noises and frequencies - Page 1
Forum section: electronics / projects

author: Ogitek
author_rank: Regular Contributor
timestamp: December 12, 2023, 05:05:58 am
content: If a circuit has say less than 2 microVolt peak-to-peak noise at DC to 100 Hz with 10,000 Ohm source.How would the noise increase at 1000 Hz? Would the noise become 8 microVolt or 30 microVolt? For a 1-10 mV signal. What must be the minimum noises in microVolt to make it produce good signal?Please give actual example of other amplifiers to see how the noise behavior changes with frequency.Thank you.

author: IanB
author_rank: Super Contributor
timestamp: December 12, 2023, 05:24:31 am
content: Homework question?It would be good if you provide some of your own input, what information sources you have looked at, what analysis you have done yourself, where you are stuck with how to proceed?I think people will be willing to help you solve problems where you are stuck, but just giving you an answer will help nobody, especially yourself.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 12, 2023, 05:49:25 am
content: There are many types of noise. Most of the noise is thermal noise. At low frequencies, the 1/f noise is dominant.See if this answer your questions:https://en.wikipedia.org/wiki/Johnson%E2%80%93Nyquist_noise

author: Ogitek
author_rank: Regular Contributor
timestamp: December 12, 2023, 08:10:34 am
content: I'm scouting for a Bioamplifier for my medical school studies. Manufacturer spec is it has noise of less than 2 microVolt at 100 Hz. But noise becomes 8 microVolt or 8 millivolts at 1000X gain at 1000 Hz. Why does noise increase (much higher) at higher frequencies?But the signal I'm after is only1 to 5 millivolt. So won't the 8 milliVolt noise swamp the signal?Is it very hard to create amplifier with 1 milliVolt resolution yet with noise of less than 2 microVolt at 1000 Hz instead of 100 Hz?

author: magic
author_rank: Super Contributor
timestamp: December 12, 2023, 09:58:52 am
content: What's the source impedance that the amplifier sees, i.e. the output impedance of the sensor (or whatever it is that you are amplifying)? Is it really 10kΩ?How exactly are you measuring the noise? Did you also verify that it goes down to 2μVpp at 100Hz as expected?What do you mean by "at 100Hz" or "at 1000Hz"? Does the amp have a bandwidth switch with options like 100Hz, 1kHz, 10kHz which means "from DC to 100Hz" etc and changing this setting changes the total amount of output noise? If so then your observation is not far off from expectations, the typical increase being proportional to square root of the increase in bandwidth (3.16x more noise for 10x more bandwidth).

author: David Hess
author_rank: Super Contributor
timestamp: December 12, 2023, 11:30:54 am
content: Quote from: magic on December 12, 2023, 09:58:52 amExactly, for a flat noise spectrum which is typical at higher frequencies, noise is proportional to the square root of the bandwidth.Sqrt(100)=10Sqrt(1000)=31.6The ratio of noise is 31.6/10 so a difference of 3.16 times.At low frequencies flicker noise increases except in chopper stabilized amplifiers, but corner frequencies below 100 Hz are likely so flicker noise should be insignificant in this case depending on the design.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 12, 2023, 11:33:49 am
content: Quote from: magic on December 12, 2023, 09:58:52 amYes, they have adjustment switches.For 100 Hz, the noise is 2 microVolt peak-to-peak with 10 kOhm source impedance.For 1000 Hz, the noise at 1000 gain measured at output is 8 milliVolt peak-to-peak with 100 Ohm source impednance, which they said should correspond to an approx. 8 microVolt noise figured referred to the the input.I'd like to know something. If my input is 10mV, or 50mV or 100mV, is the noise input noise always constant at 8 microVolt? or is it related to the input voltage, given a fixed frequency?

author: magic
author_rank: Super Contributor
timestamp: December 12, 2023, 01:27:28 pm
content: If the source is resistive then it has Johnson noise which is amplified by the amplifier like any other signal. This should be about 1μVpp for 10kΩ and 100Hz, or 0.3μVpp for 100Ω and 1kHz. This is a limit which cannot be reasonably improved upon with resistive sources. (An effective but usually unreasonable measure is cooling the resistor significantly).This amplifier is probably not optimized for 100Ω source impedance. It is also possible that your source has noise other than Johnson noise. You could find out by connecting a 100Ω resistor to the amplifier (between signal input and ground).

author: CaptDon
author_rank: Super Contributor
timestamp: December 12, 2023, 02:39:21 pm
content: To answer one of the questions, the noise (floor) would remain constant regardless of input signal amplitude. What you get is 'Signal to Noise Ratio' or how much bigger the desired signal output is in relation to the noise floor. Sounds like you are monkeying around with ECG / EKG type amplifiers. For medical use an 8uv input side noise floor is pretty good when referenced to a 1mv input 'chest' signal. There will be lots of filtering involved for certain signals like parsing out the breath rate very low frequency of 40Hz or less. The ECG / EKG signal requires greater bandwidth for the sharp QRS transitions and often you have 50Hz / 60Hz sharp notch filters to get rid of power line influences. If you have these questions and don't understand the math or specs involved why have you been chosen as 'the guy' to select a proper amplifier chain?

author: Ogitek
author_rank: Regular Contributor
timestamp: December 12, 2023, 10:59:41 pm
content: The school didn't plan to buy a new one, but they have budget and I'm just suggesting them to buy one, and hence choosing what to write in the proposal.The manufacturer told me "You are asking us to copy your A-D system for this request. The noise figure I gave was for broad-band, and we can't narrow it any more than that. In addition, your A-D system will have some impact on noise, and we have no way of knowing that."What does "A-D" mean? My request for them was to test at 1000 Hz, because their only data is 100 Hz. So it seems they tested the broad-band which is 10kHz (because the adjustments can reach 10kHz). But from 100 Hz to 10,000 Hz, the noise only increases many times instead of just 4.sqrt (100) = 10sqrt (10000) = 100100/10 = 10X, but the noise increase from 2 uV (at 100Hz) to 8 uV (at 10,000Hz) is only 4X (8uV/2uV) not 10X. Could it be because the source impedance used for the 10kHz test was 100 Ohm only)?Also the source is just skin, so how could the A-D (whatever this is) system will have some impact on noise?

