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Choosing an op-amp for your project

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Re: Choosing an op-amp for your project

#41

Post by kragen »

dragontamer wrote: post #14

Vos doesn't matter until it does. I find it surprising to see Vos mentioned as likely unimportant right next to CMRR, as CMRR and Vos are innately related in some circuits. In particular, any low voltage current sense circuit is going to require very precise Vos. Let's say you have a 0.01 Ohm current-sense resistor on a 5-Amp or so circuit. Your current-sense is now in the range of +/- 0.05Volts (!!!!). So a Vos of 0…

I was puzzled about that too, so I appreciate you confirming what I thought.

I'd think the main benefit of using lower-value current-sense resistors in this application would be that the resistor would heat up less, so its resistance would be more stable?

Re: Choosing an op-amp for your project

#42

Post by kragen »

kazinator wrote: post #26
auxym wrote: post #21

Earlier quoted context omitted.

Correct. A 10% error can be calibrated out if it is constant (in practice it probably varies a bit with temperature). But for measuring thermocouples or strain gauges, for example, 50 mV (your example) can be 100-500% of the signal, which becomes impractical to calibrate-out (due to maximum output levels, etc). For these applications, Vos is one of the first things to look at. Another one is the temperature coefficie…

Power supply and common mode noise can be 60 Hz.

And its harmonics, if you're standing near a fluorescent lamp.

Re: Choosing an op-amp for your project

#43

Post by kragen »

Tade0 wrote: post #27
dragontamer wrote: post #14

Vos doesn't matter until it does. I find it surprising to see Vos mentioned as likely unimportant right next to CMRR, as CMRR and Vos are innately related in some circuits. In particular, any low voltage current sense circuit is going to require very precise Vos. Let's say you have a 0.01 Ohm current-sense resistor on a 5-Amp or so circuit. Your current-sense is now in the range of +/- 0.05Volts (!!!!). So a Vos of 0…

Back in college I was told to not think too hard about Vos, as it's typically temperature dependent so you need to assume it's non-zero or will be in some circumstances and compensate with feedback anyway.

How do you compensate for Vos with feedback?

Re: Choosing an op-amp for your project

#44

Post by kragen »

dragontamer wrote: post #16
kragen wrote: post #8

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 lo…

> 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 m…

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.)

Re: Choosing an op-amp for your project

#45

Post by kragen »

peteforde wrote: post #7

I've watched the videos and agree that the OPA2323 does appear to sound amazing by subjective comparison to the TL071. However, it's not enough to say that it costs ~8.7x as much in local tokens because that doesn't reflect how much it costs as a percentage of the other components around it, many of which have a more profound impact on the outcome. This wouldn't stand as a good argument except that in many devices, y…

I think Zalewski's post is aimed at electronics hobbyists rather than high-volume producers or competitors. In that context, an OPA2323 might cost 98.99¢ https://www.digikey.com/en/products/detail/texas-instruments... vs. 7.958¢ for the TL071 https://www.digikey.com/en/products/detail/texas-instruments... but the Saturday afternoon you spend building and debugging your circuit costs hundreds of dollars in foregone income—if it's just an afternoon and not every weekend for a month.

So the BOM cost may not be a significant consideration for hobbyists, especially when weighed against things like familiarity or being able to keep a stock of a smaller number of less specialized components, as Majromax points out.

I'm surprised to see you say that a US$1 opamp is as good as we can currently achieve. Presumably there are Analog Devices chips that are better than the OPA2323 even for audio? Even if you can't hear the difference, you ought to be able to measure it.

Re: Choosing an op-amp for your project

#46

Post by hn4352 »

dragontamer wrote: post #14

Vos doesn't matter until it does. I find it surprising to see Vos mentioned as likely unimportant right next to CMRR, as CMRR and Vos are innately related in some circuits. In particular, any low voltage current sense circuit is going to require very precise Vos. Let's say you have a 0.01 Ohm current-sense resistor on a 5-Amp or so circuit. Your current-sense is now in the range of +/- 0.05Volts (!!!!). So a Vos of 0…

I was kinda shocked by the Vos comment as well.

On CMRR, in some mathematical treatments it's modeled as a change in offset voltage with respect to common mode, which indirectly effects output voltage of course so at the end of the day it's the same result. (See: https://www.google.com/books/edition/Design_With_Operational... highly recommended )

It's also odd that the 741 was dismissed, as it should be, but the TLV9301 was not recommended. This part is specifically called out on TI's 741 page as what to use instead in 2025. Not only does it perform better in basically every possible spec, it's also a drop in replacement for most, if not all, applications.

https://www.ti.com/product/LM741 https://www.ti.com/product/TLV9301

TLV9301 ($0.5) is also cheaper than a MCP6272 ($0.88)

Re: Choosing an op-amp for your project

#47

Post by magic_smoke_ee »

ckocagil wrote: post #29
formerly_proven wrote: post #22

Earlier quoted context omitted.

It and the LME49720 (same part) were discontinued a couple years ago along with the entire LME Overture series.

Potential replacements for ultra low THD would be: OPA1612, OPA1656 and OPA1642 with bipolar, cmos and jfet inputs respectively.

There's a massive (old) list of them in Horowitz & Hill TAOE 3e Table 8.3a p. 522. I'm sure you can just go to Digikey or Octopart and do a parameter search for high-voltage, low-noise, BJT-input op-amps too. If one wanted to "use a Ferrari to go the grocery store", they could always use a $20 AD797 or LT1115 for audio applications. :o)

Re: Choosing an op-amp for your project

#48

Post by hn4352 »

kragen wrote: post #43
Tade0 wrote: post #27

Earlier quoted context omitted.

Back in college I was told to not think too hard about Vos, as it's typically temperature dependent so you need to assume it's non-zero or will be in some circumstances and compensate with feedback anyway.

How do you compensate for Vos with feedback?

You probably don't, but your opamp might. https://www.analog.com/en/resources/technical-articles/to-ch...

Technically you probably could do it externally in most cases but it would require a bunch of extra stuff, and be a pain, so usually it's best to use the stuff built into the amplifier itself.

Re: Choosing an op-amp for your project

#49

Post by hn4352 »

perriera wrote: post #5

>f you’re designing a new op-amp circuit, here are some decent, all-around alternatives made in the 21st century None of the listed opamps would be an alternative to where an LM324 or TL071 are typically used - low cost, 20V-30V supply, infinite/guaranteed availability. They are 5V only and cost 10x as much.

Try the https://www.ti.com/product/TLV9301 $0.5 @ 1s of units w/ modern specs and a 40V supply range.

Re: Choosing an op-amp for your project

#50

Post by pythonguython »

buescher wrote: post #40

Be aware in a microcontroller design, an lm324-type op amp might outperform the on-chip ADC you’re using anyway. Even the cheapest op amps from major players are better characterized and specified on their data sheets than most microcontroller ADCs. There’s no virtue in spending other people’s money and supply chain risk to use a “better” part just because it wins a bench race or it’s newer or whatever.

What does it mean for an op amp to outperform an ADC?
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