Author Topic: Low frequency, very low level, DC biased, noise measurements  (Read 34263 times)

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Offline Marco

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #25 on: April 20, 2015, 05:23:53 am »
I've seen a report that the noise corner for new BF862 is pretty bad (China vs. Taiwan). They have two SKU's for it, one of which is impossible to get ... murphy's law suggests it's the good one.

J113 doesn't seem half bad, very cheap and tightly enough specced that you can parallel it with source resistors without matching.
« Last Edit: April 20, 2015, 05:29:36 am by Marco »
 

Offline MK

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #26 on: April 20, 2015, 07:30:59 am »
The die is doped and scribed in europe, it is then sent to one of several packaging plant, I think it is just a poor batch or two. On another forum I visit, one of the members their explained that they are only assembled in asia.
 

Online splin

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #27 on: April 20, 2015, 01:35:43 pm »
Another option may be using some batteries in series to bing down the voltage from the ref., than have a DC coupled amplification (e.g. 10 fold with an OP27) and have AC Coupling only after this.

I like the battery idea :-) But wouldn't batteries generate noise, even if current is virtually zero?

Yes, but not much depending on which sources you believe. From:

[url=http://tf.nist.gov/timefreq/general/pdf/1133.pdf]http://tf.nist.gov/timefreq/general/pdf/1133.pdf[/url]

Quote
Using this low noise measurement system we have characterized the voltage noise of 5 different battery types and compared them to a popular voltage regulator and a high performance power supply. We found that the voltage noise of the chemical batteries measured was many decades better than that of traditional power supplies. The lowest noise battery tested was a AA Ni-Cd , with Vn = -205 dBV/Hz at 1kHz. Different battery types exhibit vastly different noise voltage. In the batteries measured the broad band noise voltage appears to be approximately equal to the Johnson noise of their internal resistance.

The internal resistance of Ni-Cd cells is extremely low (a few milliohms typically, depending on capacity) – their results show that Ni-Cd cells have extremely low noise levels - .2nVrms/sqrt(Hz) at 1Hz.

Another source which has almost identical results for an AA Alkaline is page 218 of:

https://books.google.co.uk/books?id=DctAAQAAQBAJ&pg=PA218&lpg=PA218&dq=alkaline+battery+low+frequency+noise&source=bl&ots=zOKnuaYH1x&sig=1yZkjU90_N9JHZcEYGJ2rPQXnGY&hl=en&sa=X&ei=qZMxVfFsy99q_P6BwAM&ved=0CEsQ6AEwCTgK#v=onepage&q=alkaline%20battery%20low%20frequency%20noise&f=true]https://books.google.co.uk/books?id=DctAAQAAQBAJ&pg=PA218&lpg=PA218&dq=alkaline+battery+low+frequency+noise&source=bl&ots=zOKnuaYH1x&sig=1yZkjU90_N9JHZcEYGJ2rPQXnGY&hl=en&sa=X&ei=qZMxVfFsy99q_P6BwAM&ved=0CEsQ6AEwCTgK#v=onepage&q=alkaline%20battery%20low%20frequency%20noise&f=true]https://books.google.co.uk/books?id=DctAAQAAQBAJ&pg=PA218&lpg=PA218&dq=alkaline+battery+low+frequency+noise&source=bl&ots=zOKnuaYH1x&sig=1yZkjU90_N9JHZcEYGJ2rPQXnGY&hl=en&sa=X&ei=qZMxVfFsy99q_P6BwAM&ved=0CEsQ6AEwCTgK#v=onepage&q=alkaline%20battery%20low%20frequency%20noise&f=true

However this source has completely different, much noisier, results:

www.hoffmann-hochfrequenz.de/downloads/NoiseMeasurementsOnChemicalBatteries.pdf]http://www.hoffmann-hochfrequenz.de/downloads/NoiseMeasurementsOnChemicalBatteries.pdf]www.hoffmann-hochfrequenz.de/downloads/NoiseMeasurementsOnChemicalBatteries.pdf

Since all three sets of results plot the noise on different scales, and the last has results for 4 cells in series, I have normalised them in the attached spreadsheet. I have highlighted in blue the AA Alkaline results from the 3 sources.

