Noise in JFET analogue switches - Page 1
Forum section: electronics / projects

author: Cerebus
author_rank: Super Contributor
timestamp: May 21, 2016, 12:20:43 am
content: Sorry, I'm going to do this without diagrams - they're not really necessary for something this simple but I know they make it 10x faster to grasp what's being described.What I'm after is an analytical description of how to calculate noise in JFET analogue switches. Just to be clear, the type of switch I'm talking about is where you connect a signal you want to switch to one side of the channel of a JFET as an input and take the output from the other side of the channel - controlling whether the signal passes or not by the voltage on the gate - follows the input for 'ON', tied to a negative rail for 'OFF' (for an N-channel JFET). So, your standard JFET analogue switch.From what I can figure out the noise this adds to the signal when it's 'ON' is the thermal current noise from the channel which is going to be the same as for the more usual common source amplifier, or source follower -2/3gm4kT A2/Hz - although for the more usual case it's translated into input referred voltage noise by convention. Obviously in the analogue switch case the noise can also be viewed as voltage noise by just taking the channel resistance into account.I pretty confident about that bit. What I can't figure out is the noise when the switch is 'OFF'. My instinct says that it is less, as the channel is at very high resistance for the most part and most of the route that current can flow to set up a noise current is leakage paths. What I can't do is quantify this.So. Am I right about the 'ON' case, and can anyone provide a proper description of what happens noise-wise in the 'OFF' case and ideally a method for calculating it?

author: uncle_bob
author_rank: Unknown
timestamp: May 21, 2016, 12:49:25 am
content: Quote from: Cerebus on May 21, 2016, 12:20:43 amHiModel it as an amplifier and use what ever noise data you have for the JFET. A normal analysis program will work.Bob

author: Cerebus
author_rank: Super Contributor
timestamp: May 21, 2016, 01:17:17 am
content: Quote from: EmmanuelFaure on May 21, 2016, 12:32:40 amFirstly - I'd just like to know anyway.Secondly - in the circuit that prompted this, which I want to work out a noise budget/estimate for, the only parallel resistances there will be are:The input source resistance, which in this particular case could be literally anything.The gate of a JFET that's the input amplifier for the next stage and that's specifically engineered for very low bias current (~60 fA if my simulation can be believed)Several other JFET switches in parallel (only on the 'output' side) with this that make up an input multiplexer and the cumulative noise from several switches could be significant.Thirdly - although it might be negligible in many or even most cases I'd feel uncomfortable waving my hands and saying so, only to have someone ask "How do you know it's negligible? What's the noise voltage or current?".

author: danadak
author_rank: Super Contributor
timestamp: May 21, 2016, 01:18:13 am
content: There are parasitics associated with analog switches that couple noiseinput to output.http://www.analog.com/media/en/training-seminars/tutorials/MT-088.pdfRegards, Dana.

author: Cerebus
author_rank: Super Contributor
timestamp: May 21, 2016, 01:37:27 am
content: Quote from: uncle_bob on May 21, 2016, 12:49:25 amAgreed it can be modelled as a common base stage. Noise data? Extraordinarily rare in JFET datasheets for parts that are marketed as switches but when it does appear it's always for common source/source followers and expressed as einor a noise figure.Let's be clear. There's a gap in my understanding of what's going on here. If it was a common source amplifier or source follower I wouldn't need to ask anything. I might even manage to figure out what was going on a small signal common base amplifier (I have a bit of a blind spot with common gate/common base stages) but the switching voltage makes it look like a very peculiar large signal input if you model is as a common base stage and at this point I'm lost.QuoteThe normal analysis program here is me, pencil, paper and calculator. I want to properly understand what is going on and be able to manually put figures to it before I go anywhere near SPICE or friends. Aside from fear of being haunted by the ghost of Bob Pease, I have a healthy distrust of circuit simulators until I know enough about what's being analysed to know when SPICE is lying to me.

