# How to measure power supply noisefloor



## FOXY

How to measure power supply noisefloor with computer sound card? 

 Like this pic:


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## tomb

I don't think you're going to get there with this. It may output something, but I doubt that it's real. There's nothing readily available on the market that will measure ripple down into the microVolts, which is what we're interested in for audiophile power supplies.

 Here's a good start: Low Noise Measurement Preamplifier. We used Tangent's excellent design to tweak the power supply on the MiniMAX to really sterling performance:
Millett MiniMAX History


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## peranders

Foxy, why do you want to measure the noise from the PS?


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## FOXY

To build a very quiet power supply for DAC.

 I tried batteries, it's improved the sound very much compared to standard power supply with 7805.


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## peranders

A 7805 is the noisiest power source you can get. You can use a LM317 or some LDO regulator or perhaps my super regulator which produces 921 nV.


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## frex

Hello FOXY,

 If your are (like me) interested to PSU noise measurements, you can see the AN83
 Linear Technology App here : http://cds.linear.com/docs/Application%20Note/an83f.pdf
 This document is very useful.
 It also describes all the electronic materials nedded to do this measurement.
 I made it, and it works very fine !
 You can find other interesting documents on the Linar Web site.
 Good luck.

 Frex.


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## amb

Quote:


  Originally Posted by *peranders* /img/forum/go_quote.gif 
_A 7805 is the noisiest power source you can get. You can use a LM317 or some LDO regulator or perhaps my super regulator which produces 921 nV._

 

Actually, 7805 and LM317 are about the same ballpark with respect to noise.

 For example, compare Fairchild's LM7805 and LM317 datasheets, you'll see the LM7805 having 42μV/Vo (typ) noise, which, for a 5V regulator, translates to 210μV. The LM317 is rated at 0.003-0.01%/Vo (typ-max), which for 5V output, works out to 150-500μV. Thus the 7805 is right in that range, but note that the 7805's noise spec was rated between 10Hz-100KHz whereas the LM317's is only from 10Hz-10KHz. If the 7805 was measured with the narrower bandwidth, then I would expect its noise spec to be better, perhaps besting the LM317.

 There is also the LM7805A (in the same datasheet) which is a lower noise version, with 10μV/Vo, so for 5V it would be only 50μV. Pretty good for a cheap three-terminal regulator.

 Surprised? I was too, given the often-cited "conventional wisdom" that LM317 is "better" than the 78xx series. Now let's look at ripple rejection. LM7805 is 73dB. LM7805A is 68dB. LM317 is 60dB (or 75dB with the addition of the ripple reduction capacitor). These numbers don't demonstrate a clear superiority of LM317. One could only say that LM317 is worse, and only achieve roughly equivalent performance with added parts.

 National Semiconductor's LM340T-5.0 and LM340AT-5.0 (pin-compatible with 7805) is even better with noise of 40μV and ripple rejection of 80dB.

 There are some who consider the Linear Technology  LT1083/1084/1085 series LDO adjustable regulators to be superior substitutes for the LM317, but one look at the datasheet shows that the noise and ripple specs to be the same as the LM317.

 These numbers were no doubt taken in lab conditions with fancy shielding and other techniques to minimize external factors from influencing the results, so in reality we'd most likely see a bit more noise than in the spec, but still, the numbers speak for themselves and is a reason why I chose the 78xx series regulator instead of LM317 in my "cheap and simple" σ25 PSU. 78xx-based circuit has the advantage of lower parts count, too.

 The σ11/σ22 at 5V is about 20x quieter than this, but we're talking a whole different league of PSU there. In audio circuits with high PSRR, σ25 could be very cost-effective.


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## tangent

Quote:


  Originally Posted by *amb* /img/forum/go_quote.gif 
_Actually, 7805 and LM317 are about the same ballpark with respect to noise._

 

When I was designing the TREAD, I started with the idea of using a 7824, since the idea was that the TREAD was to be simpler and cheaper than the STEPS, which I still offered at the time. I didn't want the two to compete with each other, but when I saw that the noise was 4x higher despite having all the recommended bypassing, I decided to go with the LM317 after all. This ended up killing off most of the demand for the STEPS, but I still feel it was the right call.

 It could be that I happened to test an inferior 78xx sample, or perhaps there is some difference in manufacturer's renditions of the design to explain it. I don't think the 317 was exceptionally good, because I've not seen large variations in performance among them.

 Consider this, too: both the AD797 and the LT1028 claim 0.9 nV/rt.Hz noise in their datasheets, but try both in an LNMP and you'll see the base noise floor be higher with the LT1028. The datasheet doesn't tell all.