Hoffman’s results are pretty much identical to the others at 100Hz but increase very much more rapidly below that. Hoffman used his own amplifier design using 20 paralleled ADA4898-2 op-amps to reduce the voltage noise, described here:

http://www.hoffmann-hochfrequenz.de/downloads/lono.pdf]http://www.hoffmann-hochfrequenz.de/downloads/lono.pdf]http://www.hoffmann-hochfrequenz.de/downloads/lono.pdf

He measured his amplifiers's voltage noise density to be 220 pV rms/sqrt(Hz) at 1kHz, rising to 1nV rms/sqrt(Hz) at .1Hz. The current noise will be rather high though with sqrt(20) x the noise of one op-amp (1/f characteristic – 2.4pA/sqrt(Hz) at 1kHz rising to 10pA/sqrt(Hz) at 1Hz) so only suitable for low impedance sources as he notes in his article. However I believe his amplifier to be seriously flawed – I calculated that the noise voltage generated by the current noise flowing through the 10k//160uF input filter will be around 45nV rms/sqrt(Hz) at 1Hz rising to 800nV at .1Hz! This goes a long way to explain the huge discrepancy between his results and the others for the Ni-Cd cells. However Hoffman’s results for AA Alkaline batteries are so much worse than the other sources, (397nV versus Walls’s 4.2nV at 1 Hz) that there must be something else going on as well.

Note that his article has a chart showing the very respectable noise floor with the amplifier input grounded; however the short is applied after his 160uF, 10kohm DC blocking filter so only the voltage noise is shown. The significant current noise can be seen in the same chart showing the noise floor with the input grounded via a 50 ohm resistor – again after the input filter. At .1Hz this is 18nV/sqrt(Hz) compared to 10nV for4 the voltage noise alone; this implies the noise due to the current noise in the 50 ohm resistor is sqrt(18nV^2 – 10nV^2 – 1nV^2), or 14.7nV/sqrt(Hz). The DC filter has a resistance of 7kohms at .1Hz so the current noise totally dominates.

He’s not alone though – looking at the data sheet for the ADA4898-2, on page 14 Analog Devices seem to have made exactly the same mistake! Their test circuit shows an AD743 used to amplify the noise x1000 using an input filter of 1uF/1612kohms!! The current noise of the AD743 will swamp the ADA4898-1’s noise - as shown in their 500nV pk-pk result shown in Figure 45.

The real value .1 to 10Hz AD4898-1 noise I calculate to be around 82nV pk-pk. Since AD generally know what they are doing, this may be a simple mistake showing the wrong values in the schematic etc. but I’m not convinced given that their 500nV pk-pk noise is almost exactly what I calculate it would be from the AD743 current noise spec into the 1uF//1612k input resistance. Don’t believe everything you see in the datasheets.

So, using Ni-Cd batteries to offset the LTZ1000’s 7.2V so that the first stage of the noise amplifier can use a DC-coupled low noise bipolar op-amp such as an LT1028, AD797 or LME49990 should be feasible. However as always there is a catch. The battery voltage is very temperature dependant. As an example I applied the shank of a screwdriver (shank approx 3mm dia.) warmed in my hand, across the side of an AA alkaline battery approx half way between the ends to minimise problems due to thermal EMFs at the connections. Within 3 seconds the voltage was changing by around 1uV/second. Not very scientific, but I believe it would be difficult to control the battery temperature sufficiently for making sub-microvolt measurements over periods of more than few seconds without the battery drift obscuring the results. Using 5 or 6 batteries in series increases the problem proportionally. A bigger battery with its larger thermal mass has slower drift, but also takes longer to stabilize when the test environment changes – if at all.
 

