Uber low noise current source suggestions - Page 1
Forum section: electronics / projects

author: grouchobyte
author_rank: Regular Contributor
timestamp: July 03, 2019, 11:27:59 pm
content: I have to design a laser driver for a very expensive (>$10k) high performance laser. It needs 3 volts of compliance and deliver around 650 mA CW. Long term stability and drift is not too important but current noise below 100 khz needs to be below 2 nA p-p. What I am concerned with for now is short term stability and current noise. The laser will be temperature stabilized and there are multiple APC loops for stability. Dont ask me what the application is......unless you want some men in black suits showing up at your door.Suggestions are appreciated and of course questions are welcome. I have limited info from my client at this point but I will attempt to answer what I can@grouchobyte

author: David Hess
author_rank: Super Contributor
timestamp: July 04, 2019, 01:58:48 am
content: Without feedback and filtering, I do not think you can get even close to that specification but that is ok. Besides filtering, use a feedback loop to lower noise within the feedback loop's bandwidth. Then this problem becomes one of voltage noise at the sense resistance and the feedback amplifier.

author: grouchobyte
author_rank: Regular Contributor
timestamp: July 04, 2019, 02:04:30 am
content: I did a little more calculation and I can probably live with less than say 10 nA of p-p noise in the specified 100k BWi think what David Hess is suggesting is probably the most reasonable approach plus careful component selection@grouchobyte

author: David Hess
author_rank: Super Contributor
timestamp: July 04, 2019, 02:27:55 am
content: If a bipolar transistor was used as the pass element, then just the shot noise of the output transistor would clobber that specification but like I said, filtering the output and feedback can control output noise so the characteristics of the pass device lose some importance. Someone who knows about noise in power MOSFETs can comment on if they produce less noise which would make meeting your requirements easier.Also consider building in a way to verify the current noise as part of your development circuit. That may include an amplifier with a specific bandwidth to drive your oscilloscope.

author: Marco
author_rank: Super Contributor
timestamp: July 04, 2019, 03:04:38 am
content: Quote from: blueskull on July 04, 2019, 02:39:16 amIt seems inescapable, butthese guysget close at 470 mA with 15 pA/rtHz (lots of 1/f because no auto-zero, but lets ignore that) and I see no massively heatsinked sense resistor. How do they do it?PS. or maybe what I saw as 1/f is a consequence of using inductance in the feedback loop?

author: Vgkid
author_rank: Super Contributor
timestamp: July 04, 2019, 03:20:21 am
content: I cant see the shunt they are using, but do see a lm399.

author: Marco
author_rank: Super Contributor
timestamp: July 04, 2019, 04:18:03 am
content: I don't quite see how that's going to work ... a shunt source can't remove the noise of the high current source without actually measuring it, so you still need a high impedance in the high current path to measure it.Best I can come up with is generating a high voltage and chopping the current to the high voltage through a high value sense resistor at low duty cycle to limit the power necessary.

author: Marco
author_rank: Super Contributor
timestamp: July 04, 2019, 04:39:34 am
content: How do you build a current source without an amplifier? An emitter/source follower is still an amplifier, because the transistor has a much larger area than in any opamp a relatively low noise one, but still an amplifier. That said, maybe I'm overestimating how low noise the amplifier needs to be. They have 10V to play with, so presumably ~20 Ohm sense resistor, so they need 0.3 nV/rtHz ... that's doable.Of course, groucho needs less than 2 nA RMS ... which is about 10x better than the SMC11 (minus 0-100 Hz noise even). So that's getting harder.How about something completely silly. Ping pong between two inductors, when one is connected to the diode you dump the remaining energy in the other into the power supply and then connect it across the supply the other way for a fixed time before switching the inductors. No current is measured at all, the average current delivered is only dependent on the on time while they are charged and the power the laser diode absorbs.

author: coppercone2
author_rank: Super Contributor
timestamp: July 04, 2019, 06:13:52 am
content: rofl outsource military design to EEVBLOGso what is it, i don't mind the goverment assholes

author: Marco
author_rank: Super Contributor
timestamp: July 04, 2019, 06:39:24 am
content: If you want to do this with the traditional sense resistor, opamp, MOSFET design I think you'll be forced down a two path amplifier design if you want to avoid burning a ton of power in the sense resistor.A bunch of ADA4528 in parallel for the low end and a BJT-opamp composite for the high end.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 04, 2019, 12:56:21 pm
content: Quote from: grouchobyte on July 03, 2019, 11:27:59 pmAlright, but they might show up at yours if they find this out.

author: mikerj
author_rank: Super Contributor
timestamp: July 04, 2019, 01:06:23 pm
content: Have a look at the LT3045 voltage regulator, this is a very low noise voltage regulator that can be configured for constant current operation. It's used in a similar though lower power laser application. Only rated at 500mA so you may need a couple of them.