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## dsavitsk

Quote:


  Originally Posted by *amb* /img/forum/go_quote.gif 
_Actually, 7805 and LM317 are about the same ballpark with respect to noise._

 

Nice writeup, Amb -- any measurements to indicate how these things differ in terms of transient response?


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## cobaltmute

A thread about battery powering a DAC

DIYHiFi.org &bull; View topic - Battery Powered DACs?

 If you know your load draw, the zener + emitter follower is apparently good :

Simple Voltage Regulators Part 1: Noise - [English] (part way down the page)


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## amb

Quote:


  Originally Posted by *dsavitsk* /img/forum/go_quote.gif 
_Nice writeup, Amb -- any measurements to indicate how these things differ in terms of transient response?_

 

No, I did not measure transient response on any of the 3-terminal regulators. It's an exercise that I would be interested in doing some time down the line. I suspect that the output decoupling cap will have a very large influence on the outcome.


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## AudioCats

very interesting thread. I wish somebody have the noise data for the eneloop Nimh rechargeables. 

 I supposed since most opamps have very high PSRR, the power supply noise floor is not super critical for an opamp-based amplifier, correct?


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## luvdunhill

Quote:


  Originally Posted by *AudioCats* /img/forum/go_quote.gif 
_very interesting thread. I wish somebody have the noise data for the eneloop Nimh rechargeables_

 

I could measure it for you, but I can tell you that it's an order of magnitude higher than the numbers mentioned in this thread.... give me the parameters you are interested in and I'll measure it for you.


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## AudioCats

is it really that bad? noisier than even the LM317? 
	

	
	
		
		

		
		
	


	





 can you measure the noise level of eneloop vs standard Nimh from 500K~1.5Mhz? or maybe it doesn't really matter, as the intended use will be a noisy PWM switching (tripath amp) anyway.

 Thanks


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## frex

Hello,

 After this discussion and for my own interest, i have decided to make some noise measurements for differents voltage regulator.
 The three tested voltage regulators are :
 LM7815CT fixed 15V regulator (ON semi, old Motorola) in TO220 package.
 LM317 positive adustable regulator (STmicro) in TO220 package too.
 LT1764EQ positive adjustable "low noise" regulator (Linear Tech) in D2PAK package.

 All of those voltage regulators have been welded in a copper board with decoupling capacitors, as you can see in the schematic below:






 To make those measurements, i used an home made instrument (yes, DIY!).
 It's a low noise high gain amplifier with band-pass filtering (10Hz-100kHz ).
 The amplifier gain is 80dB (10000x) in frequency range, and it's own noise is only 500nVrms (-127dbVrms), with it's input shorted by a 50Ohms plug.
 (This amplifier is fully decribed in the AN83 from Linear Technology).
 Then, i used my external audio A to D converter for compute and display the noise spectrum.
 The ADC is calibrated, and the spectrum is directly displayed in dbV (0dbV = 1Vrms, -60dBV= 1mVrms.)
 The sampling rate of the ADC is 192kHz(so ~90kHz max allowable signal), it is good for a measurement bandwith of 10Hz-100kHz.

 Below, the brief description of the measurement procedure :






 The FFT software display the averaged noise spectrum and the RMS value.
 At verification purpose, the rms value computes by the sofware is always compare with the measurement done by the 9300F millivoltmeter.
 For finding the real noise of the regulator, now i must add 80dB at this value (or divide it by 10000).
 Measurements results for each regulator are grouped in the table below.





 Note:
 Column 2 and 3 are the same value but in dBV and mV.
 Same for column 4 and 5.
 Column 2/3 is the measured voltage at the output of the 80dB amplifier.
 Column 4/5 are the calculated equivalent output noise of the regulator (x10000 ratio).

 As we can see, the LT1764 specified for low noise operation is the worse!
 Without a good ripple rejection capacitor, the LM317 is not the best, but became best with a good filtering capacitor.
 The 7815CT is also good.
 All of those regulators have low output noise. It is unlikely that this level of noise become a problem with any audio analog designs.

 Nevertheless, for the hoobyist who want to design an ultra-low noise PSU (for supply ultra low phase noise oscillator for example),
 there are many active noise reduction circuits. This actives circuits allow to reduce the noise up to 40dB(100x).

 Below, a second DIY board where 2 voltage regulators are used. One LM317 for +15V and one LM337 for it's opposite of -15V.
 At left, you see each regulator, and at right the active noise filter section ..





 With this board, the output noise result is listed in the table below.





 The first result with noise reduction circuit "OFF" is roughly the same than the previous result (-7dBv).
 If the NR is active, the noise become very very low, 27dB lower !..

