Origins of the graphs in datasheets - Page 1
Forum section: electronics / projects

author: tree
author_rank: Regular Contributor
timestamp: February 19, 2014, 05:32:53 am
content: I've always wondered where the graphs in datasheets originate. Is it from measurement or from simulation?If it's from a measurement then how does bootstrapping work? How do you measure the performance of one IC whose performance is much higher than that of the instrument?

author: nickm
author_rank: Regular Contributor
timestamp: February 19, 2014, 06:18:51 am
content: I imagine all reputable manufacturers will either make the graphs from measurements, or at least verify with real world measurements. One example I know of where they show graphs with performance much better than any instrument is with high performance op-amps and their THD specs. TI and National explain the process in their datasheets and basically they would hinder the error correction capabilities of the opamp by like 40dB. They take the measurements and it will still be close to the noise floor of the instrument but they then manually change the numbers by 40dB. This way they have increased the resolution of the instrument by 40dB. I know the OPA1612 datasheet has the explanation and at one point I did the math and convinced my self what they were doing made sense.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2014, 07:01:28 am
content: Should always be measurements. Now, which ones isn't always apparent, whether they've taken the response of a single average device, or averaged the response of a sample of devices (and if so, what kind of statistical variation that sample set exhibited).The good ones include histograms (e.g., many BB, ADI and LT op-amp datasheets that give Vos frequency plots) and min/max margins on important curves (e.g., some National voltage regulators plotting Vout vs. T).As for means, the instruments are always better. This can sometimes be a daunting task, one which is occasionally detailed in an appnote (e.g., LT'sAN-124). Whether you're lucky enough to have such info depends on the manufacturer.I wonder from time to time about just how faithful the renderings are, as well. There was one MOSFET datasheet where the Coss looks nothing like the real device (nevermind the SPICE model, which doesn't even model that property at all). On close magnification, it looks like the disparate section is drawn with a separate bezier curve or something like that. It could very well be they best-fit a prettied-up curve and it simply came out wrong.Tim

author: JuKu
author_rank: Frequent Contributor
timestamp: February 19, 2014, 01:17:32 pm
content: Very often they even specify the measurement circuitry.

author: luky315
author_rank: Regular Contributor
timestamp: February 19, 2014, 01:28:37 pm
content: I worked for a semiconductor manufacturer and I can say that the values in the Datasheet are based on real measurements, but made under ideal conditions with test boards optimized for the specific measurement. So its nearly impossible to reproduce them in a real circuit. Some say that the graphs are even further "optimized" to look "nicer". Others say they lie...

author: tree
author_rank: Regular Contributor
timestamp: February 19, 2014, 03:13:22 pm
content: So how would one verify the noise floor of an op amp that will go into "the best" spectrum analyzer? Wouldn't the noise floor of the spectrum analyzer be much higher than the noise floor of your new low-noise amplifier?This is very much a question of what came first, the chicken or the egg?

author: c4757p
author_rank: Super Contributor
timestamp: February 19, 2014, 03:29:02 pm
content: Quote from: luky315 on February 19, 2014, 01:28:37 pmI don't have a problem with that, myself. Thosearethe properties of the device itself - I'd get similar measurements if I built up the same test jig. It's up to the person applying the device to figure out how a specific circuit will degrade its performance.Quote from: tree on February 19, 2014, 03:13:22 pmDid you read LT AN-124? It has some good information on that. Basically a combination of understanding the physics that are trying to interfere with your measurement, and verifying the properties of the measurement equipment itself in a way that does not depend on the DUT.The egg came first.

author: KJDS
author_rank: Super Contributor
timestamp: February 19, 2014, 03:30:45 pm
content: Quote from: tree on February 19, 2014, 03:13:22 pmThe noise floor of a spec ana will be set by many things, the input switching, mixer, post mixer matching circuit and the noise figure of the first IF amplifier are the dominant sources, but the circuitry beyod that usually still adds significantly. The one thing that doesn't contribute is an op-amp in any spec ana that I'm familiar with. If you want lower noise from your spec ana then it's common to put an amplifier in front of it but that will usually be designed using a discrete transistor (pHEMT) for lowest noise.For op-amps, they have so much gain that measuring their noise is fairly easy as you use the op-amp to increase the noise. IC testing is a bit of an art, mostly because testing time costs money, so during the first few batches then devices will be carefully measured and some taken by engineering to fully characterize. After that the means and spreads of some selected parameters will be constantly checked and SPC used to ensure that the process remains in control.

author: krivx
author_rank: Frequent Contributor
timestamp: February 19, 2014, 04:02:26 pm
content: Quote from: tree on February 19, 2014, 03:13:22 pmQuite often the op-amp is configured to amplify it's own noise floor to a level that can be easily measured, then the measurement is scaled down. Again, this is often specified in the datasheet.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2014, 05:59:32 pm
content: Even if you can't measure the noise floor because you're already using the best (like measuring the noise of a PHEMT with a PHEMT..), you can do an additive measurement, where you observe the RMS output for some time, with a terminated input, then with a noise generator of known amplitude, and bandwidth equal to or greater than the circuit under test. Adjust the noise amplitude so that the RMS output goes up by 41.4%. Now the noise generator is putting out the same amplitude as the input referred noise of the circuit under test.Note the input is terminated during the first part: this contributes Johnson noise from the termination resistor, which is present in any application of that circuit (e.g., an RF amp with 50 ohm matched transmission lines). This noise remains constant when connected to the noise generator's output resistance (which, whether it's an actual resistance -- from an attenuator perhaps -- or the equivalent output resistance of an amplifier, 50 ohms is 50 ohms), so that it counts towards the circuit's inherent noise floor. It can be subtracted out later if desired.I actually have a noise generator -- used for this purpose -- that I pulled from the junk. Rackmount, hybrid (tubes and, I think, mesa germanium or early Si transistor) design. Lovely. It uses magnetically biased glow discharges to produce copious amounts of noise, and it's gaussian to, I think it's supposed to be 3 or 4 sigma, which isn't bad, statistically speaking.Tim

author: calexanian
author_rank: Super Contributor
timestamp: February 19, 2014, 06:32:25 pm
content: A long time ago (all the way back to when tubes were invented) it they would calculate the theoretical curve and then try to confirm it to the best of their ability, typically picking data points and just using meters and plotting a curve the traditional way, and calling it good enough. By the late 30's actual scope camera pictures were appearing and actual electromechanical plotters were being devised. When tek came out with curve tracers and such it was off to the races.

author: tree
author_rank: Regular Contributor
timestamp: February 19, 2014, 08:11:15 pm
content: Quote from: c4757p on February 19, 2014, 03:29:02 pmYes. It uses a pre amp and another filter/amp to amplify the noise (via AC coupling). I read the part about the test jig and shielding used to reduce external noise as much as possible. But what about those amplifiers? They have inherent noise as well.Also, noise is not the only graph usually present in a datasheet. How about offset voltage drift metrics? How are those put together? What about input bias currents? How can we confirm sub-nA range currents?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 19, 2014, 08:56:50 pm
content: Pease can tell you about that one,http://electronicdesign.com/test-amp-measurement/whats-all-femtoampere-stuff-anyhow