author: grouchobyte
author_rank: Regular Contributor
timestamp: July 04, 2019, 02:34:17 pm
content: Quote from: coppercone2 on July 04, 2019, 06:13:52 amWho said it was for a government agency or was a military project? My accountant wears a black suit.My lips are sealed. I do appreciate the discussion and suggestions BTW. Thank you allIf you really have to know, I suggest you re-watch Breaking Bad@grouchobyte

author: Marco
author_rank: Super Contributor
timestamp: July 04, 2019, 03:30:48 pm
content: Quote from: mikerj on July 04, 2019, 01:06:23 pmThe standard way of doing it seems to besimple opamp/MOSFET designswhich are substantially lower noise than that.

author: coppercone2
author_rank: Super Contributor
timestamp: July 04, 2019, 03:46:57 pm
content: don't lie bro your scared some colonel is gonna show up at the office and smack you aroundperhaps a new precision fryer for KFC?

author: Kleinstein
author_rank: Super Contributor
timestamp: July 04, 2019, 03:57:50 pm
content: 10 nA noise for some 500 mA of current is 0,02 ppm level. No way one get this with normal room temperature electronics. If possible at all this would be the range where superconducting devices are needed. So think about a Josephson junction reference squid amplifiers and NMR based current reference.I kind of doubt the requirements are really such a stable current, more like a very stable light output or a very stable temperature. But than it would probably better to directly control the intensity of temperature, but no the current. Current would than be only a control input.

author: tszaboo
author_rank: Super Contributor
timestamp: July 04, 2019, 04:11:01 pm
content: So, you design a current source for ~1ma, make sure it is low noise.You know, JFETs, low noise opamps and stuff like that.Get a good contract from your customer.Connect 650 of it in parallel, and let statistics take care of the noise.Maybe you will need larger number and lower current.

author: Marco
author_rank: Super Contributor
timestamp: July 04, 2019, 05:24:59 pm
content: Quote from: Kleinstein on July 04, 2019, 03:57:50 pmI'm not sure if that's true as far as thermal noise is concerned.Bigger problem is that you could only ever deliver the current noise through a resistor ... through a laser diode you are shot noise limited to ~7 nA RMS, so ~50 nA peak to peak. Throwing it in liquid helium won't help you.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 04, 2019, 07:04:34 pm
content: Quote from: Marco on July 04, 2019, 05:24:59 pmThat's not true. Laser diode driven by a low noise current source can easily be below the shot noise limit. Think about it: if you have a sub shot noise current through a resistor and the resistor is in series with a diode, where is the noise current going to go?Shot noise applies to a diode that is voltage biased.At high frequency you will see shot noise because at high enough frequency the diode capacitance is low impedance. But up to 100 khz is no problem.Achieving the specified noise parameters is going to be difficult but doesn't seem impossible.

author: Marco
author_rank: Super Contributor
timestamp: July 04, 2019, 11:27:04 pm
content: Quote from: ejeffrey on July 04, 2019, 07:04:34 pmOops, guess you're right.Well in that case 20 current sources consisting OPAx189s and MOSFETs with 400 Ohm sense resistors should just about get you there in theory. Only 50 Euros of opamps.PS. I guess if <0.1 Hz noise doesn't matter so much you could also just use precision BJT opamps, though you can't really reduce the number of current sources much if you want to keep the supply headroom the same (ie. ~10V) because the thermal noise of the sense resistors starts dominating.

author: 1audio
author_rank: Frequent Contributor
timestamp: July 05, 2019, 02:30:48 am
content: Would this work?Use a transformer with the primary designed to not saturate at 650 mA with a 10:1 or 100:1 (1000:1 may not have the bandwidth needed) turns ratio. Sense the DC current on the primary and use the secondary for sensing the current noise and feedback a correction for the current noise. Similar to this:http://www.wenzel.com/documents/finesse.htmlYou would also need to sense the temperature of the primary to correct for the change in DCR of the copper in the primary.

author: awallin
author_rank: Frequent Contributor
timestamp: July 05, 2019, 05:27:05 am
content: here's some noise-analysis found by google:http://atomoptics-nas.uoregon.edu/odom_current/libbrecht-hall-noiseanalysis.pdfit would be good if someone could make an effort and plot the different specs in the same figure!!?Steck's Libbrecht-Hall designsimple opamp/MOSFET designsSisyph link abovehttp://www.sisyph.com/smc11-puy-mary-ultra-low-noise-current-sourceErickson's Libbrecht-Hall designhttps://arxiv.org/abs/0805.0015the Vescent producthttps://www.vescent.com/products/electronics/d2-105-laser-controller/

author: grouchobyte
author_rank: Regular Contributor
timestamp: July 05, 2019, 05:00:01 pm
content: Some good suggestions and ideas here. Thank you. I am now reviewing the prelim docs of the laser to try and understand the sensitivity to noise and the various mechanisms that need be considered in the overall operational transfer function.My original guesstimate was mostly theoretical based on some matlab sims, but I might need to get the noise initially as low as I can and then either ease up on the noise requirement or find find some way to mitigate any unwanted noise-induced behavior.At this point I think I can get away with about 10nA p-p of noise in a 50khz BW and many of your suggestions are a step in that direction.Again, thanks to all the regular folks, some of whom I easily recognize, like Marco, Blueskull and David Hess, etc.BTW: The Government of Canada thanks you for your help.