Offline Rupunzell

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #28 on: April 20, 2015, 05:54:34 pm »
medium power BJT being used as a "low noise" device is rather conditional. It appears "low noise" due to sheer die size which lowers it's bulk resistance resulting in what appears to be low noise. Being a Bipolar, it will have current noise dependent on collector current (beta related) adding to this leakage currents can add to this effect making this a very source impedance dependent "low noise" device. Other problems are die size which has an effect on it's BW, collector base capacitance will be high and if used as a common emitter amplifier miller effect will stunt's if frequency response. Both collector-base capacitance and beta modulates with current and temperature changes. All these reasons an more are why medium sized bipolar transistors are not often used if at all for low noise input stages.

Devices like the LM394 would have been use if a circuit needed a device like this. Alternatively, super beta match diff pairs are also use as low noise input devices. Super beta bipolars goes a ways to reducing noise current, but does not eliminate it.

Want to use individual matched FETS or bipolars as a diff pair/ They will need to be thermally coupled to foster tracking. In the case of FETS, their IDSS or VGS will need to be matched over a range of temperature or there can be serious temperature drift problems. This problem is less with bipolars, but not to be ignored. Oh, any thermal gradient between the two diff pair devices aggravated this problem.

As for power supplies, if the circuit cannot achieve low noise (less than 1nV/root-Hz @ less than 100Hz) the circuit design will be difficult to implement into many systems. Designed properly low noise input sections have good power supply noise rejection. Batteries powered low noise input sections regardless of they batteries internal impedance and possible low noise is simply not the way to get there.

Using a coupling capacitor, specially a electrolytic or similar will cause a host of distortion and signal errors at the signal levels being discussed. EE caps have extremely high dielectric absorption (put a volt meter on a few hundred uF EE cap and measure it's voltage. Charge and discharge it and measure it again) and do not behave any where near like a ideal capacitor. If forced to use a coupling capacitor, it must be polystyrene or teflon film with few exceptions.

Noise from grounding is a serious problem that cannot be ignored. The problem is more difficult and more serious for input sections with a chassis grounded-single ended input. If a differential input section is needed, the impedance for both + and - inputs must have the same impedance, frequency/phase response and gain to idealize common mode rejection.

Knowing what the source impedance is makes all the difference to achieving low noise. Designing an input section for a low ohmic value thermocouple is very different than designing a low noise input section for a electrometer, charge amplifier or similar very high impedance device.

Any digital device like a DAC (i.e., DAC in the FB loop) will produce glitches as they make their switching transitions. These glitch transients will find their way back into the input section where they will be integrated by the devices involved resulting in more noise and distortion. This is one of the many reasons why mixing digits with low noise high fidelity analog can be so extremely difficult to do properly.

A "low noise" device is only the beginning to making a good, low distortion, signal accurate low noise input section. More often than not, really good low noise, low distortion, high performance input sections do not come from an IC, these are done the old fashion way using individual devices.


Bernice


The Art of Electronics, third edition, has a very long chapter on low noise techniques. It contains many practical and theoretical results, some of which are well-known, some of which came as a surprise to me.

For example, I would never have guessed that (for some applications) the lowest noise transistor would be a medium power BJT!
 

Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #29 on: April 21, 2015, 04:23:49 am »
Hi,

Did some leakage tests on caps.

First: the failed path: Measuring leakage current by having the cap charged at a constant voltage, measuring the current, turned out to be quite un-practical except for on the large and leaky caps. For others, leakages are sub-nano-amp and beyond my capability.

It's much more practical to charge them all up to some voltage, leave them to rest a while, measure the voltage, rest, measure againg, see the voltage drop, calculate the rest.

Attached you'll find some results. I have measured on some 20+ caps, ranging from 2.2uF up to 33.000uF. Voltage ratings from 35V to 700V. So far I have measured at 100V and 30V. And for a few, for other voltages.

