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What is your favorite most versatile op-amp?
Zero999:
--- Quote from: MasterT on June 09, 2019, 11:39:53 pm ---Noise easy to null out by averaging in software. Though ADC sampling rate needs to be higher.
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Not true. If you're talking about averaging, then all you're doing is reducing the bandwidth, which is only acceptable in a DC application. Then there's 1/f noise, which increases at lower frequencies and equates to offset drift, which needs to be frequently nulled.
--- Quote from: Neomys Sapiens on June 09, 2019, 11:23:56 pm ---
--- 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.
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Just don't make it sink :). I mean, if we are talking about ADC, crossover distortion is irrelevant, isn't it?
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I don't see why not. Crossover distortion is only irrelevant for DC or when the output is continuously sourcing/sinking.
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Crossover distortion doesn't make it into the discrete samples.
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It does if some of the samples are taken at the point when the output current is changing direction.
exe:
--- Quote from: MasterT on June 09, 2019, 11:39:53 pm --- It is affected by frequency, but it does not go to 0% (THD) at DC level. So, no, LM358 and NE5532 are not good above 12-bits.
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DC offset can be nulled. I did a couple of experiments on lm358 and ne5532 from TI measuring offset voltage vs input vs supply voltage on a breadboard at room temperature (unity-gain configuration, no load). In addition, I also heated the opamp with hotair gun to ~110C. I include the best and the worst opamp. I used soic opamps, mounted on a dip adapter, the circuit assembled on a breadboard using crappy wires, flux wasn't cleaned. Please find the data attached. In total, I tested two samples of ne5532 from TI, two samples from OnSemi, and four samples of lm258/lm358 from various vendors. All bought from reputable distributers (Farnell and TME). For NE5532 I include my best sample, as well average sample. For lm358 I just include "typical" performance.
I did measurements in 2017, so don't remember all the details. The goal was exactly to understand if they are suitable for 12bit ADC. My NE5532 samples showed that Vos is stable within 0.05mV if enough voltage headroom is given (as measured by UT61C). Temperature coefficient is about 1.5-3uV/C (but I measured only two datapoints: at 25C and ~100C on a breadboard, thermocouple issues etc were not taken into account, I let the temperature stabilize for ~1min to avoid die stress due to non-uniform temperature).
It's up to the reader to decide how good they are. But I disagree that 12bit is their limit.
SiliconWizard:
Averaging for noise reduction only works as long as the noise is reasonably random, or at least uncorrelated to the signal (probably a workable assumption here) AND the signal of interest is identical from one capture to the next. Otherwise you're just basically low-pass filtering and will lose bandwidth.
Averaging is often used for repetitive signals triggered by some event, not as a noise-reduction technique for non-repetitive or unsynchronized signals.
MasterT:
--- Quote from: David Hess on June 10, 2019, 03:57:04 am ---
--- Quote from: MasterT on June 09, 2019, 11:39:53 pm ---
--- Quote from: Zero999 on June 09, 2019, 04:36:01 pm ---Noise can't be accounted for in software, so it's pretty important in my opinion.
--- End quote ---
Noise easy to null out by averaging in software. Though ADC sampling rate needs to be higher.
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If flicker noise could be averaged away, then there would be no need for chopping.
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"Hardware averaging" - putting a few devices in parallel ?
What I'm getting at, there are solutions to minimize noise & offset, but distortion /non linearity are different species. This is why I'd move to #1 in my priority list.
Probably, DC accuracy may get better by increasing open loop gain using 2+ OPA as combine or composite whatever it's called. Anyway it's not so easy like paralleling for noise reduction.
Zero999:
Some types of non-linearities can be corrected in software, as long as they are known and repeatable. For example, a single supply op-amp might be non-linear near the negative rail, even at DC, but if we can compile a table of the input and output voltages, compared to the expected (ideal) output voltages, then we can apply a correction factor later.
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