### Author Topic: Low frequency, very low level, DC biased, noise measurements  (Read 34273 times)

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#### Rupunzell

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #50 on: April 22, 2015, 03:01:08 pm »
OPA827, not much lower noise than the OPA140 at 6nV/root-Hz @ 10Hz.
See figure 1, input noise density -vs- frequency.

How many op-amp can be paralleled before the problems occur? After the first pair, the noise does not go down much with successive op-amps added.

Bernice

OPA140 is great for measurements with RC highpass filters on the input. As the current noise of this device is very low, you can parallel devices to achieve even lower noise.

OPA4140 is the quad version of OPA140, great for paralleling.

From my analysis a OPA827 (Lowest voltage noise) is also even greater for an AC-coupled / low frequency / low noise amplifier.

#### janaf

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #51 on: April 22, 2015, 04:45:01 pm »
Whatever comes out with >= 250nVPP, 0.1Hz to 10Hz and gives a suitable input cap size, is good enough for me for now.

For the OPA140 they have specified 0.5pA input bias & offset. Some quick calculations show that with 1000 ohm Rin and D/C blocking, the voltage noise still outweigh current noise with a large margin; paralleling pays off in this case.
my2C
Jan

#### Rupunzell

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #52 on: April 22, 2015, 05:38:03 pm »
250nV at the output between 0.1Hz to 10Hz implies gain, question is how much gain?

Very generally speaking, FET input op-amps have low current noise (this is why FETs are used for low noise amplifiers more often than not as it allows lower noise to be achieved over a broader range of source impedances. See my initial reply).

LT1007 has lower noise. What matters is the 1/f corner freq. not just the volts/root-Hz or advertised output noise volte pk-pk. In all cases, spec sheets are designed to sell parts and play the game of spec-manship to move parts.
http://cds.linear.com/docs/en/datasheet/100737fbs.pdf

or

Do consider NOT using an coupling capacitor and using a low noise current at the feedback summing junction to offset the DC input as required, Not using the coupling cap avoids a host of problems. With +/- 15 volt supplies, there should be enough dynamic range to accommodate a 10volt ref. given the op-amp has enough common mode range.

Do keep the feed back impedance low as this will go a ways to keep the overall noise low and keep in mind the relationship between feedback loop impedance -vs- source impedance.

Grounding, shielding, overall layout, powering and all those tin details will make or break how well it works regardless of device specs.

Devices in parallel gains a few db in lowering noise with rapidly diminishing returns as the number of devices increases.

Lower impedance generally lower noise, Lower temperature generally lower noise, Lower bandwidth lower noise. For higher source impedances, bipolar input devices are at a dis-advantage.

Beyond this gets into what type of noise aka spectra and a host of other specifics.

Side curiosity, LVA zeners. They are low noise by virtue of their low dynamic impedance, sharp knee at low zener current.
http://ams.aeroflex.com/metelics/micro-metelics-prods-zener-LVA.cfm

Bernice

Whatever comes out with >= 250nVPP, 0.1Hz to 10Hz and gives a suitable input cap size, is good enough for me for now.

For the OPA140 they have specified 0.5pA input bias & offset. Some quick calculations show that with 1000 ohm Rin and D/C blocking, the voltage noise still outweigh current noise with a large margin; paralleling pays off in this case.
« Last Edit: April 22, 2015, 06:10:15 pm by Rupunzell »

#### dom0

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #53 on: April 22, 2015, 06:47:33 pm »
Whatever comes out with >= 250nVPP, 0.1Hz to 10Hz and gives a suitable input cap size, is good enough for me for now.

I don't think you can go to <250 nVpp(!) with op amps. Even the very good discrete design linked earlier in this thread has about 300 nVrms, about 3000 nVpp input referred noise.

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
,

#### dom0

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #54 on: April 22, 2015, 06:55:23 pm »
I'd also guess that it could be quite challenging to devise a suitably low noise, low drift comparison / subtraction voltage. It probably makes more sense to have a I-controller with very low bandwidth to do that (eliminates drift). As integrators with low bandwidths become unhandy quickly a digital controller would be an option, too.
,

#### Marco

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #55 on: April 22, 2015, 07:58:33 pm »
I don't think you can go to <250 nVpp(!) with op amps. Even the very good discrete design linked earlier in this thread has about 300 nVrms, about 3000 nVpp input referred noise.