author: rfeecs
author_rank: Frequent Contributor
timestamp: May 21, 2016, 01:40:12 am
content: Quote from: Cerebus on May 21, 2016, 12:20:43 amI think that equation is for the JFET in the saturation region. When it is acting as a switch, it is in the linear region, around Vds=0. Here it is acting like a resistor, so model the noise like resistor. The resistor value is the on resistance of the channel.When the JFET is off, the leakage current is the current of a reversed biased diode junction, whatever noise current that might be. It is likely to be very small.

author: Cerebus
author_rank: Super Contributor
timestamp: May 21, 2016, 01:45:10 am
content: Quote from: danadak on May 21, 2016, 01:18:13 amYes there are, and yes they do, but that doesn't help with working out how much noise the JFET itself is generating.And to be completely clear, I'm not talking about using prepackaged MOSFET or JFET switches here, but raw, build it yourself JFET switches. It's a lowish level semiconductor question, not a 'how do I do this' kind of question.Oh, and anyone who says "just use a prepackaged switch and get the figure from the datasheet":Has missed the pointWill be haunted by the ghost of Bob Pease

author: uncle_bob
author_rank: Unknown
timestamp: May 21, 2016, 02:11:54 am
content: Quote from: Cerebus on May 21, 2016, 01:37:27 amHiWell if you are going to do it all by hand anyway, just set up and measure the noise parameters on a few devices. That is the only way to be *sure* you have it right.Bob

author: Cerebus
author_rank: Super Contributor
timestamp: May 21, 2016, 02:24:21 am
content: Quote from: rfeecs on May 21, 2016, 01:40:12 amGood point, qualified by Vds=0 only being necessarily true when the switch is 'ON'. That equation does model the noise from the channel resistance it's just in the more unfamiliar current noise form, you just use the gmthat's appropriate to (Vds=0, Vgs=0) which just happens to be 1/RDSon. In the more usual version of that equation which models the noise as voltage noise it's more obvious that you're dealing with the channel resistance.QuoteThe noise due to gate leakage current is the shot noise due to that leakage current and it's therefore the square root of bugger all* - which is great. The resistance it would cross to get to the gate in the 'OFF' state from the drain and source is not so nice in terms of converting that trivial current noise into voltage noise.I just need convincing that this is the only source of noise in the 'OFF' state and that there's no mechanism beyond this generating noise in the channel as well. My assumption has been that the channel essentially becomes a no movement zone for charge carriers when it's 'OFF' otherwise the off resistance of the channel would generate masses of thermal noise (around 127uV for a 1012ohm channel resistance which isn't unrealistic).*Assuming we are at room temperature. At 125C all bets are off on JFET leakage currents being small.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 21, 2016, 02:33:51 am
content: 'ON' noise should be simply channel resistance (a resistor is a resistor!) plus gate leakage.'OFF' should be channel leakage, but, contingent on the input voltage. Obviously(?), gate leakage will be comparable, too.Tim

author: Cerebus
author_rank: Super Contributor
timestamp: May 21, 2016, 02:39:16 am
content: Quote from: uncle_bob on May 21, 2016, 02:11:54 amI'm sure you're trying to be helpful but "just measure it" isn't a substitute for an analytical understanding of what's going on which is what I'm trying to achieve here. Yes, I'm trying to get the understanding to refine a rough design (i.e. there is a practical end here somewhere) but I'm more concerned initially with extending my theoretical understanding.

author: rfeecs
author_rank: Frequent Contributor
timestamp: May 21, 2016, 02:39:57 am
content: Quote from: Cerebus on May 21, 2016, 02:24:21 amIn my mind, gmis dId/dVgs, which is a function of Vgs and Vds. At Vds=0, gm=0. It increases as Vds increases until it saturates in the saturation region.Anyway, it is easy to get RDSonfrom the datasheet, so much easier to use.