author: CaptDon
author_rank: Super Contributor
timestamp: December 13, 2023, 12:20:32 am
content: Because they are referring to an Analog-to-Digital conversion which they are assuming you are doing to the analog chest signal. The 'noise' in your A to D converter would to a large degree depend upon how many bits of resolution your converter has. For example an 8 bit converter vs. a 12 bit converter. The 12 bit would have smaller step increments and would appear to have less noise as the least significant bits of resolution tend to dither back and forth even though the sample remains constant. Also a faster sample / conversion rate would tend to appear as a less noisy signal. They have no way of knowing what you are doing with the post amplification signal and perhaps you are not even running it into a digitizing circuit? Open skin contact measured as a human with normal skin moisture holding DVM probes (one in each hand) and squeezing the probe tips with the fingers shows typically 10K ohms to 50K ohms. With ECG /EKG 'sticky pads' attached to a shaven chest the 'source impedance' could run 1K ohms to 10K ohms but usually on the lower side once skin acids and sweat enter the picture! Cheers mate, hope this info helps.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 15, 2023, 11:46:02 pm
content: The manufacturer told me he used oscilloscope to check for the 8mV output at 1000X gain so he figured the noise is 8uV referred to the input (this is using a 100 Ohm source impedance). The unit has frequency adjustments of 20Hz, 50Hz, 100Hz, 1000Hz, 5000Hz, 10,000Hz.But is it not a spectrum analyzer is the right tool to check for noises? How accurate is an ordinary oscilloscope compared to spectrum analyzer?We know most spectrum analyzers (except the very expensive ones) have minimum 9kHz. But then the unit has a 10kHz setting. If he used spectrum analyzer, does the following relationship still hold?Sqrt (10000Hz) = 100Sqrt (100) = 10So if the noise at 10,000 Hz is 8uV. Then the noise at 100Hz is 0.8uV (10 times less)?I think he just used an ordinary oscilloscope which may be less accurate?

author: CaptDon
author_rank: Super Contributor
timestamp: December 16, 2023, 03:14:20 am
content: A spectrum analyzer could show the frequencies at which the noise occurs which may be of some concern. The oscilloscope will simply show a Peak to Peak signal and sophisticated scopes can make the measurement automatically. You may gain some frequency information from a scope. Singer Metrics made an 'ultrasonic' Spectrum analyzer that went from D.C. to 600KHz. Low frequency tailored analyzers are common and used to examine bearings in machines and often in very high speed devices like turbine engines and turbochargers. Certain peaks in frequencies can often indicate a bearing approaching failure. Low frequency (audio range) analyzers are also used during engine testing looking for pre-detonation, piston slap, piston wrist pin slap, and characterizing engine sounds which are normal and should be ignored by the E.C.U.'s F.F.T. analytics as well as abnormal sounds that indicate an abnormal running condition or destructive failure. Sounds like your medical lab needs to hire a good electronics technician.

author: David Hess
author_rank: Super Contributor
timestamp: December 16, 2023, 07:18:05 am
content: The oscilloscope makes a "spot noise" measurement which is the total noise over a range of frequencies, or over a noise bandwidth, in peak-to-peak and in RMS. In this case the noise bandwidth is determined by the device itself assuming that the measured spot noise is greater than the noise from the oscilloscope itself.Over 100 MHz, the best typical oscilloscope input noise is about 30 microvolts RMS, but 100 microvolts or higher is more common. In your case the input noise measurement can be made because it is being amplified first, and limiting the oscilloscope bandwidth to 20 MHz or lower further helps.An oscilloscope with the FFT function (and averaging of the FFT results, which is not a given) can then make a noise density measurement, which is what a spectrum analyzer can do. In both cases the noise density measured depends on the resolution bandwidth (RBW) of the oscilloscope FFT or spectrum analyzer. A spectrum analyzer should have a "noise marker" function which takes this into account to return the noise density within a 1 Hz bandwidth, but almost all oscilloscopes lack this function (1) so you have to do the math yourself after determining the correction factor which depends on the FFT bin width and window function. This article discusses the problem:https://www.edn.com/dsos-and-noise/Basically it is going to come down to using a DSO to make spot noise measurements instead of noise density measurements because they lack the functionality to do the later properly, but a spot noise measurement is probably more useful anyway in your application and oscilloscopes work fine for this.RF spectrum analyzers have a lower limit of about 9 kHz because of input coupling limitations (2) and because the mixing process moves the high phase noise part of the local oscillator to low frequencies, so they are typically very limited at low frequencies anyway. A low frequency dynamic signal analyzer can produce a low noise measurement down to DC, but is essentially doing the same thing that an oscilloscope FFT is doing, but properly.(1) The lack of oscilloscopes which support this properly is a real shame since they should be good at it. The DSO should also allow averaging of the FFT result *without* averaging the input signal, but many modern DSOs cannot get this right either.(2) Some old spectrum analyzers had an option which swapped the leads of the first mixer and removed the input coupling so they could operate down to DC, but they still had high noise at low input frequencies.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 16, 2023, 10:22:43 am
content: So if the manufacturer uses an ordinary oscilloscope and measured 8mV noise at output using 1000 times gain deriving 8uV noise referred to input. So the 8uV is the spot noise? what frequency is the 8uV given that his amplifier/unit has frequency adjustments of 20Hz, 50Hz, 100Hz, 1000Hz, 5000Hz, 10,000Hz? Remember he said "The noise figure I gave was for broad-band, and we can't narrow it any more than that."

author: magic
author_rank: Super Contributor
timestamp: December 16, 2023, 10:27:01 am
content: Sounds like it's the total noise at all frequencies from DC (or some lowest frequency the amp can pass, maybe a few Hz, if it isn't DC coupled) up to 20/50/100/1000/10000 Hz.Also, 8μVpp from 100Ω source at 1kHz bandwidth is not a good result, it could be less than 1μV. Look for a different amplifier if it makes a difference for you.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 16, 2023, 10:47:11 am
content: How is input impedance related to it. I mean. For example at spec of 100 Hz and the noise is 2 microVolt peak-to-peak with 10 kOhm source impedance. What happens if it is 100 Ohm source?