I'm sure it's not trivial to build it to that quality, but there are scope shots in the datasheet.

These Burr Brown opamps seem to be just plain magic ... so were the Linear BJTs at the time (still are really, by a factor 2x over the competition) and so is the LTC2057. I wonder if Jim would even have bothered going discrete if Linear made an integrated JFET/CMOS amplifier that good (JRC does a CMOS amplifier with similar specs, the NJU77806, but it's lower voltage range and like most of their parts unobtainium.)

PS. if you don't go overboard with the amplification I don't think you need to worry about DC and ultra-low frequency effects too much, with the PP film caps you can use in this case leakage won't be an issue (as I said 220uF is plenty, that gives you a 0.01 Hz with 100kOhm and thus less than 10 kOhm equivalent noise over the entire relevant spectrum). Any remaining DC offset can be cleaned up in a subsequent high pass filter.
« Last Edit: April 22, 2015, 08:09:49 pm by Marco »

#### janaf

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #56 on: April 22, 2015, 10:10:32 pm »
250nVptp 0.1Hz to 10Hz IS input referred level and comes from the datasheet of the OPA140, compared to 1200nVptp for the LTZ1000. Paralleling a quad brings it down to half, 125nVptp.

Influence by gain is Rf only, as it's noise gain is unity, I.e. typically totally marginal.

Check the datasheet for this amp, voltage noise is unusually low for a FET amp. With "normal" low noise FET op-amp data, this noise level at low frequency would be impossible.

With DC blocking, the input impedance becomes a serial noise source (like a serial resistor on the inverting input) With FET amps, it can be kept low while paralleling improves voltage noise.

Ultra-low noise amps like LT1028 have <50nVrms (0.1-10Hz) but the current leakage ruins it with DC blocking.

The problem with biasing voltage is that the bias voltage has to have lower noise than the one measured or the bias voltage noise will ruin the measurements. Unless one comes up with some noise cancelling magic.

Also, I'd want high gain in the very first stage, to get out of the noise sensitive domain.

250nV at the output between 0.1Hz to 10Hz implies gain, question is how much gain?

Very generally speaking, FET input op-amps have low current noise (this is why FETs are used for low noise amplifiers more often than not as it allows lower noise to be achieved over a broader range of source impedances. See my initial reply).

Whatever comes out with >= 250nVPP, 0.1Hz to 10Hz and gives a suitable input cap size, is good enough for me for now.

For the OPA140 they have specified 0.5pA input bias & offset. Some quick calculations show that with 1000 ohm Rin and D/C blocking, the voltage noise still outweigh current noise with a large margin; paralleling pays off in this case.
my2C
Jan

#### janaf

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #57 on: April 22, 2015, 10:38:09 pm »
I think we are past the general good advise, now hard calculations and real tests count....
my2C
Jan

#### janaf

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #58 on: April 22, 2015, 10:41:31 pm »
So did I get this right?
Urms(0.1-10) = UW2x(10+fc2x1000)

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
my2C
Jan

#### splin

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #59 on: April 23, 2015, 12:23:02 am »
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.

True.

Quote
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

And yet the SR560 is a lot noisier than the OPA140 below 10Hz - 42nV/sqrt(Hz) @ 1Hz compared to 15, approx 590 nV pk-pk 0.1 to 10Hz v 250nV? (I extrapolated the SR560 noise down to .1Hz using Vn = 1/f^.5 * 43.7 - 1.7 which is a close fit to the 1 to 10Hz curve)

#### Marco

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #60 on: April 23, 2015, 12:42:50 am »
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.

No, but it's close to the limits of what the high pass filter can do without the capacitors getting ridiculously expensive ... so it's good enough. The only way to do much better without ridiculous capacitors is to make a differential amplifier and servo the output to zero ... for a measurement where an opamp with a relatively cheap passive high pass filter suffices that's just wasted effort.