One figure of merit is the leakage expressed in seconds (Mohm*uF). Some manufacturers publish this value. It's typically between 5.000 and 100.000 at the rated voltage. Divide this value by your capacitance to get the leakage resistance for your cap size. I give this value for the measurements I have made, but at the measured voltage. I've made other measurements (not included here) that show that the leakage resistance can be 10x higher when the voltage is half of the rated voltage.

So some results:

Polystyrenes and Polyprops are the best performing as expected. In my measurements, a Russian mil spec polystyrene 10uF was the very best (per uF) but an identical one ended last of all, obviously has a defect, damage. The 10uF polyprops are huge. A large e-cap, 33.000uf RIFA (now Kemet) did very well too. I'ts a very heavy duty, long life type. I did not have any polyesters at hand and not many regular ecaps either. The ecaps I tested so far where mostly either large power supply caps, 10.000 and up, or smaller 1000-3000uF ultra-high ripple/low ESR types.

Right now I'm preparing to measure at around 10V, might be done tomorrow.

Well, results attached.
my2C
Jan
 

Offline Marco

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #30 on: April 21, 2015, 05:07:45 am »
EE caps have extremely high dielectric absorption

There is no significant voltage across it in frequencies of interest. The apparent leakage current it causes can easily saturate the amplifier and/or load the source, but other than that it's irrelevant for AC coupling.
« Last Edit: April 21, 2015, 05:09:35 am by Marco »
 

Offline Rupunzell

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #31 on: April 21, 2015, 01:06:09 pm »
Magnitude of signals involved are in nano volts & pico amps.

If the E'lytic or cap in question is theoretically ideal for a capacitor, there might be zero volts across it. Keep in mind a coupling capacitor is in series with the input source and the measuring device's input. In the real world, that E'lytic capacitor behaves more like a battery in series between the input source and measurement device input, at these levels than a device no longer has zero voltage drop due to the effects of dielectric absorption, leakage and other non-linear behavior real world capacitors exhibit.

Don't believe this, take a E'lytic capacitor, put a DC Volt meter with no less than 10 Megohm input resistance across it's terminals and measure the voltage across that E'lytic capacitor. Then charge it up, discharge it, rinse and repeat a few time and see of the initial voltage measured is the same..

Try building a precision sample-hold or precision integrator using a E'lytic or other capacitor with significant amounts of DA, does it work, yes, how well.. that becomes a question of what is acceptable or not acceptable.

Will an E'lytic cap work as an AC coupling capacitor, absolutely. Question is, how much signal error, distortion and more is acceptable?

This analog stuff is not about does it work, it is more a matter of how well does it work, how much does the circuitry add or subtract to the original signal and all of those realities. It is very much shades of gray and the deal one makes with nature with a full understanding that nature will do what nature does regardless.


Bernice





EE caps have extremely high dielectric absorption

There is no significant voltage across it in frequencies of interest. The apparent leakage current it causes can easily saturate the amplifier and/or load the source, but other than that it's irrelevant for AC coupling.
« Last Edit: April 21, 2015, 02:07:42 pm by Rupunzell »
 

Offline Rupunzell

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #32 on: April 21, 2015, 01:19:45 pm »
Try doing some dielectric absorption test at the capacitor's rated voltage.

Leakage test tell one small part of actual capacitor behavior.


Bernice


Hi,

Did some leakage tests on caps.

 

Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #33 on: April 21, 2015, 05:42:44 pm »
There will be a DC across these caps, if measuring noise on a voltage reference.

All caps have Dielectric Absorption (memory effects). The question is how much is acceptable. E-caps have much more DA than film caps. Teflon and C0G ceramics probably best, but both are available up to something like 0.22uF

So far, from what I measured and read datasheet, depending on cap availability upper limits, ball park values:

- Up to 1uF, NP0 or teflon? (not measured)
- Up to 10uF: mil Polystyrenes
- Up to 100uF, possibly up to 1000uF:
   - Polyprops, not all huge "DC-link" types
   - Mil rated Tantal-Silver? (not measured)
- Higher than 1000uF: select e-caps
my2C
Jan
 

Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #34 on: April 21, 2015, 06:02:09 pm »
Batteries/ecaps:

Direct current measurements I did on a 1200uF 35V ecap using a 9V PP3 battery;

The e-cap pre-conditioned for a day at 9V, the battery connected for another day.