author: G0HZU
author_rank: Super Contributor
timestamp: December 16, 2023, 02:19:21 pm
content: QuoteIf it helps, I have an old Tektronix 8GHz spectrum analyser that works really well down to AF frequencies. It's a 50 ohm system and I usually fit a low noise preamp ahead of it if I want to look at very small signals. The advantage that the spectrum analyser offers is that it can show how the noise level changes with frequency right down to AF frequencies.The LF preamp has about 28.6dB gain and the noise figure is about 4dB. If I place a very sharp 2.5MHz LPF after the preamp I can define the noise bandwidth as 2.5MHz. If the gain is 28.6dB and the noise figure is 4dB it should be possible to calculate the Vrms of the noise at the output of the amplifier.The noise bandwidth is 2.5MHz or 2.5e6 Hz. 10*log(2500000) = 64dBHzSo the output noise power should be -174 + gain + noise figure + 64dBHz = -174 + 28.6 + 4 + 64 = -77.4dBm. This is 1.811e-11W. In a 50 ohm system this will be 30uV because power = (V*V)/RSo a 50R terminated Vrms meter connected to the output of the preamp + LPF should read 30uV rms for the noise at the amplifier+LPF output.This all assumes that the noise figure of the LF preamp is flat from 2.5MHz down to audio frequencies. The old Tek spectrum analyser shows that the noise level is flat although it struggles a bit below about 1kHz due to its own internal spurious terms. There will be better analysers available than my old Tek, especially if they have been designed for low noise audio work. However, my analyser is OK to use as a quick demo here. I've used it to measure the noise figure of various low frequency amplifiers in the past, but usually down to about 1kHz rather than 100Hz.

author: G0HZU
author_rank: Super Contributor
timestamp: December 16, 2023, 02:22:59 pm
content: One thing to watch out for is ground loop noise/spurious especially if you connect various bits of test equipment together. I usually power the LF preamp and any DUT with a battery to minimise this issue. Also be wary of out of band noise and spurious up at higher frequencies. These can dominate any measurements if you are not careful and this can cause confusion.My background is in RF, not AF but I do have some experience measuring low noise stuff down at audio frequencies.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 16, 2023, 02:54:30 pm
content: Quote from: magic on December 16, 2023, 10:27:01 amPlease tell me. Why is a 8μVpp from 100Ω source at 1kHz bandwidth not a good result when the input signal I'm after is 1mV and not 1 uV?The amplifier unit has this spec:Input impedance: 10 megaOhmsFreq response DC to 10kHz +/- 3 dBFrequency rolloff -12 dB/octaveCommon mode rejection 90 dB minimumNoise Less than 2 uV peak-to-peak, DC to 100 Hz, with 10kOhm sourceOutput configuration Single-endOutput impedance 1000 ohms.I know that with 100 Ohm source impedance instead of 10 kOhm, it would have better voltage divider performance with respect to the 10 megaOhm input impedance. But with 10 megaOhm, is it not 100 Ohm vs 10,000 Ohm is not much difference?What other amplifier do you or anyone heard with better performance?

author: magic
author_rank: Super Contributor
timestamp: December 16, 2023, 03:12:54 pm
content: Quote from: Ogitek on December 16, 2023, 02:54:30 pmSimply because it could be a few times lower, under these particular circumstances.Quote from: Ogitek on December 16, 2023, 02:54:30 pmIf you care about 1mV signals which will become 1V after 1000x gain then this level of noise is probably harmless.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 16, 2023, 03:25:55 pm
content: Quote from: magic on December 16, 2023, 10:27:01 amIf 8uV is the total noise. How is it derive with respect to each of this frequency? 20/50/100/1000/10000 Hz? I mean you average them like the 100 Hz has 1 uV noise, the 10000 has 20uV noise. So 8uV is the average of them?How does the oscilloscope algorithm compute for it?

author: donlisms
author_rank: Frequent Contributor
timestamp: December 16, 2023, 06:33:43 pm
content: I will make one observation. You ask many questions using the terms like "at this frequency." The answers refer to "bandwidth." I have a feeling you don't really understand this yet.I'm quite sure there are some pedants here who will object to my description. My assumption is that you know medicine, not these things. I suspect your intuition has lead you off track, and it might help to go through it step by step.Noise measurements are really only useful across a range of frequencies. When the specs say 2uV "from DC to 100Hz", they're not saying "choose any specific frequency between 0 and 100 Hz, say 37Hz, and the noise will be 2uV." No. It's not at some single frequency.Conceptually, the input noise signal is filtered so that fluctuations in the signal that happen faster than 1/100 second are removed. Only the slower fluctuations remain. This is the significance of the various frequencies mentioned in the specs, and in most of the discussion here. It is the dividing line between the slower fluctuations, which will be measured, and the faster fluctuations, which are ignored. It is the upper speed limit. With this amplifier, the lower limit is always 0.Once the fast parts are removed, we're left with one signal, which is to say, one voltage at any instant in time. The voltage wiggles up and down.If we keep only motions slower than 1/100 second, the voltage wiggle is less than 2uV. If we raise the limit to keep only fluctuations slower than 1/1000 second, the wiggle is always less than 8uV.There is only one voltage. It changes. We are not measuring those changes AT a specific frequency. We're just measuring the voltage fluctuations from instant to instant. The frequency choice was used earlier, to decide the upper speed limit for the noise of interest.When we choose the filter frequency, we're saying "I don't care about any fluctuations faster than ...such... Just ignore them. The only meaningful variations are below that speed." I don't know medicine, but I guess that frequency selection for the amplifier is much more about the meaning of the voltage changes, rather than specifically about electronics-induced noise. In other words, it's more interesting to filter out the high frequencies of the actual input voltage coming in from the sensor, than to get all tied up in the impact of noise generated in the circuit, which, if I understand it, is much smaller than the signal coming in, and therefore can be ignored. Probably. Maybe.