#### Kleinstein

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #61 on: April 23, 2015, 02:46:15 am »
These low noise JFET OPs are an option. However one may have to careful, as the 250 nV_pp noise number is a typical value. The 1/f noise of JFETs is known to vary between wafers. So not all OPs may meet that number.  It likely would not help much getting 10 from one supplier - these are expected to be similar (all good or bad). Chances are better when getting one OPA4140 and two OPA2140.
Using the typical numbers an amplifier with 4 of the OPA140 would be at about 125 nV_pp, which is similar to what was achieved with a 3.3 mF coupling cap. and a LT1037 (German thread), and slightly better that what LTs AN124 got (about 150 nV_pp). All these numbers are well good enough for an LTZ1000 and likely most other references except the chemical cells - even a single typical OPA140  would work. To a limited extend one can calculate back the noise of the amplifier: So even if the noise of the source is as low as the noise of the amplifier one can still get some useful data.

One can not directly compare the numbers of OP-amps to a bare JFET. The ground referenced JEFT amplifier has much higher thermal effects - so thermal setup is much more demanding. Even the differential circuit will likely have a higher temperature dependence and already 3 dB more noise.  The problem with typical numbers also applies to individual JFETs - they can be good ot bad unless expensive special FETs for LF use are used. The rather good IF9030 is something like $16 at mouser. #### janaf • Frequent Contributor • Posts: 339 • Country: ##### Re: Low frequency, very low level, DC biased, noise measurements « Reply #62 on: April 23, 2015, 04:15:12 am » Do you have the resistor values used with these? Using the typical numbers an amplifier with 4 of the OPA140 would be at about 125 nV_pp, which is similar to what was achieved with a 3.3 mF coupling cap. and a LT1037 (German thread), and slightly better that what LTs AN124 got (about 150 nV_pp). my2C Jan #### Kleinstein • Super Contributor • Posts: 5968 • Country: ##### Re: Low frequency, very low level, DC biased, noise measurements « Reply #63 on: April 23, 2015, 06:31:11 am » The circuit with the LT1037 used 3.2 mF and 1 K ground. Feedback for 100 times amplification via 10 K and 100 Ohms. Source protection is through a resistor at the input, that is manually bridged with a switch. If higher frequency stability is a problem and no need for amplification above 1 kHz, a LT1007 (compensated for G=1) may be the better option. The resistor to ground could well be larger, as the 0.1 Hz lower is set by the 2nd stage. A larger resistor would not increase noise in the 0.1 Hz and up range, but only the settling time at the start. Link to simplified circuit: http://www.mikrocontroller.net/topic/207061#2060389 #### Andreas • Super Contributor • Posts: 2333 • Country: ##### Re: Low frequency, very low level, DC biased, noise measurements « Reply #64 on: April 23, 2015, 07:03:57 am » Hello, Interesting is also the noise floor (including leakage current when measuring 8*Eneloop) around 100-120nVpp (0.2uV/Div) And a LTZ1000 can be easily measured above the noise floor (1.02uVpp @ 0.2uV/Div) A photo of the amplifier is shown in a follow up thread: http://www.mikrocontroller.net/topic/250656#2679335 with best regards Andreas « Last Edit: April 23, 2015, 02:37:37 pm by Andreas » #### paulie • Frequent Contributor • Banned! • Posts: 849 • Country: ##### Re: Low frequency, very low level, DC biased, noise measurements « Reply #65 on: April 23, 2015, 08:31:16 am » Quote Hi, Thanks for the feedback! What I'm looking for now is a Unity gain, very high impedance amplifier. Noise is not an issue here. An amp with 1x gain but 1000Gig+ inpunt impedance. I'm measuring capacitor self-discharge and even the >10Gohm 10V range on my DMM is not high enough. Is that something you could help me point at a design? Ideas? I'll reply here because there don't seem to be any way to post photos in the PM area. This is part of the circuit I've been using for ultra high impedance radiation detector projects. IDK if this can be modified for your cap leakage measurements. The single issue for me was the very high value (aka VERY expensive) feedback resistor and solved by taking advantage of leakage of other devices like JFETs and LEDs. The good news is in your case no feedback may be needed since you are interested in absolute voltage or current. The bad news is if you need unity gain some reconfiguration