I need a 10V reference with a noise level below 5nV/sqrt(Hz). My current thinking is that the only way to get this is to add a huge amount of filtering to a normal reference. Are there any references with a low noise spec. A care most about the frequencies above 0.1Hz, Below that point, the noise can increase by 20dB/decade without causing too much trouble.
-- -- kensmith@rahul.net forging knowledge
I was going to try to take a few references and adding them. It will work but takes many along with the filtering.
greg
Take a look at the Linear Technology LT1031
The data specifies a typical 11uV rms from 10Hz to 10kHz which is
110nVper root Hz.The voltage noise plot puts the noise corner is at 40Hz, and show the high frequency noise as closer to 120nV per root Hz.
The LT1031 is one of the better buried-zener based references around, and does specify broad-band noise, unlike the Thaler VRE310
If you want to do better you may have to buy an integrated array of Josephson junctions from the National Burea of Standards, and a bucket of liquid helium to keep them at their working temperature.
-- Bill Sloman, Nijmegen (but in Sydney at the moment)
Yep, that sounds right. Resistor noise from the impedance of a Zener is higher than your target (see AD587 data sheet for instance; about
100 nV/sqrt(Hz) for a Zener-based reference). I hear that the old Weston standard cells have good low output impedance (and therefore lower noise).The guys down the hall from me put together a 300-cell battery for one experiment (but they were aiming for ten or eleven decimal places... got there, too).
I assume you mean LT1021 not LT1031. That is a mere 22 times bigger than I need. It is better than many. The LT1236-10 seems to be in that same range. Most of the rest I have found are above 300nV/sqrt(Hz).
[....]I think the customer may object. I'm already getting rumblings about the size of the battery cart needed for the wrist unit.
-- -- kensmith@rahul.net forging knowledge
I knew that either the LT1021 or the Lt1031 was good, but it was from my time at Cambridge Instruments - 1982 - 1991. The broadband noise spec for the LT1021-10 looks identical with the spec for the LT1031.
Then you probably don't want to know about the voltage reference for electron microscopy in Review of Scientific Instrruments in the late
1960s. Some guy took a vacuum insulated standard capacitor - about 100pF, mounted in a bell-jar - and charged it up to his reference voltage, and monitored the charge with a Carey vibrating reed electrometer.Capacitors don't generate Johnson noise, so the basic idea was okay, but they are microphonic ....
Texas Instruments still sells the Burr-Brown low noise FET input op amps OPA-657
with 4.8nV/root Herz noise and 2pA leakage current, but that would need a capacitor cart dragged along behind your battery cart.
-- Bill Sloman, Nijmegen (but in Sydney at the moment)
Ken Smith a écrit :
I had some similar requirements for really low noise PSUs two year ago and I had to get down to 0.1Hz too but I managed to have this point somewhat relaxed.
The best ref. you can come with (well, the best I could find) are the XFETs one.
All the refs I found were about 100nV/rtHz but that's not hard to filter... except for the low freq part. Also they are not all 10V, so for you the interesting data is the ratio of low frequency noise to ref voltage.
The good old AD586 at 4uVpp/10V is still a good value, which only equaled by the more recent 5V ADR445 (2.25uVpp/5V)
Are you really wanting under 5nV/rtHz flat down to 0.1Hz? That's pretty tough. Over a 0.1Hz-10Hz BW this is about 20nVrms or 120nVpp which is just within reach (guaranteed) of LT1028/1128 and AD797. Unfortunately the low LPF corner frequency needed to filter the ref voltage requires high impedances which won't do any good with these bipolar opamps high current noise. The OPA227 with 3nV/rtHz and 90nVpp(typ) has a 2.5 to 5 time better current noise, but is still high at 0.4pA/rtHz and skyrockets at
20pA/rtHz at 0.1Hz which will require high value caps with their own noise pbs (over 10000uF, no even speaking of LT1128 or AD797).Jfet opamps are really bad at low frequency. The best is probably the AD745, but will still miss the target by a 10 factor at 0.1Hz
-- Thanks, Fred.
Fundamentally a very tough requirement. Excluding the exotic stuff like Josephson junctions, the two basic reference-generation methods use either Zener diodes (noisy) or bandgaps (noisier). The bandgap loses because the basic voltage you generate is fairly low, so lots of gain is needed to get where you want to go. Zeners are just intrinsically noisy gadgets, although the so-called buried Zeners used in most IC refs these days are much better because at least they solve the surface-noise problem.
No matter which you use, you still need an op-amp (more noise, but probably far from the dominant contribution). If you really really need the noise level you mention, I think filtering is your only practical option. Watch out for cap leakage in the filter, these can give you shot noise across your (presumably) large filter resistor.
steve
I could use an extra battery in the battery cart. That would give me a fairly low noise but I'd have to regulate its temperature to prevent drift.
I was thinking of using something like the LSK389. If I make the capacitors out of nested spheres, the microphonics will cancel to teh first order.
Check the 1/F point on the noise graph. It really sucks when you get down to 0.1Hz.
It is too bad that the great big power MOSFETs don't come with a noise spec. I'd bet the noise voltage is quite low.
In real life, I think I'm going to end up with a large capacitor as a filter. This will prevent the noise current from making too much noise near the 0.1Hz point. Bipolar op-amps like the LT1007, have a much better
1/F point than the TI fet input one. The typical LT1128 meets the spec but the worst ones don't.Note for any Linear folks who happen to read this: Because of LT spice, all things being equal a Linear part goes in the design.
-- -- kensmith@rahul.net forging knowledge
[....]
I'll look this up.
[....]If it was easy I'd be done now. Lucky for me, I have room for a large capacitor or two and no "instant on" requirement.
If I use a truly huge capacitor I can get around that issue. Right now I'm thinking about using a discrete part to do the 0.01Hz to 1KHz band and an op-amp to set the DC level. Sort of like this:
Ref in --------!+\\ !---+15V ! >--+--/\\/\\---+---->! ---!-/ ! ! !----+------ Ref out ! === === ! ! ! ! ! ---------+--/\\/\\--------------+ ! ! GND \\ / \\ ! -15V
-- -- kensmith@rahul.net forging knowledge
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