author: CaptDon
author_rank: Super Contributor
timestamp: December 16, 2023, 11:47:28 pm
content: Ogitek, I think you have us puzzled here. If you are in a BioMedical Electronic Technician course then your curriculum should be explaining all of this to you. If you are in a medical course then why have you been selected to specify and purchase an amplifier when you obviously know very little about electronics and amplifiers? If you are in a medical course of study why doesn't the university have everything needed on hand? When the supplier of the amplifier gave you a test spec of 8uv peak to peak and for medical studies your total bandwidth after amplification will probably be 10KHz or less the noise isn't likely to rise much above that at any input impedance. Wrap your mind around 'Signal to Noise Ratio' (SNR). Your signal of interest is around 1mv and your apparent noise is 8uv or let's say 10uv to make the math easy. So the SNR is mathematically is 100:1 or in decibels I think that equates to 40Db in voltage ratio (not power ratio). In the medical field that 8uv is tiny compared to the ambient noise from power lines, electrosurgical units nearby, and possible E.M.I. from other machines and even the lighting in the room. If someone has tasked a surgical medical student to specify the purchase of an amplifier then they missed the boat on assigning tasks. If you are a BioMedical student then you should be able to give us the answers as opposed to asking questions. Tell us what you are doing or how you intend to do it and maybe we can offer more targeted advice. You seem to ask the same questions in different ways that prove you have no understanding of electronics and as such probably should not be tasked with purchasing a particular piece of electronic amplification equipment. I hope we can help you but so much of this makes no sense? I have 5 years as a B.M.E.T. in a hospital with 18 O.R.'s + 3 trauma surge tables and 3 more N.I.C.U. tables + Cath Labs and OB/GYN. The majority of 'amplifiers' I dealt with as a repairman / technician / calibrator were from a company called SpaceLabs. There were also some G.E. / Marquette stuff and a few oddballs.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 17, 2023, 12:15:58 am
content: Quote from: David Hess on December 16, 2023, 07:18:05 amWhen you measure a signal in an oscilloscope like an audio source. Does it also do "spot noise" measurement of the entire audio? Is it not it just plots the amplitude and then frequency. How does the oscilloscope know how to distinguish if it is measuring signal or noise and need to decide whether to use "spot noise" like measurement process which is the total noise over a range of frequencies, or over a noise bandwidth? You don't measure audio or signal over a range of frequencies, or over a bandwidth, do you?

author: CaptDon
author_rank: Super Contributor
timestamp: December 17, 2023, 03:29:21 am
content: I would say "Of course you measure audio frequencies over a given bandwidth, 20Hz to 20KHZ. Although in today's digital recording world you could measure from D.C. to 50KHZ in the higher sampling rate devices. But as to your question, you could measure the noise across the entire bandwidth which is often done or you can either 'bandpass' a portion of the audio or 'band-reject' an unwanted portion through filters and then measure remaining noise. So, you have never seen or used an oscilloscope in a lab environment? The modern digital sampling scopes can do any sort of math imaginable to a series of digital samples and you can even take those .CSV files and import them into MatLab and other math programs. You still are hung up on processes and terms you don't understand and ask the same sort of questions that do nothing but go round and round in circles. Tell us, what are you trying to do and how are you trying to do it. You have gotten endless answers including formulas to calculate noise vs. bandwidth and endless explainations of noise vs. impedance and you still haven't moved off of home plate towards first base. Give us some info, what the heck are you actually working on? You don't appear to be the correct choice for moving forward in the project. If your math skills are good there is endless information about noise and sources of noise and formulas that would make Einstein's head spin just by doing google searches. Good luck.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 17, 2023, 04:45:45 am
content: Although a medical student just starting. I'm interested in emotional feedback systems for integration to AI so I'm studying the emotional nervous and biopotential feedback control system in humans so can integrate to machines or prototypes similar to the Cyberdyne T-800.Thanks for the many information. I'll study them at length and hopefully get a device for my own use and understanding. My school department doesn't use the machines yet because they are used only in other departments not accessible to me.

author: David Hess
author_rank: Super Contributor
timestamp: December 17, 2023, 05:37:53 am
content: Quote from: Ogitek on December 16, 2023, 10:22:43 amThe amplifier's frequency adjustment sets the upper frequency limit in this case, so the spot noise measurement applies from DC, or whatever the low frequency cutoff is for the amplifier, and the selected high frequency limit.Quote from: Ogitek on December 17, 2023, 12:15:58 amA spot noise measurement is the total noise over a range of frequencies with a lower and upper frequency limit which depends on filtering. Sometimes spot noise measurements are made with a bandpass filter centered at one particular frequency, like 1000 Hz, but there will still be a bandwidth.The oscilloscope is actually measuring the total amount of noise over its own input bandwidth, like DC to 20 MHz or DC to 100 MHz or whatever the upper limit is. However the total noise from the oscilloscope itself over 100 MHz might be 50 microvolts, so if it is connected to a source which is significantly larger, like something with 8 microvolts of input noise but with a gain of 1000 producing 8 millivolts at the oscilloscope input, then the noise from the oscilloscope itself can be ignored because it makes no significant contribution to the total noise. Then the spot noise bandwidth is that of the source instead of the bandwidth of the oscilloscope.Note that the shape of the filtering has an effect also. The spot noise measurement includes the skirts of the filter outside of the -3dB points, and this is why the "noise bandwidth" is greater then the filter bandwidth. For example a spot noise measurement from DC to 1000 Hz which relies on a single pole RC filter for the 1000 Hz -3dB bandwidth has a noise bandwidth 1.6 times greater, so 1600 Hz.The same applies to the resolution bandwidth of an oscilloscope FFT or spectrum analyzer measurement which is why a correction factor has to be applied to get consistent units as explained in the article that I linked. Spectrum analyzers which have the noise marker function are suppose to compute this for you. For an oscilloscope FFT this correction depends on the FFT bin width and window function.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 17, 2023, 07:18:46 am
content: Quote from: David Hess on December 17, 2023, 05:37:53 amThanks. pls give actual values of the total noise. For example..If noise at 100Hz is 2uV. And noise at 10,000Hz is 10uV. Is the total noise 2uV + 10uV = 12 uV or is the total noise average of 2uV and 10uV or 6uV?Quote