probably required. Also there will be an offset. This should not be a problem since you are looking at rough values not precision. I think at room temp TC drift may not be a big issue there either. Thanks again for the 2n4117. Those electrometer grade devices solved issues that were a problem with the 2n3819 parts tried previously. However for some strange reason they did not works as well as the 2n3819 for G0HZUs "Worlds Simplest Osc" in dannyf thread. Similar HF specs (transconductance, Vgs, etc) but something must be different. #### Marco • Super Contributor • Posts: 4273 • Country: ##### Re: Low frequency, very low level, DC biased, noise measurements « Reply #66 on: April 23, 2015, 09:47:11 am » A temperature controlled 2n7002 source follower if you really want no leakage (a really good JFET reverse biased junction diode holds off current nicely, but not as nicely as a layer of silicon oxide). A LMC6001 if you want an opamp with guarantees ... but I'd save some time and money and just get a LMC662. « Last Edit: April 23, 2015, 09:50:47 am by Marco » #### paulie • Frequent Contributor • Banned! • Posts: 849 • Country: ##### Re: Low frequency, very low level, DC biased, noise measurements « Reply #67 on: April 23, 2015, 12:09:49 pm » I'd save some time and money and just get a LMC662. Yes, a very good compromise between low noise and high input impedance to allow smalll inexpensive blocking caps. Exactly the same conclusion I came too when implementing the noise amp for a multiplexer/logger as seen in the top center of this photo from my own low cost voltage reference experiments: https://www.eevblog.com/forum/projects/low-cost-voltage-reference-experiment/msg638485/#msg638485 #### Andreas • Super Contributor • Posts: 2333 • Country: ##### Re: Low frequency, very low level, DC biased, noise measurements « Reply #68 on: April 23, 2015, 02:42:53 pm » I'd save some time and money and just get a LMC662. Yes, a very good compromise between low noise and high input impedance to allow smalll inexpensive blocking caps. Really? when I look at the datasheet I see 90nV/sqrt(Hz) at 10 Hz. so effectively 300 nV RMS or 2uVpp for a 0.1 to 10 Hz bandwidth. While we are looking for below 0.3uVpp. Did you really measure the low frequency noise? pictures? with best regards Andreas #### paulie • Frequent Contributor • Banned! • Posts: 849 • Country: ##### Re: Low frequency, very low level, DC biased, noise measurements « Reply #69 on: April 23, 2015, 08:18:09 pm » Yes, I did measure the noise level and yes, it is not on a par with some parts costing 10x or 100x more. Replacing a previous lower noise OP07 but I needed the higher impedance input to keep coupling cap cost down and dual part to make room on limited board size for a peak detector. I also needed r-r output yet better than 5v supply which is hard to come by. However two points: First like in my previous photo, a much lower noise discrete is placed in front which is the real key to keeping circuit noise down. In my case biplolar transistors like S9014, NE85633, or even MPSA18 perform very well for a fraction the cost of special purpose devices. Secondly I must admit to not being a volt-nut. In fact pretty much the opposite, more like a penny-nut. Instead of most expensive/complicated solution, simplest and lowest cost compromise. Achieving 90% or less of the benefits for 1% of the cost is what floats my boat. In the case of voltage references an order of magnitude less precision at thousands of times less cost. For example as seen in that thread 2 cent bandgaps or one cent zeners. No box full of 50$ LTZ1000 here.

Anyway like janaf noise measuring was nowhere near as demanding as the voltage readings. For me pretty much an afterthought.

ps. I do admit buying an LM399. Not so much for evaluation but rather to use as a maximum limit to test methods and circuits. Only because they were available at 4 dollars and change. For me an outrageous luxury.
« Last Edit: April 23, 2015, 08:39:46 pm by paulie »

#### Marco

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #70 on: April 23, 2015, 08:42:23 pm »
In case it's not clear, I suggested the LMC662 as a high impedance buffer ...

#### paulie

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #71 on: April 23, 2015, 08:48:35 pm »
Me too. Originally there was no gain after the discrete front end but only need for high impedance buffer to allow a cheaper film cap. Eventually put some gain in as I realized the dynamic range was excessive and I could afford it to get a little more resolution.