author: Cerebus
author_rank: Super Contributor
timestamp: May 21, 2016, 02:54:50 am
content: Quote from: T3sl4co1l on May 21, 2016, 02:33:51 amPretty much what I thought, I just wanted someone to confirm that it's just Johnson noise and there were no hidden surprises.QuoteSo noise in the 'OFF' state is just going to be shot noise appropriate to the leakage currents going essentially (source->gate) and (drain->gate); current going that way because to be 'OFF' the gate has to be at a lower potential than the source and drain (N-channel assumed, swap everything for P-channel). And those leakage currents I know how to calculate. And there's no other noise source lurking in the channel because there's little to no charge carrier movement possible.All correct?

author: Cerebus
author_rank: Super Contributor
timestamp: May 21, 2016, 03:28:22 am
content: Quote from: rfeecs on May 21, 2016, 02:39:57 amAt this stage of the night I'm sufficiently past my bedtime that my brain is now officially made of guacamole and I'm not sure whether I should be saying "Yup, you're right", or trying to tactfully correct you, or just offering the leprechaun who's just appeared a whiskey. Time for bed...I think T3sl4co1l has hit the nail on the head - I should have thought of him first, as he often appears in semiconductor physics discussions. So hopefully, I'm just waiting for him to confirm that I've properly understood him.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 21, 2016, 04:54:15 am
content: Yeah, gm shouldn't be important because as long as it's in cutoff, gm is approximately zero.Pinchoff should be exponential, not arithmetic, i.e., Id goes as exp(Vg-Vp), as is the case with most things (MOSFET and vacuum tube "subthreshold", BJT over the full range). I don't know offhand if this is the case, or if it's usually modeled as such. (Modeling isn't necessarily a good example, because SPICE models for JFETs might be rather neglected. There's a slew of MOSFET models built in to standard SPICE tools today, and even those aren't good enough for the newest e.g. SuperJunction power FETs.)And there should be residual D-S leakage (as well as D-G and S-G), in whichever directions. I don't know how much, or if this is significant in comparison to gate leakage. Or detectable by correlation with the input.Quote from: Cerebus on May 21, 2016, 03:28:22 amThanks! Say, guacamole and a whiskey sounds great right about now...Tim

author: uncle_bob
author_rank: Unknown
timestamp: May 21, 2016, 12:53:41 pm
content: Quote from: Cerebus on May 21, 2016, 02:39:16 amHiIf you are trying to extend into an area where there is no data and only sketchy modeling, measurement *is* the first step. Trying to come up with a model based on no data is guaranteed to lead to disaster. Understanding is great, it needs to be based on something other than an un-verified hypothesis. An hour or three doing some testing will give you almost all you need to validate this or that model as correct. A loop back through the testing after the modeling is likely to get you closer to perfection than you need to get.Bob

author: Cerebus
author_rank: Super Contributor
timestamp: May 21, 2016, 01:22:11 pm
content: Quote from: T3sl4co1l on May 21, 2016, 04:54:15 amYou're not kidding. it's not just that the underlying model in SPICE is a bit dusty, which is what I take it you're saying. it's also that manufacturers issue sloppy models for their parts. In my recent travels I've found that most, not some but most, JFET SPICE models don't have correct basic temperature modelling parameters. I keep finding parts in which the Vt0ceisn't specified in the model and Betatce is left to take the defaults. Hopeless if you're trying to model drift against temperature after carefully picking the operating point to minimize the same.

author: Cerebus
author_rank: Super Contributor
timestamp: May 21, 2016, 01:52:02 pm
content: Quote from: uncle_bob on May 21, 2016, 12:53:41 pmI don't think I'm heading into uncharted territory (at least I bloody well hope not) just somewhere I've never been before and I hoped someone else had a map. It looks like they did - Tim seems to have put me on the right track.And I fear I'd be into more than an hour or three of testing. We're in the rough territory of a few nV/sqrt(Hz) here and making those measurements with any confidence would tax better equipment than I have and the ingenuity of people with far more low level measurement experience than me.As I now understand it we're just looking at the thermal noise from the channel for an 'ON' channel, so 127 nV (-138 dbV) of noise in a 10 kHz bandwidth for a 2N4393, and the shot noise from leakage currents in the 'OFF' condition. For a 2N4393 that's a gate leakage of ~ 50 pA at 10V reverse bias which gives a shot noise of 4 fA/sqrtHz or 400 fA in a 10 kHz bandwidth. Them's mighty small numbers and I'd not have much confidence in my ability to measure them well.