author: magic
author_rank: Super Contributor
timestamp: December 17, 2023, 08:07:05 am
content: I think you don't understand some basicmost basics.The whole "measurment" performed by your amplifier vendor works like this:1. connect an oscilloscope to the output of the amplifier2. connect a resistor of particular value to the input of the amplifier3. switch the amplifier to one bandwidth option or another4. look at the oscilloscope screen and measure peak to peak amplitude of whatever's thereThere is nothing more to it and nothing to add or subtract or average.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 17, 2023, 08:37:42 am
content: Quote from: magic on December 17, 2023, 08:07:05 amI know. What I was asking was how the oscilloscope compute the noises, does it add them like 2 + 10 = 12uV or average them like (2+10)/2 = 6uV?I own a small oscilloscope but was only looking at signal before. I'll set it up to look at noises next week.Ok. What happens if you disable the high pass filters in the circuit so all frequencies to RF or higher gets into the amplifier and oscilloscope. I actually did it before. Have you done this before? Would the noises get into milliVolt and swamp all the signals? I will measure them at details now.I also got one of the most noise free ADC in the world that beats even Audio Precision. The ADC manufacturer EIDA told me it can measure below 1 microVolt using REW.https://www.linsoul.com/products/e1da-cosmos-adc

author: magic
author_rank: Super Contributor
timestamp: December 17, 2023, 08:46:46 am
content: An oscilloscope doesn't compute anything. It samples the input voltage N times per second and plots the resulting waveform on the screen. Noise is random variations in the output of the amplifier and that's what you'll see on the display.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 17, 2023, 08:54:49 am
content: Ok. Anyone, please give me an actual circuit that can do 1mV at 1000 Hz with less than 10 microVolt noise. Because the current manufacturer is selling me one for $1200. I can get a $20 unit but it has only 100 Hz limit. Can you get any less than $100 unit that can do 1000 Hz at microvolt noises?is this easy to build? What IC has this spec?

author: David Hess
author_rank: Super Contributor
timestamp: December 17, 2023, 02:20:48 pm
content: Quote from: Ogitek on December 17, 2023, 07:18:46 amUncorrelated RMS values are added using the square root sum of the squares. So if you have two noise sources added together, you square each one, add, and then take the square root.(2^2+10^2)^(1/2)=10.2If your oscilloscope has a noise of 50 microvolts over 100 MHz, and your x1000 amplifier has an output noise of 10 millivolts over its bandwidth, then the measured noise would be:((50x10^-6)^2+(10x10^-3)^2)^(1/2)=10 millivoltsWhich is why I said that the oscilloscope noise can be ignored in your case. The input referred noise of the amplifier is then 10 millivolts / 1000 or 10 microvolts.

author: MathWizard
author_rank: Super Contributor
timestamp: December 17, 2023, 05:58:09 pm
content: Hey Mr. Hess, what's a good book for transistors with advanced topics like noise and temperature effects ? I don't want to get into Poisson equations and all that stuff yet, but I do want the text book with all that in it. I'm not deep enough into the semiconductor physics, I guess that's the kind of book it will have to be. But that's ok.None of my books really get into noise and any amount of changing frequency and non-linear frequency effects.

author: David Hess
author_rank: Super Contributor
timestamp: December 17, 2023, 09:32:32 pm
content: Quote from: MathWizard on December 17, 2023, 05:58:09 pmFor noise and noise in connection with operational amplifiers, I suggest the various application notes published by Analog Devices.For transistors and noise, the most approachable book I know of is Small Signal Audio Design by Douglas Self which covers voltage and current noise in bipolar transistors and JFETs in just a couple pages.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 17, 2023, 11:27:05 pm
content: My pocket oscilloscope has vertical resolution of 10mV/div to 10V/div. The moderately price Rigor DS1054Z has vertical resolution or scale range of 500uV/div to 10V/div. Where will I get an oscilloscope with 1uV/div and cheap? What models are you guys using that can do this?Also is it so hard to build a 1mV with 1000Hz with 1uV noise unit? Can't anyone do it? Is this harder than the Manhattan Project? (see the movie Oppenheimer to get an idea what this is)

author: Someone
author_rank: Super Contributor
timestamp: December 18, 2023, 05:52:40 am
content: Quote from: Ogitek on December 17, 2023, 11:27:05 pmIt is very easy, but you are yet to understand the electronics/physics behind it. Once you understand the principles it is obvious how to undertake the task.Suggested starting points:https://www.ti.com/lit/SBOA345https://download.tek.com/document/LowLevelHandbook_7Ed.pdfYes, you can buy an already built/assembled circuit/IC that will magically work. But you pay for the expertise or excess performance beyond what you actually need. There are very cheap solutions but they require skill to understand where shortcuts can be taken. Your supervisor should be directing your study on the topic, rather than asking the internet for the end solution.

