JFET/opamp noise questions - Page 1
Forum section: electronics / beginners

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 01, 2015, 07:13:24 am
content: i want to find out, how does 1 compare the specs of say2SK369, which the noise spec = 1dB @ 1kHztoLSK170, which the noise spec = 0.9nV(sqrt Hz) @ 1kHzis there a way to convert 1 to another? or an approx approach to compare?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 01, 2015, 10:29:39 am
content: AFAIK, both are low.Noise figure/factor is dependent on the circuit. That is, with everything matched ideally, what do you get?Noise voltage is an unmatched parameter, which might make it more useful in some applications (especially where power and impedance might be arbitrary, like a scope front end), but doesn't really tell you about its performance as an amplifier (i.e., if the matched impedances were quite low, then that 0.9nV/rtHz would be a relatively larger part of the circuit's signal, and therefore worse NF).If you really need vanishingly low noise, consider cranking up the amps and using many in parallel. Don't worry about matching, it'll work out.Tim

author: dom0
author_rank: Super Contributor
timestamp: December 01, 2015, 10:33:27 am
content: If this is about a measuring amplifier for LF, thenthis postmight be of interest to you.Average noise density is ~0.7 nV²/Hz over a .1 Hz - 100 kHz bandwidth.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 01, 2015, 01:06:17 pm
content: Quote from: dom0 on December 01, 2015, 10:33:27 amtnx domo n tim (yep its a DMM front end, more like switch), i saw that thread before, only managed to understand bits n pieces (my electronics foundation is not good)my purpose is to explore possibility of replacing interFET IF1320 (SOT23) into TO92 package. however it also seem the actual board is using many models of the JFET as switch (ie : Q104), even though it was suppose to be a single part number, maybe it was fixed PCB before i got itin anycase, could anyone ID what those 2 physically are? because the IF1320 are not really those 2 (which they are suppose to be) T8W2A and what looks like 6Cx ?i read another website that there were replacements done using 2N4393, w/o any noise specs.any help appreciatedty in advance

author: Kleinstein
author_rank: Super Contributor
timestamp: December 01, 2015, 01:33:03 pm
content: The IRF1320 looks like a poor choice in this circuit. As a switch, the voltage noise is not relevant. The more critical parameters are gate leakage and DS- Leakage in off state. Also only 20 V maximum gate voltage could be a problem.A more typical one would be something like 2N4392 though not TO92 but TO18 (metal).There are other similar JFETs made for switching.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 01, 2015, 06:10:18 pm
content: Quote from: Kleinstein on December 01, 2015, 01:33:03 pmthanks for your input again kleinstein, hmmm a quick looky look at the pdfs, they dont seem to specify it. i found this (http://www.seekic.com/circuit_diagram/Basic_Circuit/index200.html) , i am guessing this is a high variance parameter?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 01, 2015, 07:23:41 pm
content: Leakage current is often not specified with a typical value. It can vary a lot, and testing it is relatively expensive, because measuring such small currents takes quite some time.So it's a good idea to have a few more FETs and select good samples. There is a good chance to find some with leakage much lower than specified maximum.Anyway, I see not advantage in finding a To92 type replacement. The Fets used as switches are not that critical and produce essentialyl no heat.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 01, 2015, 07:37:51 pm
content: hehe noise hunting, poking at everything

author: Kleinstein
author_rank: Super Contributor
timestamp: December 01, 2015, 08:50:03 pm
content: The FETs are responsible for much of the input bias. The trouble is that this are currents in the 1-10 pA range, and some may compensate. Before changing anything in this area one chould check the input bias - e.g. by measureing how fast a small capacitor (e.g. 1 nF) charges (with AZ off !).The tricky part is that the auto zero circuit might also contribute signalifcantly via charge injections. So some of the Fets might need to be same type (capacitance) to minimize the charge injection. So it likely wil not be possible to change just one of them.Best case ony could gain some margin to allow for a lower noise (but higher bias) OP.

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 03, 2015, 09:15:47 am
content: sanity check Qnto obtain "amount" of noise from nV/sqrt.Hz spec, i just need to (nV.Hz x bandwidth)^0.5 = approx nV of noise for that bandwidth?but this amount is not related to the p-p noise which is separately stated in the specs, but then how would i "classify" the calculated noise as? a RMS value?since even the p-p noise also carries normally a stated 10Hz bandwidth, shouldnt a 10Hz noise density have some kind of math relation to the p-p noise? (but from many op amp specs i saw, it doesnt seem to be related)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 03, 2015, 09:51:49 am
content: e_N * sqrt(BW)not parens around everything. Which would give you root volts, which aren't helpful.Noise density is RMS. p-p is normally about 3x higher, but obviously varies because statistics. It's higher for wider bandwidth (effectively, more trials of MAX(RAND) in that period).LF noise is typically in the 1/f (flicker) regime, where noise density is not meaningful, hence the alternate rating method.See if there's a e_N vs. freq graph. This is illustrated.Tim

author: TiN
author_rank: Super Contributor
timestamp: December 03, 2015, 10:31:11 am
content: I hadbodged JS calculatorfor that conversion.You can take a look, perhaps it's useful?