author: uncle_bob
author_rank: Unknown
timestamp: May 21, 2016, 09:19:52 pm
content: Quote from: Cerebus on May 21, 2016, 01:52:02 pmHiWell, there's a lot of gear out there that will get down below 1 nv/Hz. It's hardly state of the art these days. With some care, you can do it with a sound card, a couple op amps and a bit of calibration. Done that sort of stuff many times with many generations of parts over many decades. It's a lot easier than you might think.Bob

author: Zeranin
author_rank: Regular Contributor
timestamp: May 22, 2016, 12:15:54 pm
content: Quote from: Cerebus on May 21, 2016, 02:24:21 amI agree with others that in the ON state, you can probably wave your hands and say that what matters is the Johnson noise of the ON resistance. We can state with certainty that the ON noise contribution will not belessthan the Johnson noise of a resistor of the same resistance.But you asked about noise from the OFF state, and it's true, 1E12 ohms generates 127 uV/rootHz. A number of replies more-or-less said, the OFF resistance will be very high (it will), so the extra noise contributed will be small to negligible because other parallel resistance will be much smaller. Yes, I thought initially, that sounds reasonable.But its wrong. For example, these other parallel resistance(s) might equal 1 kohm, which might for example be the input impedance of the amplifier. OK, so you add another 1E12 ohms in parallel, and the overall Johnson noisedecreases, not increases, because paralleling resistors decreases net resistance, and thus noise.Next, or so it seems to me, we need to consider rather more exactly what circuit you had in mind, rather than just hand-waving and saying that you don't know how many such Jfet swtches there might be multiplexing one or several voltages to the input of the amplifier, and not stating what the input impedance of the amplifier, and so on. IMHO, your noise questions can only be answered in the context of the overall details of your circuit and what you are doing.If you have multiple Jfet switches, and only one (but not zero) will be ON at any time, then the noise from the OFF switches will be somewhere between negligible and irrelevant, because the dominant 'noise' will more likely be cross talk from the signal sources behind the OFF switches. If the OFF resistance is high enuff to make cross talk negligible, then the noise from the OFF resistance will be even more negligible.However, if there is a possibility that ALL the Jfet switches might be OFF in some situations, then the whole ballgame changes for the worse,muchworse. In this case, the noise at the input to your amplifier could be incredibly high, limited only by the input impedance of the amplifier. Unless you amplifier has <50 ohms impedance (unlikely) then the only way you will get low noise when all your switches are OFF is to use an additional Jfet to short the amplifier input when all other switches are OFF.So really, we would need to know exactly what you are doing, with more circuit details, and only then can we meaningfully analyse noise. Talking aboutnoise contribution from the OFF stateis meaningless when discussed in isolation.

author: Cerebus
author_rank: Super Contributor
timestamp: May 22, 2016, 02:39:38 pm
content: Quote from: Zeranin on May 22, 2016, 12:15:54 pmI was careful todeliberatelynot talk about circuitspecificsat the more system level because it wasthe general behaviour of thedevicein that kind of configuration1I didn't fully understand and I couldn't find clear explanations from the usual sources. Handling that as part of a system is relatively straightforward once you know how the device works in isolation.The deliberate obfuscation of irrelevant detail and sticking to just the specific topic was because these questions often wander off into "let's design his circuit for him" discussions rather than answering the original question. Oh look, that's what you want to do. I rest my case.I think the original, essentially device physics, question has been answered. If you  haven't got challenging enough design problems with your current shunts and want to have a circuit design discussion to get your teeth into I'd be happy to oblige, but in a new topic2I think.1As Bob pointed out, essentially a common gate amplifier.2I always find that terminology awkward. All the other discussion platforms I've ever used had a different name for it and used the word "topic" for the collection of what are called "topics" on this platform.