author: David Hess
author_rank: Super Contributor
timestamp: December 18, 2023, 08:14:35 am
content: Quote from: Ogitek on December 17, 2023, 11:27:05 pmOscilloscopes run out of noise performance before they run out of vertical sensitivity. 500 uV/div and better is only useful when combined with suitably low noise.Old Tektronix oscilloscopes which supported the 7A22/5A22 differential amplifier had a sensitivity of 10 uV/div and bandwidth of 1 MHz, but not at the same time which is why this amplifier has variable user selected bandwidth. Tektronix also made this amplifier as a standalone unit with the part numberAM502which could be used with any oscilloscope.QuoteIf a singled ended input is acceptable, then most precision operational amplifiers can meet those requirements easily. Use the simple non-inverting configuration and add a feedback capacitor to control bandwidth, so all of 4 parts total, plus decoupling capacitors and a power source. An LT1007 with a 1000 Hz bandwidth limit will yield a noise of about 0.25 uV rms noise and will be suitable for source impedances around 1 kilohm. A more modern OPA140 which has JFET inputs will do a little better with higher source resistances because of lower input current noise, but noise from the source will dominate as the source resistance increases.The AM502 mentioned above also delivers about this level of performance but has a differential input. Duplicating this with parts and a circuit is more involved. I might use a pair of OPA140s to make a low noise differential preamplifier (5 parts), followed by a difference or instrumentation amplifier to produce a single ended output which can drive an oscilloscope or other test instrument, but this will be slightly higher noise than an AM502, although still well below 1 microvolt up to 1000 Hz.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 18, 2023, 09:35:33 am
content: Quote from: David Hess on December 18, 2023, 08:14:35 amSingle ended means 1 channel only?If they would be built using breadboard only, what noises are introduced?Should they be built using 0603 SMD to lessen noises? The smaller the components the lesser the noises?If you would build them, what PCB and components would you use?Can you build me one? Would $300 or $500 be ok for the service? Anyone?

author: David Hess
author_rank: Super Contributor
timestamp: December 18, 2023, 02:34:29 pm
content: Quote from: Ogitek on December 18, 2023, 09:35:33 amSignal ended means a signal and ground wire, like with a coaxial cable.A differential input would mean two signal wires, positive and negative, and a third ground connection which may or may not be present.QuoteNone, but it may benefit from a metal enclosure for shielding.QuoteIn theory the smaller layout which surface mount parts allow will pick up less noise, but at this bandwidth and sensitivity it will not make much difference. It will likely pick up your hand waving around it from electrostatic coupling, but it isn't something I would worry about.QuoteNewer parts like the OPA140 are only available in surface mount, so SO-8 style packages might be considered the expected standard for construction now.QuoteFor that much you could buy a used Tektronix AM502 and TM501 or TM503 power supply mainframe to power it.A quick search turned up these:http://www.aricorp.com/OSP-1.htmlhttps://www.amazon.com/Amplifier-Portable-Oscilloscope-Electronic-Engineers/dp/B0B7XML13Nhttps://www.ebay.com/itm/165597846810The OSP-1 linked above, which should do exactly what you need, is basically a remake of the Tektronix AM502. What I proposed with a couple of operational amplifiers has similar performance but is not as fast.Approximately where are you in the US? I am in New Hampshire.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 18, 2023, 02:55:19 pm
content: Quote from: David Hess on December 18, 2023, 02:34:29 pmAll Biopotential amplifiers use differential input. Why, which of them is single ended?You can ship it to Delaware. So what ICs and components you'd use with differential input besides AM502 and 1mV, 1000 Hz, with 1 uV noise?Thanks.Quote

author: CaptDon
author_rank: Super Contributor
timestamp: December 18, 2023, 03:35:01 pm
content: Dave, I like the fact that you mentioned the AM-502 since it is able to do single end or differential with bandwidth selection. It truly is a useful piece of the Tek 500/5000 modular series! I wish it would have had more available bandwidth, greater than 1 MHz. I use mine rarely but 'When you need it there is nothing else like it'. I probably use mine at most once per year but I would never sell it!! I have the RM-506, TM-503 and TM-502. I also have a stripped down TM-502 mounted to a wood base with bottom guide rails which I use when calibrating the 500 series modules. Wish I had the RM-5006 so I could take advantage of the 5000 series modules!! Cheers!! (B.T.W., I often see TM-506 and RM-506 units surplused out of medical institutions with all 6 slots filled with AM-502's. Sad that often these institutions just put their old equipment in the crusher, I think it is a requirement!)

author: David Hess
author_rank: Super Contributor
timestamp: December 18, 2023, 03:37:56 pm
content: Quote from: Ogitek on December 18, 2023, 02:55:19 pmBased on your earlier post:I'm scouting for a Bioamplifier for my medical school studies. Manufacturer spec is it has noise of less than 2 microVolt at 100 Hz. But noise becomes 8 microVolt or 8 millivolts at 1000X gain at 1000 Hz. Why does noise increase (much higher) at higher frequencies?I think you need an amplifier with a differential input like an AM502 or the OSP-1 that I linked.

author: David Hess
author_rank: Super Contributor
timestamp: December 18, 2023, 03:48:46 pm
content: Quote from: CaptDon on December 18, 2023, 03:35:01 pmThe AM502 was based on earlier designs so Tektronix had a long history with that type of amplifier. They also made some oscilloscopes that had the AM502 functionality built in, which indeed were intended for the biomedical community. I acquired several 7A22s, which are the same thing as an AM502 but for the 7000 series oscilloscopes, before I managed to purchase a AM502, and they are indispensable in some applications where before I would have had to design and build my own custom amplifier.Bandwidth is limited to 1 MHz because despite its low input noise, noise still limits its sensitivity at the highest sensitivity ranges. Of course this is not a problem because it has user adjustable bandwidth to reduce noise. You can find the same thing with the Tektronix 7A13 100 MHz amplifier which has too much noise to make good use of its 1 mV/div sensitivity, so it includes a switchable 5 MHz bandwidth limit; a 20 MHz bandwidth limit would have left it too noisy.I think a modern design could have about 5 times lower noise than the AM502 using a pair of low noise audio JFETs from Linear Systems. It should be possible to retrofit an AM502, but I am not aware of anybody trying it.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 18, 2023, 08:26:26 pm
content: Quote from: David Hess on December 18, 2023, 03:37:56 pmCan one also create 1mV differential circuit at 10,000 Hz (instead of just 1000Hz) with only 1uV spot noise instead of 8uV spot noise. What components will you use at 1mV, 10000Hz with below 1 uV spot noise?

author: MathWizard
author_rank: Super Contributor
timestamp: December 18, 2023, 08:33:52 pm
content: Quote from: Mechatrommer on December 18, 2023, 04:18:30 amI bought some legit "low noise" parts, like BC550, and COS77/277 opamps ages ago, but so far didn't try any til I made a 100MHz JFET buffer probe. IDK if it's any better IRL, but in the sim, for it barely makes a difference what BJT's I used. But I'm not up to adding in noise sources yet.But I want to build something to see what those parts can do.