 The graph shows the noise spectrum between 1Hz to 100kHz for each regulator type and configuration
 This speaks for itself.






 Last year, i had bought a JSR06 "low noise DC supply board" from Per-Anders Sjöström (sjostromaudio.com - Home).
 It is a Pcb wich include positive and negative DC supply rail with very low output noise.
 For the fun , i have also make some measurements on it's output noise.

 The result is nearly the same than the LM317 with noise reduction.






 The resulting spectrum is illustrated in the graph below.
 In red, i have add the noise floor spectrum of the measurement system.
 It's level is down to 500nVrms reffered to input in the measurements bandwidth.







 I hope some of this measurements help to clarify some preconceived idea about noise issues.


 Frex.


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## amb

Nice work, Frex, I am curious how a Fairchild LM7815ACT and National Semiconductor LM340T-15 might compare.


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## KT88

Nice job Frex. Can you please link to a schematics of the NR section you've used?

 It does a great job of removing noise. Is it limiting the maximum output current/inrush current somehow?


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## frex

amb, i will try to find this regulators for test it.
 But, i think the result will be nearly the same.


 KT88, the schematic of noise reduction system used:





 More info about this circuit at Library
 I have simulated this circuit (before wiring) with Microsim PSpice, and it work fine. In my circuit board, the AOP used is a LT1115 because it have less noise and higher bandwith. The best results is done with the LT1028 or AD797.

 This circuit doesn't really affect the output current of the regulator.
 But the serial shunt (low value) can change the transient response.
 However, good transient response is reached primarily using local decoupling near each IC.


 Frex.


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## TimJo

Very interesting results Frex. Thanks for posting your investigations...


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## glt

Many thanks for the experiments and report. It would be interesting to see if this circuit indeed has nV-range of noise: Ultra-Low-Noise LDO Achieves 6nV/√Hz Noise Performance - Maxim


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## frex

glt,
 I know this application note.
 You can easily calculate the output of this circuit if it's voltage noise is ~6nV/sqrtHz. 6e-9 x sqrt(100kHz)=1.9µVrms. (3.3V)
 It is a good approximation (spectral noise density varies with frequency).
 This level of output noise is very good.
 Beware, here the voltage noise is given for a low output voltage (3.3V).
 The output noise is proportional to the closed loop gain.
 For a 15V operation, the output noise will be multiplied by the ratio !
 So you have now ~ 8.6µVrms (1.9µV x (15/3.3)).
 Sorry, i don't have those parts for test it. 
	

	
	
		
		

		
		
	


	





 Frex


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## digger945

The test load is 100 ohms and test current 150mA?(sorry if I got this wrong)

 Have you tried any tests with a switching load, like a transistor or mosfet, or an ampifier of some kind with a load connected to the output?

 Have you ever tried to parallel 3-pin regulators to see what happens?

 Just curious. I'm in the middle of testing some different kinds of power supplies to see how stout they are as a load is switched on and off at audio frequencies. Rather crude, I know, but I'm coming up short on information about power supply testing and evaluation.


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## tangent

Quote:


  Originally Posted by *frex* /img/forum/go_quote.gif 
_It's a low noise high gain amplifier with band-pass filtering (10Hz-100kHz ). The amplifier gain is 80dB (10000x) in frequency range, and it's own noise is only 500nVrms (-127dbVrms), with it's input shorted by a 50Ohms plug. (This amplifier is fully decribed in the AN83 from Linear Technology)._

 

The AN83 amplifier has 60dB gain. What did you change to get 80? If you increased that first feedback resistor by 10x, that puts a lot more Johnson noise in the first stage. If you added another stage, that adds noise, too. The best place to make a change would be to increase the gain of the final stage.

 Any other differences to report?

 Would you consider trying the AD797 for the first two stages instead of the LT1028? I found them to be quieter, despite so many datasheet similarities.

  Quote:


 The FFT software... 
 

What software is this? It looks like something I might be interested in getting.

  Quote:


 The 7815CT is also good. 
 

Interesting. I looked back at my 7824 test PCB, and it has some of the same layout problems as the Velleman K1823 I was trying to replace when designing the TREAD. So, I hereby withdraw my 4x-the-noise report. Clearly the test was compromised.

  Quote:


 I had bought a JSR06...The result is nearly the same than the LM317 with noise reduction. 
 

Interesting. What happens if you follow the JSR06 with the NR shunt?

  Quote:


 amb, i will try to find this regulators for test it.
 But, i think the result will be nearly the same. 
 

I'm not sure about that. I've found durability problems in the past with Fairchild regulators relative to NatSemi parts. The question AMB is posing is sensible: are there regulation differences, too?