author: Cerebus
author_rank: Super Contributor
timestamp: May 22, 2016, 11:03:19 pm
content: Quote from: Zeranin on May 22, 2016, 09:58:54 pmYou need to either read or quote more carefully. The original sentence was:QuoteTheotherwisein the middle completely turns the meaning of the second half around as it's a counter-example, it's saying "this doesn't happen". If it did JFET switches would be next to useless just because of the noise levels involved.QuoteIs such confrontational language necessary? Why should we be judging one another? I thought we were just having a friendly technical discussion here, not a "who's right" contest.

author: Marco
author_rank: Super Contributor
timestamp: May 23, 2016, 12:23:42 am
content: I think it's best to simply consider it a voltage noise source with a high internal resistance, in my opinion that makes it easiest to conceptualize why it doesn't contribute significant noise.

author: Zeranin
author_rank: Regular Contributor
timestamp: May 23, 2016, 01:06:08 am
content: Quote from: Cerebus on May 22, 2016, 11:03:19 pmBut wait a minute, you can’t have it both waysIn the OFF state, does the channel behave as a very high value resistance such as 1E12 ohms, or does it become ano-movement zone for charge carriersmeaning infinite resistance? Your reasoning appears to be that it is the latter, becauseif it did (if it did behave as 1E12 ohms) JFET switches would be next to useless just because of the noise levels involved.With sincere politeness and respect, I believe you are wrong, and still apparently don’t ‘get it’. JFET switches when OFF donotbehave as a no-movement zone for charge carriers at all. On the contrary, they have a high resistance, such as 1E12 ohms, with corresponding Johnson noise. By your reasoning that would make themnext to useless because of the high noise levels involved, but that is also not true in any realistic circuit implementation, and was largely the thrust of my posting. Rather than repeat myself here, please read my previous posting carefully. If what I say above is wrong, then of course I want to know about it, and knowwhyI am wrong, that’s what I am here for, that’s how we learn.On your second point, I agree 101%. Confrontation was not intended, and we both enjoy technical discussion. FWIW, your rely to my first posting did come across as dismissing what I had said a little too lightly, and I maintained (and still do) that the points I made were both valid and relevant, and perhaps even interesting to some. In such cases, rather than ‘argue’ it is often best to ‘agree to disagree’ and let others judge. Agreed it should not become awho’s rightcontest, but the very nature of engineering and science means that we are and should be involved inwhat’s rightdiscussions. One of the beautiful things about engineering discussion/debate is that personal opinions and personalities are irrelevant. With patience and sufficient discussion, it is very rare that engineers won’t agree. Now I’m just raving so I’ll stop. Let’s enjoy our technical discussions, and I apologize for appearing confrontational.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2016, 02:42:57 am
content: *Cough*...Yes, you can model it as a very high resistance, but at that point, you're probably better off modeling it as a current source, which is what I implied ("leakage current shot noise").You'll get so-and-so fA/rtHz or whatever, which simply develops a noise voltage V = I*R on your system impedance R. Well okay, Z to be general, not R, but whatever.There's nothing wrong about Thevenin-Norton source equivalency, it's just a matter of which is most appropriate for the case.Tim