author: David Hess
author_rank: Super Contributor
timestamp: December 18, 2023, 10:28:26 pm
content: Quote from: Ogitek on December 18, 2023, 08:26:26 pmWell, let's do an not unduly complicated paper design using the best operational amplifiers available for this application. We want low noise from DC to 10 kHz, which also means low offset voltage drift, and low input bias current and low input current noise to support higher source resistance.A two operational amplifier differential input stage will provide the highest possible input impedance and common mode noise rejection, but double the noise. Current noise can be ignored for JFET inputs with the resistances we will calculate. Actual noise is a little bit higher because it rises below 10kHz, but not by a lot.OPA828 0.57uVrms to 10kHz 0.82uVrms 8.2nV/SqrtHz 4 kilohmsOPA140 0.72uVrms to 10kHz 0.69uVrms 6.9nV/SqrtHz 2.8 kilohmsADA4625 0.47uVrms to 10kHz 0.88uVrms 8.8nV/SqrtHz 4.6 kilohmsThe first number is the voltage noise from the amplifier, the second number is the maximum noise from the source to stay below 1uVrms noise, the third number is the associated noise density to stay below 1uVrms noise, and the last number is the source resistance which would produce that noise.So for each amplifier type, the last number shows the maximum source resistance which will keep the total input noise below 1uVrms, from DC to 10kHz. As the amplifier noise decreases, then maximum source resistance that can be tolerated increases.So noise below 1uVrms to 10kHz is very feasible, at least with low source resistances. If the bandwidth only extends to 1kHz, then higher source resistances are tolerable.Note that the source resistance applies to both inputs of each operational amplifier and the feedback networks add their own noise, so I am assuming that the feedback network has an arbitrarily low parallel resistance for arbitrarily low noise. In this case that may require that buffers be added to the output of each operational amplifier to drive the their own feedback networks, but hey, we spared no expense. For buffers I might use AD812 current feedback amplifiers, but 4 transistor diamond buffers or something more complicated could also be used. Adding the buffers may require some frequency compensation tweaks around the operational amplifiers to maintain stability.If the source resistances are significantly lower, then the JFET operational amplifiers can be replaced with bipolar input parts for lower noise. Or a low noise discrete JFET pair from Linear Systems could be used for lower noise at higher source resistances, but examine the AM502 schematics to see how complex this could be.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 19, 2023, 11:44:00 pm
content: Quote from: David Hess on December 18, 2023, 10:28:26 pmI used an impedance checker. My electrode and wire to skin impedance is about 10k to 20k. The test at 8uV noise was at 100 Ohm. If the source resistance increased from 100 ohm to say 15,000 ohm. How do you compute what the noise of 8uV would become? And how is noise related to the input resistance/impedance. The unit has input impedance of 10 megaohms (see last message for the full spec sheet). If you'd use the voltage divider concept. The 100 ohm vs 15,000 ohm would not have much significant against the 10 megaohm input impedance. But how about noise? Why is noise more affected by increase in the input impedance.Also stand alone EEG, ECG, EMG units have fixed frequency. In the UFI unit, it is adjustable from 20, 50, 100, 1000, 5000, 10000 Hz. What parts of the circuits need adjustments for these switches? And would they introduce more noises? Why. Thanks!

author: Someone
author_rank: Super Contributor
timestamp: December 20, 2023, 02:08:57 am
content: Quote from: Ogitek on December 19, 2023, 11:44:00 pmIts not going to get simpler than this:Quote from: Someone on December 18, 2023, 05:52:40 am

author: Ogitek
author_rank: Regular Contributor
timestamp: December 20, 2023, 04:45:34 am
content: Quote from: Someone on December 20, 2023, 02:08:57 amI read it. But it's not mentioned how to compute it.If 8uV noise is produced from 100 Ohm input resistance (input impedance).What will 15,000 Ohm input resistance produce? 15,000/100 = 150uV noise?

author: magic
author_rank: Super Contributor
timestamp: December 20, 2023, 07:57:39 am
content: Usethisfor Johnson noise. Use manufacturer's specs () for voltage noise and current noise of the amplifier, multiply current noise by source resistance for equivalent voltage noise contribution. RMS sum all three as instructed before. Vpp is ~7x more than Vrms.In absence of detailed specs from the manufacturer, if you know total noise in 100Hz bandwidth with 10kΩ source, going to 1000Hz with 10kΩ source should be a matter of multiplying by sqrt(1000/100) if noise is mostly "white". Johnson noise is white, and amplifier's noise hopefully is too over almost all its bandwidth (except very low frequencies, but you seem to be mostly concerned with varying the BW at the top end). Increasing source resistance typically increases total noise anywhere between sqrt(X) and X, where X is the ratio of increase (e.g. 15k/10k = 1.5). Decreasing source resistance decreases similarly, until you reach a limit imposed by voltage noise of the amplifier. In your case this appears to already happen near 10kΩ, so going lower is unlikely to make things better.And of course read the basics. You're getting nowhere without understanding what those words mean.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 20, 2023, 10:09:58 am
content: Quote from: magic on December 20, 2023, 07:57:39 amOk. So increasing source resistance really increase total noise anywhere. If the spot noise at 100 Ohm input impedance is 8uV (as actually measured by the manufacturer). The noise at 10,000 Ohm is really (10,000/100) = 100X and multiply it by 8uV = 800uV or 0.800mV or almost 1 mV! This can totally drown the 1mV signal!But something puzzling. In the unit (see pdf spec I shared before). Their known specs is 2 uV peak-to-peak, DC to 100 Hz with 10k Ohm source.Yet the real noise at 1000 Hz with 10k source impedance appears to be 800uV or 0.8mV (see first sentence), and it becomes 8uV at 100 Ohm only source impedance (instead of 10kOhm). Something is not right.Note projecting the original 2uV at 100 Hz in the spec sheet to 1000 Hz (with similar 10k Ohm input impedance as your described) would produce sqrt (1000)/sqrt 100= 31.6/10 = 3.16 times or noise of 2uV x 3.16 = 6.32 uV only. Yet the real noise appears to be 800uV in actual. What happened?Are the computations correct, magic?Ohs. David Hess. Just help build one for me, $300It's ok if no fancy knobs and only potentiometers and no dc offset adjustments. The latter is only moving the scale up 0V and not affecting the waveforms.