 By the way, I think you partially tested this in reverse already, pitting an expensive Motorola 78xx against a cheap ST LM317. Is durability the only difference here, too?

 Another thing about your test that bothers me is those first and third-harmonic AC line hum bumps in your noise floor. Can you say for certain where they're coming from? If the computer is wall-powered, it could be some kind of ground loop between the computer PSU and the DUTs. I've taken to making noise tests with laptops running on battery lately to avoid this very thing.


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## frex

digger945,

 Yes the load is 100 Ohms for 150mA output current.
 as I already said, i don't have test the transient response.
 In my own opinion, it's not the main issue in audio PSU.
 First, because the regulator in audio appplication is rarely near the
 load. So, the transient response is greatly affected by the wiring and 
 pcb traces (serial inductance in the supply path). 
 It is therefore essential to add good decoupling capacitors the nearest
 possible to the supply pin of each IC to obtain a good transient response.
 Sometime, you can add also (with care) some ferrite beads to bypass
 some HF noise generated by high speed IC.

 Unlike noise testing, transient response measurement is easy.
 You need only a oscilloscope and a pulse generator.

 If you parrallelling several regulators, probably the output noise will be
 divided by the square root of the number of regulator.
 For 4 regulators in parallell, the noise will be reduce by Sqrt4=2.
 Is is only -6dB !
 I don't have done this test, i will try it if i have some time. Ok ? 
	

	
	
		
		

		
		
	


	




 I don't understand why you want to test the PSU with switching the load ?
 What is the final application of your PSU ?



 ---------

 tangent,

 Yes, i have add a +20dB stage after the band-pass filter.
 It use a LT1007 low noise AOP. Not other noticeable differences.
 I don't have tested with a AD797. Maybe (but not sure) it could be
 slightly improved noise performance. That said, the AD797 is more
 problematic and become unstable more easily than the LT1028.
 If you want i can post more info about my measurement tool.


 The FFT software is Spectralab 4.32.17.


 I don't have try to use the NR system at the output of the JSR06.
 I will test it if i have some time. 
 It is unlikely to change much. 
	

	
	
		
		

		
		
	


	





 I will search this week other voltage regulators with differents manufacturers.
 Their noise specifications will all be measured.


 In the noise floor of spectrum, the higher harmonic peak is less than -60dBV(50Hz).
 Reffered to the input the level is -140dBV. So corresponding to 100nVrms !
 Not a good unit ?
 Of course, it is probably possible to get better noise floor using a battery for supply the measurement amplifier. But here, the noise level is not a issue. You must know at this voltage level, many problems occurs. 
 Thermo-electric noise in this range ! Any connector and parasitics resistances add noise ! 

 A another think is here we measure only in limited bandwidth. 
 The real total noise is higher. It is the main reason because the noise is expressed in noise density. This value allow to calculate the real noise level you have in your bandwith interest.


 Frex.


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## frex

Quote:


 Another thing about your test that bothers me is those first and third-harmonic AC line hum bumps in your noise floor. Can you say for certain where they're coming from? If the computer is wall-powered, it could be some kind of ground loop between the computer PSU and the DUTs. I've taken to making noise tests with laptops running on battery lately to avoid this very thing. 
 


 I have try to supply the 80dB Amplifier with the JSR06 PSU. All spurious harmonics dissappear... But, the rms noise level is stricly the same. In fact, the rms power of these harmonics is negligeable.

 I agree, it is more "pleasing" to see a flat spectrum!

 Frex


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## digger945

Quote:


  Originally Posted by *frex* /img/forum/go_quote.gif 
_
 Unlike noise testing, transient response measurement is easy.
 You need only a oscilloscope and a pulse generator._

 

That's what I am using now to test with.
	

	
	
		
		

		
		
	


	




  Quote:


 I don't understand why you want to test the PSU with switching the load ?
 What is the final application of your PSU ? 
 

A shunt PS for a small audio amp, or used as a digital supply. I am swithing the output of different power supplies just to see how stiff the outputs are and how quickly they can recover. Right now I am comparing the Twisted Pair Audio Placid bipolar shunt against the AMB Sigma 22. The Sigma 22 is not a shunt type, and is much larger and capable of far more current than any shunt I have seen. Mainly I'm just trying to see how different stuff reacts to loads on the scope.
	

	
	
		
		

		
		
	


	






 I'm still waiting for the Tangent LNMP, although I have the stuff to build one on breadboard. Pcb would be much nicer and look finished.


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## amb

digger945, see this headwize post I made about the pulse response measurement and simulations I made of the σ22, 7815 and LM317.

