whats in a keysight ultra low noise DC supply filter? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: December 29, 2018, 04:42:17 pm
content: N1294A-021http://www.farnell.com/datasheets/1936689.pdf?_ga=2.79753044.2109743412.1546101534-1593084816.1538441214https://www.keysight.com/en/pd-2265516-pn-N1294A/b2900a-series-accessories?cc=PL&lc=engIts meant for a 6.5 digit power supply. What do you think is in this low noise filter?

author: rx8pilot
author_rank: Super Contributor
timestamp: December 29, 2018, 04:48:03 pm
content: Some very impressive specifications! The cost likely matches the performance.Hopefully, Keysight will provide one for a Signal Path teardown/experiment presentation.Examining the design would be an education for most viewers.Typet purly on my fone.

author: David Hess
author_rank: Super Contributor
timestamp: December 30, 2018, 02:36:16 am
content: We can tell a few things from the specifications.Noise within the power output stage is reduced by the excess gain of the error amplifier to the level of the error amplifier's input noise which is why the output noise rises in direct proportion to the range switched feedback divider. The noise density graph indicates unremarkable low frequency performance but I think this reflects designing toward the practical resolution.The external low noise module is a damped LC filter with a 50 ohm impedance which can be used in either 2 wire mode or 4 wire mode to reduce the effective output resistance. This explains why the ultra low noise external filter is limited to lower currents and much slower settling time performance; the low cutoff frequency requires higher inductance with the associated higher series resistance and lower saturation current. The not-ultra low noise filter has higher bandwidth, lower series resistance, and supports a higher current.I wonder how the current control loop is implemented.

author: EEVblog
author_rank: Unknown
timestamp: December 30, 2018, 04:28:26 am
content: I love how it's got 100 nV resolution, but 10 μVrms noiseThat's 100 times more noise than resolution.

author: David Hess
author_rank: Super Contributor
timestamp: December 30, 2018, 05:14:28 am
content: Quote from: EEVblog on December 30, 2018, 04:28:26 amIt is not quite that bad because it is 10uVrms of noise over the entire bandwidth and this is reflected in the 5uVpp from 0.1 to 10Hz specification which would be about 1uVrms. At low frequencies, the noise is consistent with the resolution. 100nV resolution has no meaning above the measurement bandwidth which will be very low.The noise specification is not particularly noteworthy. It is low for a regulated power supply and perhaps for a source meter but attainable with a low noise operational amplifier, low noise reference, and practically any pass element with some care.

author: coppercone2
author_rank: Super Contributor
timestamp: December 30, 2018, 10:01:48 am
content: I am looking at the standard box actually, the noise level is 3mVpp.That is much worse then a 80's linear supply but it has 5X more current and less size so I guess its a switching supply.I can't find the natural noise in the manual but some other data sheet that references it says 3mVpp. A good linear supply is something like 150uV rmshttps://literature.cdn.keysight.com/litweb/pdf/B2961-90010.pdf?id=1407816There are little hieroglyphs when talking about the filter and its just a 10 or 50 ohm load and a capacitor it looks like? Page 3-19 shows the symbols, 3-21 shows some block diagramsthey just use the sense lines directly over the filter to compensate for its voltage drop. the low noise not ultra low noise one just has the sense lines shorting through it, I guess its more of a issue with cable pickup...so the filter drops 2.1mV RMS to 10uV rms over a 20MHz bandwidth?so a factor of 210.the low noise filter (not ultra low noise) is 350uVrms (factor of 6), so closer to a linear supply. But you still get to use the sense lines with it, but not with the ultra low noise ones.If the little hieroglyphs are right, then the ultra low noise ones are just a RC filter connected to the sense lines to get rid of the R. And the low noise one is just a C.Not sure why they don't just put the C in the box by default? Maybe so you don't get nasty surges when you connect it (output impedance of supply is too low for connection transient). It might be a big cap. Polymer ? high performance tantalum?You also have current limits, 105 and 500mA. Should give an idea of what the resistor wattage is. Vishay foil ?

author: coppercone2
author_rank: Super Contributor
timestamp: December 30, 2018, 03:10:06 pm
content: I kind of want to see what will happen if I put a RC filter across the back of a lambda low noise supply and do the thing with the sense that keysight does.that low noise box is 2000$, thats like a RF amp.a better datasheet gives you some frequency response plots, possibly information about 3db setting.https://cdn.testequity.com/documents/pdf/N1294A-022-ab.pdf

author: chuckb
author_rank: Frequent Contributor
timestamp: December 31, 2018, 01:47:29 am
content: The LT3042 Voltage regulator is rated at 200ma and 20V with 2nV / rt Hz of noise. Put 4 in parallel and you can have 1nV / rt Hz of noise at 10kHz and a 800ma output driver.Now the voltage reference for this ultra low noise output driver would be the normal output voltage of the Keysight PS, heavily filtered. Just ignore the LT3042 current source. The output of the LT3042 Driver will be within 2mV of the filtered source voltage.If the voltage span and maximum current are not what you need then use the LT3080 architecture to design your own. Use a very low noise error amplifier (LT1128A, AD797) and a good NPN output stage. The Ultra low noise error amplifier has to run with a gain of 1 at all times to maintain the low noise. The NPN output stage should be easier to stabilize than a PNP output stage.BTW - The NEW LT3042, LT3080 architecture reminds me of the old HP6114A Precision Power supply.

author: coppercone2
author_rank: Super Contributor
timestamp: December 31, 2018, 09:44:11 am
content: yes I thought about making a bench unit with those chips before.there is also the LT3094 that is new, it is the sister of the 42, the LT3045 is the same as the 42 pretty much but also 500mA.that might be the thing to make for low voltage RF amplifier prototyping and precision op-amp circuits. 800mA will run quite a few chips,

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2018, 12:31:07 pm
content: Quote from: coppercone2 on December 30, 2018, 10:01:48 amIt is designed as a fast voltage and current source-meter so the output capacitance has to be minimized. The low and ultra-low output filters severely compromise bandwidth and pulse performance. The output stage probably looks a lot like a class-AB audio amplifier with a low capacitance Zobel network (for lack of a better name) for stability.Clever bench power supplies intended to have good constant current performance have a very low output capacitance for the same reason. Merely "good" designs get by with 10 to 100 microfarads per amp.

author: coppercone2
author_rank: Super Contributor
timestamp: December 31, 2018, 01:04:38 pm
content: if its a capacitor in a box with some resistance, does anyone think the price is a bit steep?

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2018, 01:44:02 pm
content: Quote from: coppercone2 on December 31, 2018, 01:04:38 pmI think it is an LCR circuit with a rather expensive inductor and capacitor but part of the price includes designing the meter to work with it.

author: coppercone2
author_rank: Super Contributor
timestamp: December 31, 2018, 02:02:31 pm
content: the circuit diagrams just show a capacitor (well not really diagrams but menu options or whatever the pictures are for, i listed the page they are found on somewhere in my post. Would they just hide a inductor?)what are they designing in if it uses the sense terminals? what kind of capacitor do you think it is? Military wet tantalum? giant foil beast?

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2018, 03:59:50 pm
content: Quote from: coppercone2 on December 31, 2018, 02:02:31 pmAs far as the load is concerned, it is a capacitor.QuoteI assume the external filter modules use the sense terminals to remove the series resistance of the inductor if needed.The capacitor has to be bipolar. I assume the lower frequency filter uses a pair of big hermetic solid or wet tantalum capacitors and the higher voltage filter uses a big film capacitor.