a linear power supply that can measure how much current it's supplying is not such a circuit, unless you have very stringent precision requirements.

It would be to my benefit to figure out a relatively small set of parts I can buy, ahead of time, in bulk, to cover a wide range of possible circuits. Better still if they're so popular that local distributors have them in stock. An analog comparator probably needs to be in that set. A chip specialized for current measuring probably does not.

If you're designing a product for mass production that needs to be competitive in the market, you can't do it that way. Super-specialized parts will always have better performance, and usually better price/performance than overpowered general-purpose parts. (Also, you need to live in Shenzhen.) But hobbyists have other priorities.

This comment reminds me of a video I saw recently (not sure if I could find it) where someone broke down failure points for projects based on different aspects of engineering, using "designing a drone" as the example project.

For someone working on the systems interactions, the failure point is making sure all the bits of the project are working together.

For someone working on optics, the failure point is finding cost-effective optics -- if you can't do that, then the project isn't going to go forward.

For the hobbyist? The failure point is <i>getting the project done</i>. Every other concern takes a back seat to this!

From my vantage point as someone living in America, however, I'm probably in a similar boat to you, because of my inexperience in electronics. If I want to take a deep dive into the subject, I'd be much better off getting a lot of generic cheap parts I can accidentally burn through, but would give me 90% of what I need, and I can worry about whether or not I need something highly specialized later, as my projects -- and my knowledge and skills! -- mature.

Yeah, that's a good point, and I've actually been terrible at getting projects done recently. Maybe a key question should be how few datasheets you can get by reading?

By the way, I'm living in America, too.

>the LM7705 charge pump can generate −0.23 V

fwiw I searched and found "A newer version of this product is available Same functionality with different pin-out to the compared device LM27761"

> Then I decided I'd screwed up my design sketch by requiring one of the opamps to sink significant current very close to the negative rail, which is something even "rail-to-rail" opamps can't do; I was planning to use millivolts from ground to represent measured nanoamps.

Get an OpAmp specifically designed for current sense applications.

OpAmps for current sense applications have high accuracy near 0 Volts and Vos measured in single digit microvolts.

Oh, you WILL lose bandwidth with these designs. So make sure you are allowed to be much much slower.

Thanks! This is probably excellent advice, and I wish I could follow it.

Being slower is not a big problem, but needing specialized parts might be, due to supply-chain issues.

(More detail in https://news.ycombinator.com/item?id=42627042.)

Oh boy you need to take another look at ToEv3 if you think this has better coverage on opamp topics. It has so much about opamps in it that it’s hard to tell you where to find it all - it’s so ubiquitous and spread out in the text.

Recommend Ch5 for Precision Design and Ch8 (or 9? Can’t remember) on noise.

I didn't mean that this post was "better" overall, but rather that it contained some information I didn't see in AoE (or didn't retain). Clearly AoE's presentation of opamps is far more comprehensive.

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