Then the current flow was still from cap to battery (4nA), ie it seems the battery internal leakage / discharge and / or DA of cap still dominated over anything else...

Unusable cap or battery ???
my2C
Jan
 

Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #35 on: April 21, 2015, 06:21:33 pm »
Another thing realized

Without leakage, C=Q/V is constant. But C is temperature dependent, goes up with decreasing temperature. So the voltage over a cap without any discharge will go up when temperature drops.

At the levels measured, the voltage change caused by temperature easily swamps the voltage caused by leakage.

In my case, a one degree C change of the room temperature caused an voltage drift that was larger than the leakage over two hours. |O

Got to go temperature controlled :(
my2C
Jan
 

Offline mzzj

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #36 on: April 21, 2015, 06:27:10 pm »
Batteries/ecaps:

Direct current measurements I did on a 1200uF 35V ecap using a 9V PP3 battery;

The e-cap pre-conditioned for a day at 9V, the battery connected for another day.

Then the current flow was still from cap to battery (4nA), ie it seems the battery internal leakage / discharge and / or DA of cap still dominated over anything else...

Unusable cap or battery ???
The battery voltage may drift enough with temperature to screw your measurements. I would also measure/log the battery voltage during the testing.

Jim Williams mentioned in some appnote that selected aluminium electrolytic caps can be pretty good for this type of measurement leakage vise but was having problems with short random noise bursts. Apparently there is something weird going on inside the aluminium electrolytics. ( I also remember reading some battery noise measurements where some battery types exhibited random jumps in noise or voltage)   

Wet slug tantalum would be your best bet  according to Williams but these are crazy expensive. See AN124f from Linear www.linear.com/docs/28585
 

Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #37 on: April 21, 2015, 08:23:49 pm »
I have read what I can find on the subject, including the AN124

I could monitor the voltages, but it would require the volt meter to be switched in for short durations now and then. Any voltage measurement discharges the cap / battery. Even if the DMM has 10Gohm inner resistance, it seems that this is much less than the caps and battery.

I suppose I could build a Terra-ohm buffer 8)

I could probably use a stable voltage ref as voltage source. It would need buffering output and isolating he capacitive load.

Wet slug tantalums, aka tantalum-silver caps: there are Russian mil surplus parts on ebay: I bought a 300uF/25V $7.50, will post results when I get it. One of the first posters in this? thread used something similar.

I also found some cheep Taiwanese motor starter cap, claimed to be Polyprop?, 800uF, brand "ABC" at around $15 eBay auction: #http://www.ebay.com/itm/231334817420?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT Will post results of that one too.

One can also look for "DC-link" polyprops. These high voltage caps may be really good when voltage de-rated.

E-cap "blurp", if it's rms measurements, not long-time ptp, then those instabilities could be rejected. The temperature dependency should show as very low frequency drift, should be OK. Dielectric absorption; cant quite wrap my head around how this will work at very low noise, low frequencies. It will add some non-linearity. The RIFA ecap I got good results with are DC-link listed. But I should need have a 1000uF -size to bring leakage current down.

Need a climate / temperature chamber too. Drifting away from the voltage reference circuit  :(
my2C
Jan
 

Offline Marco

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #38 on: April 22, 2015, 12:24:39 am »
Simulating a bit, you'll need about 22 mF to get close to the noise limit with a single BJT amplifier at 100 mHz (ten times less would be good enough for JFET ... simply because even a really good one is easily 10 times worse to begin with).

If you want high input impedance you will need either some input circuitry (inductor+comparator+multiplexer) to quickly fill the cap when necessary using a temporary buffer or use a manual  method to switch in/out such a buffer (with risk of human failure). If you used Jim Williams his buffer on a 7V reference you'd be pulling 7 mA initially.