author: Cerebus
author_rank: Super Contributor
timestamp: May 23, 2016, 02:50:44 am
content: Quote from: Zeranin on May 23, 2016, 01:06:08 amOK, it seems we still haven't got to the bottom of this. [Fx: Sigh] And I thought we had.By the way, you should realize that 'no-movement zone for charge carriers' isn't mean to be taken completely literally, especially not to imply infinite resistance. Again, what I actually said was "essentiallybecomes a no movement zone" - that "essentially" you omitted means "in essence", "reduced to its essentials", it implies that I'm omitting fine details and it's often used to make a short explanation out of a long one. If you prefer, a region of reduced charge carrier mobility caused by the field effect of the field effect transistor leading to a highly increased resistance of the channel.My reason for believing that an 'OFF' JFET switch is quieter than the Johnson noise implied by the off channel resistance is that I've seen circuits that would be ruined by this and yet aren't. Take a look at the input stages of the HP 3458A. There are as many as 8 turned off JFET switches (and 1 on) feeding in parallel directly into the gate of the input amplifier (i.e. a very high-Z situation with no shunting impedance except the source resistance of the DUT which could still be very high). If we take a nominal 127uV rms as the noise of one switch, the noise contribution of those turned off switches would be on the order of sqrt (8 * 127uV^2) = 359 uV rms. That meter doesn't make high-Z readings as if 300 odd uV of noise have been thrown into the mix and has a 1uV rms noise spec (0.1 ppm of 10V range) into >10 G input impedance - equally I haven't seen a graph of noise versus source resistance for it.I think we're going to have to make Bob happy and actually do some real measurements. I can't physically do any electronics for a few days - I've got relatives visiting and that has necessitated a tidying up and clearing of the boards. Thinking about it, that's more like a week or two. When I get a chance I've got a rare hoarded U421 (that has amaximumgate operating current of 250 fA at room temperature) that I can air wire into a small high-Z pre-amp and follow it with a stage with a gain of, say, 1000 and plug it into the scope. I've got some MMBF4393s and J113s that can be used to create switches to test. I might have to order up a few quiet op amps too as the only truly quiet and low offset thing I have at the moment is a single OPA277 (220 nV 0.1-10Hz einand 20uV offset). Yup, quick check shows that the next best I have is some AD712s at 2uV einand 300 uV offset. That should put to bed once and for all whether you can see the full Johnson noise of the channel's off resistance or not.I kinda hoped (assumed really) that someone knew alotmore than me about this and would just throw out a definitive answer early in the game. I'm really surprised that is not apparently the case.

author: Cerebus
author_rank: Super Contributor
timestamp: May 23, 2016, 03:05:04 am
content: Quote from: T3sl4co1l on May 23, 2016, 02:42:57 amYou'll notice that I started early on with A2/Hz as my unit of choice. I couldn't agree more that this is the kind of situation that's much easier to think of in terms of current, well at least it is for me.I don't know why, but I've noticed a really heavy bias among electronics folks in general to always think and discuss in voltage rather than current terms whenever they can. The odd thing about this is that current (essentially electron flow per unit time) is, to my mind at least, a much more concrete idea than voltage (potential to do work). There are a whole bunch of things I never really got a handle on until I saw them in current terms rather than voltage terms although an example refuses to come to mind just now when I could use it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2016, 03:46:24 am
content: Quote from: Cerebus on May 23, 2016, 03:05:04 amAnd even more so, power and impedance. Or frequency and impedance (versus inductance and capacitance).Believe it or not, the humble LM358 is just as low-noise as the famed LT1028...on a power basis. The difference is, if all you're interested in is voltage, period, and you have a really low source impedance (1k or less), LT1028 can only be beaten by a few select discretes, or by building a parallel array. LM358 only breaks even for a source impedance of nearly 1Mohm -- in fact, a source resistance so high that you have trouble biasing the poor thing, given its input bias current (and especially input biasoffset).So the difference is, it's impractical to try to use an LM358 at its optimal source resistance, whereas in most LT1028 applications, the low impedance isn't a problem.Just as equally, you'd never use an LT1028 in an oscilloscope front end, where the input impedance must be high. The noise appearing on the input pins, when unloaded (open circuit, or at least high Z) would be huge! And anyway, you can't win against such a constraint: that is, expecting 50-ohm noise levels from a 1M probe. Though, a discrete circuit with e.g. LSK389 can do pretty damn well, especially after adding some tricks to control 1/f noise. (Some ideas in AoE3.)As has always been the case: use what is most appropriate. If your JFET switch circuit is too noisy (perhaps because of excessively high impedances being sensitive to leakage noise; or too low, and Rds(on) is adding too much), consider using relays (and deal with the slow and bouncy operation). Speaking only in terms of voltage, or only in terms of current, is meaningless. Speaking of both, however, you can at least put an optimum resistance on the part: if Rds(on) is 100 ohms and Roff ~ 1G, then the least noise will be had at a resistance which is the geometric average of these, i.e., ~320kohms. The noise factor will be quite good, because the ratio is very high; reasonable performance would be had from 1k to 100M.Incidentally, there's never such a thing as "open circuit". Despite needing a low source resistance for optimal operation, an amplifier might still not produce much noise when unloaded -- except it is loaded, into its own internal resistance and capacitance. It's very likely the bandwidth of the input stage changes with resistance. It might be that the only frequencies where e_n = sqrt(4*kB*T*Rs) holds true, is at very low frequencies (namely, where Rs dominates over any filtering effect from L or C). But the integrated noise can still be small, because the bandwidth is small (1nV/rtHz out of a 1Hz bandwidth is, *calculates furiously*, all of 1nV!). Loaded into nominal resistance, the noise will be small, but the bandwidth can be many times higher, so that similar (total, RMS) noise is present.Tim