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 03, 2015, 10:49:16 am
content: Quote from: T3sl4co1l on December 03, 2015, 09:51:49 amah thanks Tim n TiN. root of BWanother related sanity checki went to take a look at LT article on op amp as well, (DN015F.pdf), for calculating RMS noise of op amp.if say for example we use OPA140 as "noise material". the noise plot in pdf suggests 1/f corner frequency is 1kHzusing its equation 1, with NO = 8nV.sqrtHz, FH = 20 Hz, FL = 0.1Hz, and FC = 1000. am i correct to say the opamp noise approx is 583nV? it seems like too high an estimate, i assume my FC is wrong?(and RMS noise of 6x according to the LT article = approx 3.6uV) it seems the p-p noise is quite the killer !edit* applying same equation to LTC1050, it appear to have FC of 3kHz, but the nV.sqrtHz parameter is missing, how do i estimate noise like i did for OPA140?** i also have a go at LT noise.exe program. see pic

author: Kleinstein
author_rank: Super Contributor
timestamp: December 03, 2015, 05:58:04 pm
content: The noise of the OPA140 is lower: white noise at something like 5.1 nV/Sqrt(hz) and a 1/ corssover at about 10 Hz. The crossover is where the noisedensity reaches about 1.4 times the white noise, not where the curve starts to go up.So the 580 nV is to high for the OPA140. The datasheet gives something like 250 nV__pp for 0.1 - 10 Hz. For a slightly higher upper limit this will not be so much more, as the noise in the 10-20 Hz region is rather low. So expect somthing like 300-350 nV_pp for the 0.1-20 Hz range.The LTC1050 has approximately white noise (90 nV/sqrt(Hz)) is the LF region. There is also a number for DC-10 Hz which is essentially the same as 0.1 Hz - 10 Hz: about 1.5 µV_pp. For twice the Bandwidth expect about 1.4 (sqrt(2)) times the noise - something like 2.1 µV_pp.

author: splin
author_rank: Frequent Contributor
timestamp: December 03, 2015, 06:43:23 pm
content: Quote from: T3sl4co1l on December 03, 2015, 09:51:49 amI think you meant to say Vpeak = VRMS x 3; Vp-p = VRMS x 6?

author: dom0
author_rank: Super Contributor
timestamp: December 03, 2015, 07:08:06 pm
content: Both are correct (for different sigma, obviously).

author: Kleinstein
author_rank: Super Contributor
timestamp: December 03, 2015, 07:29:05 pm
content: The ratio V_pp to V_rms depends on the realtve bandwidth. The smaller the bandwidth the lower the value. The narrow Band limit is a pure sine and thus a factor of 2*sqrt(2) or about 2.8 . With larger bandwidth it gets larger so a factor of 3 would be rather low bandwidth only. The value of 6 is a more typical value.If the noise is not white but shows strong correlations the numbers can be also different (especially larger).

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 03, 2015, 08:48:59 pm
content: Quote from: Kleinstein on December 03, 2015, 05:58:04 pmok, i think my grasp of it is now slightly better, for OPA140, i switched to the 1kHz parameter (5.1nV/sqrt.Hz)and for 20Hz, i get 371nV @ 20Hz. still too high? or maybe based on the graph plot, the estimate of FC = 1kHz is also too high. if FC = 800Hz, it will give 332nVi dont know how to get 90nV, from the pdf graph? how do you estimate it?if i look at another example say LTC2057, its plot is clear, showing about 12nV. do you reverse calculate from uV p-p for LTC1050?(thanks for all tips everyone)

author: dom0
author_rank: Super Contributor
timestamp: December 03, 2015, 10:47:33 pm
content: QuoteLTC1050 data sheet has a plot of voltage noise density vs. frequency (first page, bottom right corner).Interestingly they specify long term drift in the same dimension (V²/s, 50nV²/month) as we usually do with white noise densities ... do they mean to imply that the offset drift of this device is white?

author: 3roomlab
author_rank: Frequent Contributor
timestamp: December 03, 2015, 11:35:50 pm
content: Quote from: dom0 on December 03, 2015, 10:47:33 pmok i was looking at page 3. i am too excited about all these and lack of sleepalways skipped to page 3 or 4 where there is plots