 EDIT: And more here.


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## tangent

Quote:


  Originally Posted by *frex* /img/forum/go_quote.gif 
_I have try to supply the 80dB Amplifier with the JSR06 PSU. All spurious harmonics dissappear..._

 

I saw the same thing when doing RMAA testing on the PIMETA v2: obvious power line hum in the test results when powering it with a bench supply, which then disappeared when I switched to a YJPS.


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## digger945

Quote:


  Originally Posted by *amb* /img/forum/go_quote.gif 
_digger945, see this headwize post I made about the pulse response measurement and simulations I made of the σ22, 7815 and LM317.

 EDIT: And more here._

 

Thanks. Without even reading that I used an IRFZ24 in series with a 75ohm, and 75 on the gate(for each power supply I've tested).
	

	
	
		
		

		
		
	


	




 EDIT: btw, my tests show almost the same results, the difference probably in the load resistor.


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## grenert

A few more PS noise measurements for fun. They are all 5V supplies except the YJPS, which is 12V. These were done using a 10X amplifier (very much like this one - Buffer Hardware for xoscope, but using NE5532 fed by a dual supply from a TLE2426) into an Emu 0404USB. Nothing is calibrated, so the results only are useful for relative comparisons. I used Audiotester to capture the noise spectrum. The "oscilloscope" waveform is in the upper display. Soundcard settings were 16-bit, 96 kHz sampling.

 The linear regulated wallwart is from an old PCDP. I also tested the stock 0404USB switching supply. The LM317 is a basic datasheet design with the exception of using LEDs instead of a resistor to set voltage. The TREAD was modified with RC filtering before the regulator, and use of LEDs as above. The YJPS handily beats them all.

Gallery of noise spectrum images


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## frex

Hello,

 Because it is interesting and also because somebody ask me, i continue
 to make some regulators noise measurements.

 I have test the noise performance of :
 LM317 (Motorola)
 LT1085CT (LT)
 LM2940CT-12 LDO regulator (NationalSemi)

 The measurements methodology is same of my previous post.
 (Vout=15V unless otherwise specified Rload=100 Ohms)

 Results are (10Hz-100kHz bandwidth) :

 LM317 -- 55µVrms
 LM2940CT12 -- 190µVrms (Vout=12V)
 LT1085 -- 40µVms

 These results show that the LM2940 série (Low dropout regulator) are the worst !
 Finally for low budget, the best choice seem to be the LM317 with large decoupling capacitor in ref input.


 Because i'm curious and also to answer of somebody,
 i have tested the simple noise reduction circuit from Wenzel Associates app notes.
 You can read this app note here : Finesse Voltage Regulator Noise!
 The first circuit use only one NPN transistor, and of course if the noise reduction is effective,
 it can be a right way to supply some sensitive device, like oscillators.
 Because of the high serie impedance (15 ohms) in the supply path, only very low output current can be draw by the load (about 20-30mA).
 After many tests, with same values of the original schematics and with slight modification too, i don't gain very good noise reduction. Only about 10dB of noise attenuation (1/3).
 (Note: Iout=20mA, Vin=15V, Vo~15V).
 I have also tested the second schematic using 2N5192 power transistor.
 After differents settings, i obtain in the best case 10dB of attenuation.
 Wenzel say about 40dB of attenuation is possible, but i' can't get it.


 Since my last post, I also purchased a couple of SSR01/SSR02 PCB from Peter Sjöström. It is high performance serie regulators PCB.
 As you can think, when wiring was finished, i have start to make some noise test.
 The result is impressing !
 The output noise for 15V output voltage is about 1.5µVrms ! 
 (20V dc input, 150mA out, LT1115 AOP).
 Both SSR01 (positive) and SSR02 (negative) give the approximatively same results.
 I congratule P.Sjöström for this very compact and clean design !
 I will use it for build a /-15V 1A very low noise power supply.
 It will used to supply my new design,an high performance 24/192k ADC.

 Frex.


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## peranders

Quote:


  Originally Posted by *frex* /img/forum/go_quote.gif 
_The result is impressing !
 The output noise for 15V output voltage is about 1.5µVrms ! 
 (20V dc input, 150mA out, LT1115 AOP).
 Both SSR01 (positive) and SSR02 (negative) give the approximatively same results.
 I congratule P.Sjöström for this very compact and clean design !
 ._

 

Thanks for those kind words. You should compare jackinnj's results which indicate values down to 921 nV. More info here.

 Notice that I don't listen to "Peter".


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## digger945

Good show Frex, thanks for the impressive numbers. 
 I look forward to using the SSR01/2.
 I haven't had much time for any further testing myself.


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