PS. actually if you attached 7V to Jim William's AN124 amplifier with a discharged input capacitor you'd pull ~30 mA, Q1 would get forward biased.
« Last Edit: April 22, 2015, 12:44:27 am by Marco »
 

Offline Kleinstein

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #39 on: April 22, 2015, 01:02:59 am »
At least some E-Caps seem to work (much lower noise that amplifier). The leakage current is just one thing that may be a little easier to measure than noise itself. One can expect that leakage  current will also cause some noise. So this is just a first test. The final test for the caps would be to check for the noise of the whole system with a low noise voltage source (e.g. battery). The second step that may be needed is to get a rough measurement of the battery noise, if we don't take for granted that they are low noise - this could be done by measuring several batteries back to back.

The noise level of the zeners refs. is not so extremly low, so the simple system with a e-cap and more or less conventional OP-Amp (e.g. LT1037) can be sufficient.

The effect of leakage is to give a small DC offset - unless the amplification of the first stage is really high, this should not be a problem. DA and temperature drift will just give a slowly varying background, so not much in the range of 100 mHz and up.

A large cap will need some protection for the source. However this may already be necessary at something like 1-10 µF used with a pure JFET amplifier. Even a DC coupled amplifier may need some kind of input current limiting, in case the amplifier is not powered. Nearly all low noise OP Amps have something like diode across the inputs.

 

Offline Marco

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #40 on: April 22, 2015, 01:32:15 am »
Unpowered input protection could be a relay or a back to back MOSFET combo (or two if you want to keep it symmetrical). It's the only real way to do it with low impedance.

Leakage current will cause noise, but that's shunted by the capacitance ... unless it's modulated by some other source (sound/temperature) in frequencies of interest I don't see how it can contribute much noise.
« Last Edit: April 22, 2015, 01:41:14 am by Marco »
 

Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #41 on: April 22, 2015, 05:30:40 am »
Marco, could you please give some numbers on what you where calculating, what noise limit you where looking at? Current and voltage noise versus the LTZ1000?

All caps, especially ecaps, need to be biased for long periods to get rid of DA. Or are you talking about measuring DA too?

Edit: Batteries for cap test: Alkalines are really wrong. Then best are probably Lithium's (not Li-ion) like Energizer Ultimate and other brands of the same type which have ultra-low self discharge.

Simulating a bit, you'll need about 22 mF to get close to the noise limit with a single BJT amplifier at 100 mHz (ten times less would be good enough for JFET ... simply because even a really good one is easily 10 times worse to begin with).

If you want high input impedance you will need either some input circuitry (inductor+comparator+multiplexer) to quickly fill the cap when necessary using a temporary buffer or use a manual  method to switch in/out such a buffer (with risk of human failure). If you used Jim Williams his buffer on a 7V reference you'd be pulling 7 mA initially.

PS. actually if you attached 7V to Jim William's AN124 amplifier with a discharged input capacitor you'd pull ~30 mA, Q1 would get forward biased.
Simulating a bit, you'll need about 22 mF to get close to the noise limit with a single BJT amplifier at 100 mHz (ten times less would be good enough for JFET ... simply because even a really good one is easily 10 times worse to begin with).

If you want high input impedance you will need either some input circuitry (inductor+comparator+multiplexer) to quickly fill the cap when necessary using a temporary buffer or use a manual  method to switch in/out such a buffer (with risk of human failure). If you used Jim Williams his buffer on a 7V reference you'd be pulling 7 mA initially.

PS. actually if you attached 7V to Jim William's AN124 amplifier with a discharged input capacitor you'd pull ~30 mA, Q1 would get forward biased.
« Last Edit: April 22, 2015, 05:41:37 am by janaf »
my2C
Jan
 

Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #42 on: April 22, 2015, 05:55:45 am »
It was kind of what I thought of initially. I'ts much cheaper to throw some expensive LT1128/1028 at it than spend days making a more elegant solution. I'll take a look at the calculations again.