author: Zeranin
author_rank: Regular Contributor
timestamp: May 23, 2016, 04:28:59 am
content: Quote from: T3sl4co1l on May 23, 2016, 02:42:57 amYes, think in terms of current if you prefer. If we are talking about an equivalent noise current source to a 1E12 resistor voltage source, then don't forget that the equivalent current source is in parallel with 1E12 resistance. If you don't include the parallel resistance, then the noise voltage of the current source would be infinite. The magnitude of the current source is of course Vnoise/R.Cerebus, I'll get back to you re your posting. You raise interesting questions that demand answers.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 23, 2016, 06:18:47 am
content: Yeah, but the handy thing is, 1e12 is way fuckin' bigger than 3e5, so much that you don't need to calculate the parallel combination. T'other way around, you need to scratch your head about a voltage divider.Like I said, horses for courses...Tim

author: Zeranin
author_rank: Regular Contributor
timestamp: May 23, 2016, 06:29:39 am
content: Quote from: Cerebus on May 23, 2016, 02:50:44 amHmm. Interesting. Let me come at this from 2 angles. It is a generally observed fact that the Johnson noise of any resistance (as defined by V/I) is never less than the calculated value. Very often it is more, but never have I seen a case where it is less. For example, if the ON resistance is 100 ohms, then I would be amazed if the Johnson noise of this ON resistance was less than for a perfect 100 ohm resistor, and I suspect we all agree. Just think, we could use the ON resistance of a JFET for the feedback resistors in an uber-low-noise amplifier circuit, and beat the Johnson noise that plagues ‘normal’ resistors. But I suspect Mr Johnson won’t be fooled that easily. By the same reasoning, it seems highly unlikely to me that the OFF leakage resistance could exhibit lower noise than an equivalent resistor, either. I can’t tell you how much I would like to be shown wrong on that, because boy could I build some great circuits and make a lot of money, but it’s not going to happen, I reckon.But real world specifications and measurements are the ultimate bottom line, so how can the specifications of the HP3458A be what they are? I think you hit the nail right on the head :-That meter doesn't make high-Z readings as if 300 odd uV of noise have been thrown into the mix and has a 1uV rms noise spec (0.1 ppm of 10V range) into >10 G input impedance -equally I haven't seen a graph of noise versus source resistance for it.If all I have claimed is right, even the best DVM will be relatively noisy when configured for 1Gohm input impedance, and with nothing connected to the input, and that is certainly my experience. I don’t have an HP3458 just to hand, but tried that experiment with a Tektronix and Agilent 6.5 digit DVM, and sure enough, they are quite noisy with an open-circuit input, way, way noisier than their DC spec.If you are right, then it would be easy to prove for anyone with an HP3458 at hand. You claim that with no external connection to the input, just the internal 1 Gohm, that the measured noise will be way less than calculated for 1 Gohm. Of course, bandwidth needs to be taken into account. If the input is 10 Gohm, and total input capacitance is 10 pF, then RC=0.01 seconds, for a cutoff frequency of 16 hz. Depending on the chosen measurement integration time, the actual bandwidth may be less. This experiment is easily performed, and I would be taken aback if the measured noise was less than for a 1 Gohm resistor at the same bandwidth.What do you think?Being a hands-on person myself, I support your idea of measuring the noise of the OFF channel resistance. However, it might be challenging, though doubtless possible. For a start, you will need to measure what the actual OFF resistance is, remembering that it may well be an order of magnitude less than spec. As a check on your measurement, I suggest you find a conventional resistor of about the same value, and measure it’s noise as well. Keep in mind, the measured noise is expected todecreasewhen the resistor is connected across your amplifier input.

