Earlier quoted context omitted.
Power supply and common mode noise can be 60 Hz.
And its harmonics, if you're standing near a fluorescent lamp.
Yes, but I'm trying to argue that the frequency can go at least as low as 60.
Choosing an op-amp for your project
71–80 of 90 posts
Re: Choosing an op-amp for your project
#72Vos 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…
Amplification of the "common mode" means that if we tie the + and - inputs of the op-amp together, so that it's not doing its job (amplifying the difference), it's possible that the op-amp amplifies the voltage present on the combined + - node; the voltage between that and the ground reference. Ideally, the amplifier should not do that, but they do. The gain is not nearly as much as the differential gain, so I think the CMRR (common mode rejection ratio) is something like how many decibels down from the payload gain is this unwanted common mode gain.
Common mode gain can cause or contribute to an output offset. But if the amplifier circuit is biased such that + and - are kept very close to zero, that should not be the case. We normally care about the common mode rejection because common mode noise makes the + and - inputs swing together, causing the amplifier to amplify noise, like RFI/EMI.
In your current sense example, although we have a 10% error, it is a systematic error (a displacement), not 10% uncertainty in the value. It is an inaccuracy, not an imprecision.
You're assuming that the amplifier just buffers the output of the current sense resistor, so that the +/- 0.05 V across the resistor is translated to the output of the amp via unity gain.
We could configure the amplifier to amplify the signal, so that it becomes +/- 5V (or whatever). Then the Vos of 0.0005 will be a 0.1% inaccuracy.
Re: Choosing an op-amp for your project
#73Earlier 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…
> A 10% error can be calibrated out if it is constant (in practice it probably varies a bit with temperature). Unfortunately, Vos on cheaper general purpose OpAmps is the kind of thing that varies by... voltage. Ick. > For these applications, Vos is one of the first things to look at. Another one is the temperature coefficient on the gain. High frequency noise metrics such as CMRR and PSRR are sometimes important if…
The difference in brightness between two incandescent light bulbs running on DC could be expressed in decibels. Log10 of the ratio of wattages times ten.
Decibels are useful for any intensity scale that occurs in a context where something responds in a logarithmic way, regardless of whether the power phenomenon is steady or pulsating.
Re: Choosing an op-amp for your project
#74Earlier quoted context omitted.
> A 10% error can be calibrated out if it is constant (in practice it probably varies a bit with temperature). Unfortunately, Vos on cheaper general purpose OpAmps is the kind of thing that varies by... voltage. Ick. > For these applications, Vos is one of the first things to look at. Another one is the temperature coefficient on the gain. High frequency noise metrics such as CMRR and PSRR are sometimes important if…
> It's weird because CMRR is listed in decibels but it's absolutely a DC spec. If you get the Franco book equation 5.27 (my edition is the 3rd) explains why they do that. Long story short: It's a convenient form when CMRR = dVcm/dVos due to the orders of magnitude involved.
The moral is that you have to be careful. The ratio and decibels giving CMRR might not be the same as quoted for a given closed loop gain at a given frequency of interest.
Re: Choosing an op-amp for your project
#75Earlier quoted context omitted.
You're sniffing out the fact that I didn't have an exact application in mind when I wrote my earlier posts, lol. But yes, you are correct on this front. The more I post on this subject, the more I'm "backwardsly-targetting" a solar-powered MPPT circuit. Maximum Power Point Tracking circuits improve your solar-panel's efficiency by changing the current (through the use of a buck-boost converter, changing the voltage-a…
Oh, I see! And you might very plausibly be getting 5 amps at 0.7 volts or something there, if you're controlling a single solar cell? (I might be misunderstanding how they work.) If you're controlling a whole 200-watt panel it's less of an issue because usually they have several cells in series to get a more convenient output voltage. I feel like a car's transmission or a bike derailleur may be a good analogy to expl…
I'll just explain solar cells really quick then. The gist is that solar-cells are nearly constant-voltage, until you draw too much current and then suddenly they drop to useless-levels of voltages. The scary part is that the clouds change this point severely.
So a 60W Solar Panel might be 12V 5-Amps in the best case scenario (directly pointed at the sun during clear skies). If you draw 5.1-Amps, suddenly the voltage drops all the way to 0.7V or some other nearly useless level.
What is annoying about solar panels is that this point changes depending on temperature, shadows, and other conditions. As the sun sets and hits the solar panel at a shallower angle with dimmer afternoon light, it might drop to 12V 2-Amps, and of course at night it will drop to 12V 0.01Amps or less.
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A MPPT circuit "tracks" this point where you can obtain the maximum amps at the maximum voltage (or really: the maximum volts * amps), and changes the current draw of the effective circuit to "find" this point.
It requires some kind of power-sink (ie: hooking it up to the wall power and assuming wall-power can take infinite amounts of energy). Or more likely, a lead-acid battery or LiFePo4 charging circuit that can "store the extra energy dump".
Re: Choosing an op-amp for your project
#76[dead]
Re: Choosing an op-amp for your project
#77>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.
i build and repair audio electronics and the tl071 is used in abundance in audio circuits and is a bad choice imho. But good opamps for audio are at least a few dollars
I have an M-Audio Delta Audiophile 2496 PCI sound card somewhere. The circuit board has an 8-pin socket with a replaceable op-amp. Why does a stereo sound card have only a single operational amplifier? This sound card already sounds very nice, but what kind of op-amp would I replace it with if I wanted to upgrade it?
Re: Choosing an op-amp for your project
#78Earlier quoted context omitted.
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.
Oh, sure. You could totally build a chopper op-amp out of two discrete op-amps. I'd never thought about actually doing that...
Of course National Semiconductor had a chip for this, the LMC669 Auto-Zero.
https://www.electronicsurplus.com/national-semiconductor-cor...
Re: Choosing an op-amp for your project
#79Vos 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…
For current measurement, I'd rather use a Hall sensor. But to measure the signal from that sensor a sensitive opamp is needed, and your comment applies :)
Or a high side current sense amplifier...
https://www.st.com/en/amplifiers-and-comparators/tsc1031.htm...
Hall sensors drift.