If you just want to throw expensive parts at it the LT1128 is so good though I'd say just use it as a buffer. After that you can use a HPF with a low value resistor to the next stage without issue and use a switch to quickly charge the capacitor as well. Then use a LT1028 as a 10000x amplifier and a standard 0.1-10 Hz measurement setup (ala this). It's not elegant, but should work (LTZ1000 has low dynamic resistance right?).
my2C
Jan
 

Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #43 on: April 22, 2015, 06:33:53 am »
Thanks. Looks interesting. But my German is poor. Anything one needs to know except the schematic? Like, which components / values are critical, which are not. Not used to this kind of amps...

https://www.mikrocontroller.net/topic/207061?page=2#3410803

100 mHz - 100 kHz
.5 nV^2/Hz white noise share
.7 nV^2/Hz over full bandwidth assuming white spectrum, i.e. it also has a very low 1/f corner
« Last Edit: April 22, 2015, 06:43:14 am by janaf »
my2C
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Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #44 on: April 22, 2015, 07:17:31 am »
Thanks!

So let's see if I get this right. Simplified and including only 1/fy, for y=1, we get;

Urms(0.1-10) = UW2x(10+fc2x1000)
where
UW = Uwhite / sqrtHz
fc = corner frequency

Have mercy, 30 years since maths classes...

Calculating the RMS noise shloud be realtively easy:
One can integrate the power (e.g. voltage square) over frequency. We can also treat the white and 1/f parts separately, as it is not correlated.
So the white part just gives bandwith (BW) times square of noise-voltage.
The 1/f part takes one more step: The Integral of 1/f² is  -1/f. Integrated from f_low to high frequencies this gives 1/f_low.
At the corner frequency f_c, the white and 1/f part are of same size.
So with just an white a 1/f part and integrating to well abov f_c one would get:

U_noise² =   U²_n(white) *  BW + U²_n(white)*f²_c/ f_low =  U²_n(white) * ( BW + f²_c/f_low)

So due to the 1/f part the noise power is inceased by a factor of ( 1 +  f_c² / (f_low *BW)).
For the noise density  (volts per sqrt(Hz)) take the square root of this factor.

For the LF amplifier there may well be a 1/f² part, e.g. from current noise time capacitor impedance. So we have a third anlog contribution
and get a faktor of

Sqrt(  ( 1 +  f_c² / (f_low *BW) + f³_c2 / (2*f_low² *BW))) ) to muliply the white noise density.



A "leakage" current in the 50 nA range sound good. The current itself is not so much of a problem, its just the noise that is possibly associated with it that may cause trouble. For filtering, the input AC coupling should have a time constant considerable lower than the measurement limit (e.g. 0.1 Hz). Its better to have a later stage or software filtering will set the lower limit. So 50 nA of leakage at a 10 K resistor to GND gives something like 0.5 mV of offset and limits the amplification of the first stage to something like 1000 times, possibly 100 times, which is OK.
my2C
Jan
 

Offline Andreas

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #45 on: April 22, 2015, 07:33:16 am »
It was kind of what I thought of initially. I'ts much cheaper to throw some expensive LT1128/1028 at it than spend days making a more elegant solution. I'll take a look at the calculations again.


Hello Jan,

if you read DN6 or DN140 of LT you will find that LT1128/1028 is only useful up to
a maximum input impedance of 400 Ohms below 10 Hz.
http://www.linear.com/docs/4187
http://www.linear.com/docs/4345

Above 400 Ohms the LT1037 or LT1007 is the better choice.

With best regards

Andreas

 

Offline Marco

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #46 on: April 22, 2015, 08:28:11 am »
Marco, could you please give some numbers on what you where calculating, what noise limit you where looking at?

No, I meant to get close to the noise limits of the transistor ... none of this is really relevant to the LTZ1000 measurement.