author: donlisms
author_rank: Frequent Contributor
timestamp: December 20, 2023, 06:45:56 pm
content: Yes. Yes you do measure audio or signal over a bandwidth.I don't think you are trying to understand the things everyone has taken the time to answer for you. That's not fair. If you are a student, you have to work to understand it for yourself. You're not doing that.

author: magic
author_rank: Super Contributor
timestamp: December 20, 2023, 07:03:47 pm
content: Quote from: Ogitek on December 20, 2023, 10:09:58 amNo, the only real 1kHz measurement that you have is 8μVpp (with 100Ω impedance). Which is close enough to 6.3μVpp, I guess.Again, it appears that anything less than 10kΩ isn't making much difference for this device, so extrapolating from a few μV at 100Ω to hundreds μV at 10kΩ is invalid. Most likely 100Ω is just as bad as 10kΩ, which is just as good as 100Ω, if that makes sense.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 20, 2023, 11:24:45 pm
content: Quote from: magic on December 20, 2023, 07:03:47 pmOk, so you are saying the amplifier has noises with values akin to 10k Ohm Johnson noises so anywhere below 10k Ohm won't improve it. Yes, I'm taking the ideas of donlisms into account where you have to measure over a bandwidth.Whatever, the device official noise spec is 2 uV peak-to-peak, DC to 100 Hz with 10k Ohm source. I have a Brainmaster EEG and also an EEG signal generator in my AI with emotions lab I'm setting up. I set the generator to 10 microVolt and it is what Brainmaster is displaying in the software. Now if the noise is say 2 microVolt. How much would it make the 10 uV waveform wiggle? What is the rule of the percentage of noises before it is negligible. 2uV noise/10uV signal is 20% Is it bad?Also when you mentioned "if noise is mostly "white". Johnson noise is white, and amplifier's noise hopefully is too over almost all its bandwidth (except very low frequencies)". Did you mean the "very low frequencies" as 100 Hz? and when you said "except". Did you mean that at DC to 100 Hz with 2uV noise and 10K ohm source. If I lower the source impedance to 1k, the noise would go down from 2uV to nV? Something that doesn't occur for higher frequencies?

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2023, 07:48:06 am
content: Quote from: Ogitek on December 19, 2023, 11:44:00 pmThe increase in resistance, 14,900 ohms in your example, creates two new sources of noise, the Johnson noise of the resistance and added voltage noise from the input current noise multiplied by the resistance. In the example I gave, the voltage noise from the input current noise through the resistance can be ignored because the current noise is so low, which should always be the case with a 1 or 10 megohm input. The Johnson noise dominates in this case.That leaves the Johnson noise which is equal to 0.13*Sqrt(resistance*bandwidth) nVrms, which would be added to the 8uVrms. Over 10kHz, that will be 1600nVrms or 1.6uVrms, and 8uVrms+1.6uVrms=8.16uVrms.QuoteThe input impedance of the amplifier does not really matter, except insofar as an amplifier with a high input impedance must have a low input current, which requires using parts and a design which has higher input voltage noise. When modeled, the input impedance is a resistance across the input. This resistor contributes its own noise, however since the source impedance is across the input also, it effectively shorts that resistance and noise out.QuoteThey probably use a mechanically switched lowpass filter for that, which requires switching various combinations of resistance and capacitance into the circuit after amplification. All resistances and active elements add noise, however this can be ignored because the noise level in the circuit after amplification is much higher so later contributions are negligible.

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2023, 08:00:18 am
content: Quote from: Ogitek on December 20, 2023, 10:09:58 amI think you would be better off finding a working Tektronix AM502 amplifier. I do not think I could do justice to a project like you require for a reasonable price. The best I could do is to test and sell you a Tektronix AM502 and TM500 power supply mainframe, but I could not replace mine for that price right now. AM502s do not show up at bargain prices on Ebay very often. There isthis one currentlywhich is exactly what you need, but who knows if it is working, and it has some physical damage.As an aside, several years ago I spent more than a year looking for an AM502 on Ebay and by the time I got one, I had received two or three 7A22s as part of 7000 plug-in lots. The 7A22 is the same thing as an AM502 but lacks an external output for use with other oscilloscopes. The AM502 that I got just needed to have its switches cleaned and lubricated.

author: Ogitek
author_rank: Regular Contributor
timestamp: December 21, 2023, 10:36:06 am
content: Quote from: David Hess on December 21, 2023, 08:00:18 amSome Bioamplifiers have Isolators built in. The spec like:Maximum isolation voltage 700 volts, ACTypical leakage current <5 uA at 240 VACIf you would just use a battery like 12 Volts on Bioamplifier without isolators. It can't shock or electrocute you when the electrodes are all over your body. Isn't it. So I wonder what is the advantage of models with Isolators. Could they help with noises?Since my oscilloscope has 10mV vertical scale. And since I owe one of the most noise free ADC around (the E1DA Cosmos ADC). I asked if I could use REV software to check for the microVolt noises. They said it was possible (someone shared the image below). Have you got any experiences on this? The ADC may have some lingering noises. But at least you can see the signals than using oscilloscope with 10mV resolution. I want to check noises of some cheap $20 modules I got.

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2023, 03:01:03 pm
content: Quote from: Ogitek on December 21, 2023, 10:36:06 amMedical stuff uses galvanic isolation for safety, but there can be some noise advantages, and it makes connecting different pieces of equipment together without ground loops and common mode problems easier.QuoteThe usual method is to make a standard deviation calculation with the input shorted, or connected to a suitable resistance, which returns the RMS noise.