#### Rupunzell

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #72 on: April 24, 2015, 02:30:09 am »
FET input follower (buffer) with"helper feed back to increase inout impedance. This is basically a charge amplifier.

Here is the data sheet for the 2N4117 (InterFet process NJ01), it is a tiny geometry FET with very small gate area and designed to have low gate leakage. The IDSS max is 600uA.
http://www.interfet.com/process/NJ01/

The 2N3819 is a very different FET with a IDSS of 2-20mA, about four times the input capacitance and no specification for gate leakage. This FET has much larger gate area and overall die size making it very different and not interchangeable with the 2N4117 in any way.
http://dpnc.unige.ch/tp/elect/spec-sheets/2N3819.pdf

-Or why interchanging these in the oscillator circuit is a no go.

Good high value resistors are available from Caddock. Decent parts are NOT cheap and should be use when needed.

Bernice

Quote
Hi,
Thanks for the feedback!
What I'm looking for now is a Unity gain, very high impedance amplifier. Noise is not an issue here. An amp with 1x gain but 1000Gig+ inpunt impedance.
I'm measuring capacitor self-discharge and even the >10Gohm 10V range on my DMM is not high enough.
Is that something you could help me point at a design? Ideas?

I'll reply here because there don't seem to be any way to post photos in the PM area. This is part of the circuit I've been using for ultra high impedance radiation detector projects. IDK if this can be modified for your cap leakage measurements.

The single issue for me was the very high value (aka VERY expensive) feedback resistor and solved by taking advantage of leakage of other devices like JFETs and LEDs. The good news is in your case no feedback may be needed since you are interested in absolute voltage or current.

The bad news is if you need unity gain some reconfiguration probably required. Also there will be an offset. This should not be a problem since you are looking at rough values not precision. I think at room temp TC drift may not be a big issue there either.

Thanks again for the 2n4117. Those electrometer grade devices solved issues that were a problem with the 2n3819 parts tried previously. However for some strange reason they did not works as well as the 2n3819 for G0HZUs "Worlds Simplest Osc" in dannyf thread. Similar HF specs (transconductance, Vgs, etc) but something must be different.
« Last Edit: April 24, 2015, 02:54:14 am by Rupunzell »

#### Rupunzell

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #73 on: April 24, 2015, 02:49:32 am »
At what cost to signal fidelity?

How much DC drift and other effects does adding any impedance buffer have on the overall system performance? Have a good look at what is inside that device, it is not a simple black box that is benign in every way.

Often taken for granted today, ICs specially analog ICs are considered nothing more than building blocks that are use to make up what ever is needed with little to no consideration for the finer details of what they could be doing to the analog signal's involved or how they might interact with the overall system. If the distortion or affect cannot be "measured" it is not a problem.. until it becomes a problem then there is a mad scramble to try and figure out what has gone wrong.

What is curious, the aversion to using and designing a higher performance input section using discrete parts that can offer lower noise performance over the IC solution.

After all this discussion, it turns out the goal was to measure the noise of a voltage reference over a bandwidth of about 10Hz and the noise levels are not that low after all.

The SR560, Tek 7A22, 5A22N, AM502 becomes useful once a low noise amplifier section is place in from of them increasing effective gain and allowing some LP-HP filter options with ease. DC drift over time will still need to be addressed in some way with any measurement device choice.

Bernice

In case it's not clear, I suggested the LMC662 as a high impedance buffer ...

#### Marco

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##### Re: Low frequency, very low level, DC biased, noise measurements
« Reply #74 on: April 24, 2015, 03:00:02 am »
At what cost to signal fidelity?

I don't think you followed the conversation, he wanted to measure the self discharge and dielectric absorption of the capacitor ... noise is irrelevant. Voltage offset and input current are relevant. JFET's can do a lot of things, but beat a good CMOS at input current is not one of them.

LMC662 is the go to part for high impedance buffering with some input protection.

PS. don't top post
« Last Edit: April 24, 2015, 03:02:28 am by Marco »

Smf