For the LTZ1000 measurement I found the OPA140 ... similar voltage noise to the LTC2057 but no current noise to worry about, so the impedance of an input HPF becomes irrelevant. I'd use 220 uF worth of film capacitors with 100 kOhm. Only thing which you'd have to check is what kind of current the OPA140 input pulls when there is a large differential voltage (ie. while the input capacitor is charging).

Unless you really want to build something out of discrete components :)
 

Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #47 on: April 22, 2015, 08:31:12 am »
There is an update: App note 355 where they made separate plots per frequency range. Up to 1komh, 1028 still in the lead but any of those mentioned should probably do. Noise on the non-inverting input, coming in with the DC cap and input impedance, is not included in any of those graphs. Must be added. Will do some calculations tomorrow. 

 http://cds.linear.com/docs/en/design-note/dn355f.pdf
my2C
Jan
 

Offline janaf

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #48 on: April 22, 2015, 08:38:26 am »
Thanks for the OPA140 tip, new to me.

Right on the first page of the datasheet: 250nVPP, 0.1Hz to 10Hz . Should be very close to the total for that amp and is some 14dB down from the LTZ1000, should be enough! And supplies up to +/-18V  :-+

Marco, could you please give some numbers on what you where calculating, what noise limit you where looking at?

No, I meant to get close to the noise limits of the transistor ... none of this is really relevant to the LTZ1000 measurement.

For the LTZ1000 measurement I found the OPA140 ... similar voltage noise to the LTC2057 but no current noise to worry about, so the impedance of an input HPF becomes irrelevant. I'd use 220 uF worth of film capacitors with 100 kOhm. Only thing which you'd have to check is what kind of current the OPA140 input pulls when there is a large differential voltage (ie. while the input capacitor is charging).

Unless you really want to build something out of discrete components :)
my2C
Jan
 

Offline Rupunzell

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Re: Low frequency, very low level, DC biased, noise measurements
« Reply #49 on: April 22, 2015, 02:44:53 pm »
Don't get fooled, noise is actually 8nV/root Hz @ 10Hz (fig. 11, Input noise density -vs- freq). In the world of low noise amplifiers, this is not even close to being low noise.
http://www.ti.com/lit/ds/symlink/opa140.pdf

Compare that with a truly low noise device, Interfet process NJ3600. 0.4nV/root Hz @ 10 Hz. The op amp is not even close.
http://www.interfet.com/process/NJ3600L/

To achieve truly low noise, the amplifier will need to be designed and built with extreme care using discrete parts, no way around this fact.

The coupling capacitor can be easily deleted by applying an offset to the feedback summing junction of the amplifier if the input section has enough dynamic range (discrete devices using +/- 30V rails or higher, Forget about saving power, it is a matter of performance). In the case of measuring voltage reference noise, the signal of interest is small but sitting on a DC offset and the offset voltage should be quite stable, low drift over time and the impedance of the voltage reference should be low. Key aspect of this measurement is the input section. Once the input signal has been increased with good fidelity, then it can be processed as required (filtered, peak detected or etc).

Heat increases noise.

Voltages and currents involved can be viewed as water with random particles floating in the water representing noise. A simple numeric figure does not indicate or represent how much or type of these particles (noise) are in the water. This is why dealing with noise as a simple numeric figure is not accurate or representative of what could actually happening.

Get a Tektronix 7A22, 5A22N or AM502 differential input amplifier or Stanford Research Systems SR560 differential amplifier as these are quite useful for this type of work along with filters and peak detectors as required.
http://www.thinksrs.com/products/SR560.htm


Bernice




Thanks for the OPA140 tip, new to me.

Right on the first page of the datasheet: 250nVPP, 0.1Hz to 10Hz . Should be very close to the total for that amp and is some 14dB down from the LTZ1000, should be enough! And supplies up to +/-18V  :-+


Unless you really want to build something out of discrete components :)
[/quote]
« Last Edit: April 22, 2015, 03:30:20 pm by Rupunzell »
 


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