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What is your favorite most versatile op-amp?

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Zero999:

--- Quote from: MasterT on June 09, 2019, 04:19:01 pm ---
--- Quote from: Zero999 on June 09, 2019, 03:51:02 pm ---You've misread my post. I know the voltage noise is lower. I was taking about the current noise, which is higher, than the NE5532 at 1kHz. The LM4562 has 1.6pA/√Hz vs only 0.7pA/√Hz for the NE5532. If the impedance seen by the input is greater than about 2k925, the LM4562 will be more noisy, than the NE5532.

--- End quote ---
I see, the point of this thread is to define a priority list of the OPA's parameters. Since there is no perfection in this world, no OPA that would satisfy all requirements. Current noise is the last line of all spec I'd care about.
Voltage offset is close to the middle of the list. Its' easy to fix in hardware or software. The THD is the most important, difficult to straight up and it's directly limit accuracy of the circuits. DC accuracy is no exception, basically THD is equivalent to INL / DNL in the ADC-DAC specification.  Long time THD is associated with audio, or AC in general, but IMHO it's a big mistake to use LM358 at the front end of the anything with >12-bits resolution. And input offset is not an issue.

--- End quote ---
If depends on what you're doing.

Noise can't be accounted for in software, so it's pretty important in my opinion. Current noise dominates voltage noise, at higher input impedances and voltage noise is dominant at low impedances. This is very often overlooked. Someone might choose the NE5532 over the TL072, on the bases it has a much lower voltage noise of 5nV/√Hz vs the seemingly massive 18nV/√Hz of the TL072, but the TL072 has a current noise of just 0.01pA/√Hz vs the 0.7pA/√Hz for the NE5532. Then they wonder why it's so noisy, as the input impedance of the amplifier is 100k: the correct choice would have been the TL072 of course.

--- Quote from: exe on June 09, 2019, 04:27:14 pm ---
--- Quote from: MasterT on June 09, 2019, 04:19:01 pm ---The THD is the most important, difficult to straight up and it's directly limit accuracy of the circuits. DC accuracy is no exception, basically THD is equivalent to INL / DNL in the ADC-DAC specification.  Long time THD is associated with audio, or AC in general, but IMHO it's a big mistake to use LM358 at the front end of the anything with >12-bits resolution. And input offset is not an issue.

--- End quote ---

Isn't THD frequency-dependant? If so, can lm358 still be good for low-frequency signals?

As for offset, why do you say it's not an issue? If DC accuracy is required, it is an issue.

--- End quote ---
Voltage offset is often unimportant nowadays, as the op-amp is more often than not, before an ADC. The circuit can be zeroed in software by shorting the inputs, taking a reading, storing it in memory and subtracting it from subsequent readings.

The one thing to watch out for is drift, which can't be corrected for, unless it's convenient to frequently zero the circuit.

magic:

--- Quote from: exe on June 09, 2019, 04:27:14 pm ---Isn't THD frequency-dependant? If so, can lm358 still be good for low-frequency signals?

--- End quote ---
CMRR spec is 85dB, presumably only valid with light load, meaning that 20V of input swing produces 1mV change in offset voltage. And it's not even guaranteed to be linear, its slope may be flat close to ground and get steeper towards VCC or vice-versa or any other bizarre thing.
If you look at the schematic, the output is class A up to 50µA. If you want to sink more, the voltage gain stage needs to swing 3 diode drops down to turn off the upper darlington and turn on the lower sink transistor. Given 100dB open loop gain at DC, that's a 15µV step in offset voltage just to respond to changing polarity of load current.
But you are right that crossover distortion at 1Hz or so will not be as bad as at audio frequencies, where open loop gain is lower.

That's also a problem for most of those rail-to-rail input amps, like OPAx192. If you read the datasheet, they very carefully avoid specifying any CMRR number within that problematic region from 3V to 1.5V below VCC and instead tell you to have a look at figure 13-14 in "typical characteristics", which doesn't look pretty to say the least.


--- Quote from: SiliconWizard on June 09, 2019, 04:35:02 pm ---One thing to note about the NE5532 is that (at least in the TI datasheet for the NE5532 that I have) there is no figure to be seen regarding distortion. Did I stupidly miss it?
--- End quote ---
Nope. I have datasheets from TI, Philips, Signetics, JRC, Fairchild, ON Semi - no one specifies it.


--- Quote from: SiliconWizard on June 09, 2019, 04:35:02 pm ---Does anyone have a datasheet stating distortion? And if we're going to use NE5532's made by TI, how are we going to know what to expect in terms of distortion?

--- End quote ---
One Samuel Groner published quite comprehensive distortion measurements of a few dozen opamps. I think I saw a link earlier in this thread or just search for it. Beware it's 35MB.

Or, as Douglas Self puts it:

--- Quote --- The 5532/5534 is made by several companies, but they are not all created equal. Those by Fairchild, JRC, and ON-Semi have significantly lower THD at 20 kHz and above, and we’re talking about a factor of two or three here.
--- End quote ---
:-DD

exe:

--- Quote from: magic on June 09, 2019, 07:11:11 pm ---If you look at the schematic, the output is class A up to 50µA. If you want to sink more, the voltage gain stage needs to swing 3 diode drops down to turn off the upper darlington and turn on the lower sink transistor. Given 100dB open loop gain at DC, that's a 15µV step in offset voltage just to respond to changing polarity of load current.

--- End quote ---

Just don't make it sink :). I mean, if we are talking about ADC, crossover distortion is irrelevant, isn't it?

Zero999:

--- Quote from: exe on June 09, 2019, 07:16:29 pm ---
--- Quote from: magic on June 09, 2019, 07:11:11 pm ---If you look at the schematic, the output is class A up to 50µA. If you want to sink more, the voltage gain stage needs to swing 3 diode drops down to turn off the upper darlington and turn on the lower sink transistor. Given 100dB open loop gain at DC, that's a 15µV step in offset voltage just to respond to changing polarity of load current.

--- End quote ---

Just don't make it sink :). I mean, if we are talking about ADC, crossover distortion is irrelevant, isn't it?

--- End quote ---
I don't see why not. Crossover distortion is only irrelevant for DC or when the output is continuously sourcing/sinking.

Neomys Sapiens:

--- Quote from: Zero999 on June 09, 2019, 10:01:38 pm ---
--- Quote from: exe on June 09, 2019, 07:16:29 pm ---
--- Quote from: magic on June 09, 2019, 07:11:11 pm ---If you look at the schematic, the output is class A up to 50µA. If you want to sink more, the voltage gain stage needs to swing 3 diode drops down to turn off the upper darlington and turn on the lower sink transistor. Given 100dB open loop gain at DC, that's a 15µV step in offset voltage just to respond to changing polarity of load current.

--- End quote ---

Just don't make it sink :). I mean, if we are talking about ADC, crossover distortion is irrelevant, isn't it?

--- End quote ---
I don't see why not. Crossover distortion is only irrelevant for DC or when the output is continuously sourcing/sinking.

--- End quote ---
Crossover distortion doesn't make it into the discrete samples.

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