author: Zeranin
author_rank: Regular Contributor
timestamp: May 23, 2016, 06:58:16 am
content: Quote from: T3sl4co1l on May 23, 2016, 06:18:47 amYes indeed Tim, says me chuckling, I do indeed realize all you say, but I'm a pedantic bastard, and would not like us to be imprecise or incomplete in our understanding. Most time 1E12 is so much bigger than anything else that it can be ignored, but sooner or later you'll come across a situation or argument where itdoesmatter, and then you'll get bitten unless you are at least aware that it is there.

author: Cerebus
author_rank: Super Contributor
timestamp: May 23, 2016, 04:29:43 pm
content: Quote from: Zeranin on May 23, 2016, 04:28:59 amAfter a chance to sleep on it I've spun some numbers. The short answer is that the noise mechanism is irrelevant as it's all going to be dominated by the gate leakage current as that path is relatively low impedance compared to the channel. That is going to be the path that shunts most of the noise current away from the 'output*' of the switch.To get to this point I just made the assumption, for arguments sake, that the full Johnson noise for the channel just treated as a resistor was there and the full shot noise due to channel leakage current was there too. Further I assumed Vds = 0, Vgs = -10, and Vgd = -10 and a channel resistance of 10^12 ohms, typical voltages for an 'OFF' condition and mid rail signal. Then turned the Johnson noise into current noise, calculated the shot noise caused by the channel leakage and came up with the total current noise (rss) which is going to be something on the order of 2 fA BUT I know from experience that the gate leakage current under those initial conditions is on the order of 50 pA for a typical switching JFET (MMBF4393). So although it's a paltry 50 pA it represents an equivalent resistance of 200G which ordinarily sounds large but isn't in the face of a 2fA (12,500 electrons/sec) current. It still comes out noisier than I'd have expected.Note, that really is back-of-the-fag-packet numbers and they're just for argument's sake to show the magnitude of the effect and any individual numbers should be taken with a pinch of salt.* I'm finding it really frustrating not being able to say 'source' and 'drain' but they swap around depending on which side of the switch is at lower potential.(N.B. For the most part I've not bothered with signs e.g. That leakage currentoughtto be negative.)

author: Cerebus
author_rank: Super Contributor
timestamp: May 23, 2016, 04:51:59 pm
content: Quote from: Zeranin on May 23, 2016, 06:29:39 amSee my previous post for what I think's going on. In that I derive a parallel leakage resistance for one switch of something in the order of 200G. I don't think that the 3458's input resistance spec of >10G, the fact that there are 9-10 JFET switches in parallel and that figure of 200G for one switch are coincidences. I'm ignoring the input leakage of the amplifier that follows but I suspect that it is on the order of a few 100fA based on similar designs I do know in detail. I can't be more precise because the JFETs used both for the switches and input pair are a) obsolete, b) listed with internal part numbers and c) I've never managed to unambigiously match the HP part numbers to a standard datasheet parts.QuoteI may back off on that for a bit, the reason being, although I've pushed very hard to keep this a theoretical discussion, that there is some practical end to this and it'd be much easier to measure all this on a neat shielded PCB. I came at this because I wanted to do some noise budgeting on my draft designs to see if I was getting into the right ball-park and I realised that I had zero clue how to figure out the noise contribution of the JFET switches.Oh, and Tim, don't worry, there will be relaystoo- not worth building anything if you can't hear the clack of a few relays on your calibration cycle.