White Noise from LDO - Page 1
Forum section: electronics / beginners

author: hgg
author_rank: Frequent Contributor
timestamp: April 10, 2015, 02:12:31 pm
content: Hi everyone,I am working on a small microphone preamp for my PC which can be powered either from a batteryor from a USB port. When it is powered from the battery everything is ok and the main source of noiseif the electret itself. As you can imagine the USB power line is not the cleanest... so I am trying tofilter it. I don't know the best way to do it, but I've used a Pi filter, a common mode choke and finallya notch filter for a specific frequency.Below are the results from each filter in succession. (The 5V are AC coupled with a 100nF capacitorand fed to the mic in of the PC. On the PC no mic gain and 50% levels .)5V USB, no filtering5V USB and Pi filter5V USB, Pi filter & Common Mode Choke5V USB, Pi filter, Common Mode Choke and 12.7kHz Notch filterAt this point the 5V rail is much better and you can barely hear anythingeven if you stick your ear to the HiFi speaker.Now when I use an LDO to lower and regulate the voltage to 3V,its output adds approximately 15db of white noise! LDO is the LP29050ACZand it is bypassed with a 100nF capacitor. Here is the result :You can hear it as a slight quiet hissing sound.https://soundcloud.com/hgg2k/5v-usb-noise-pi-filter2-3v-ldoIts not loud but this will get amplified through the mic preamp.Is this a usual behaviour of an LDO?Is there a better way to clean the 5V USB?Thanks!George.

author: mij59
author_rank: Frequent Contributor
timestamp: April 10, 2015, 03:34:06 pm
content: The LP2950 may need bigger input and output capacitors.Check chapter 9.2.1.2.1 of the data sheet.

author: Howardlong
author_rank: Super Contributor
timestamp: April 10, 2015, 04:07:48 pm
content: I had some great advice about using LDOs a few years ago from a sage in RF, and that was to be wary of noise from LDOs. For digital circuits it's not usually much of a problem, but for RF and audio it is.I use the TPS79901 series extensively as a result, but there are plenty of others available these days. Look for "high PSRR", often this also indicates that the LDO's output noise will be pretty low too. They often have bypass pins on them for an external capacitor to bypass their internal voltage references, reducing noise.

author: hgg
author_rank: Frequent Contributor
timestamp: April 10, 2015, 04:49:57 pm
content: mij59you are right. The 100nF cap was not enough. With a smaller value capacitor the LDO oscillatedpretty badly. With the 100nF the oscillation stopped but you could not see any hint of the white noiseon the oscilloscope! You had to use a spectrum analyser or listen to the audio signal...Its amazing how the human ear can detect such subtle changes.Below is an animated photo that shows the drop in the signal noise as I increase the capacitor size.With 330uF the signal was back to normal. (At 820uF the oscillation started again.)Howardlongthank you very much for your advice, which I will of course follow it.Instead of solving a problem I prefer not to create it in the first place. I will search for a better LDO.Thank you very much!George.(I had a look at Dave's eevblog audio stream and his noise floor is excellent. Either he is usingnoise suppression at post processing or an excellent microphone and preamp.)

author: Zero999
author_rank: Super Contributor
timestamp: April 10, 2015, 10:05:34 pm
content: Using a lower supply voltage can cause more problems with noise. Another thing to bear in mind is the electret microphone: are you sure it'll work reliably at 3V?

author: Unknown
author_rank: Unknown
timestamp: April 11, 2015, 03:54:20 am
content: Quote from: hgg on April 10, 2015, 04:49:57 pmNot surprising. From the pics, you're looking at the difference between ~-90dB & ~-110dB down; that's into the noise / below the resolution of what most scopes (analog or digital) have any chance of showing you.It's a case of knowing your tools & what they're capable of...

author: hgg
author_rank: Frequent Contributor
timestamp: April 11, 2015, 06:26:38 am
content: Hero999As far as I know most electrets work fine with 3V and even lower 1.5V to 2.0V for most ofthem. 3V for the Primo EM172. I was not going to power the microphone though from the 3V.Instead I was thinking of using fixed regulated upper and middle voltages for the op-amp in placeof the resistor divider biasing, but after hearing the noise injected by the LDO, it doesn'tseem like a good idea...Also the noise spectrum graphs were from the regulated and filtered 5V USB power alone.AC coupled with a 100nF capacitor and fed to the PC's microphone input.No microphone and no amplification.Tac Eht Xilefso it looks like that the best instrument for testing the 20hz-20khz band is our ear.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 11, 2015, 09:41:01 am
content: What does your circuit look like? Is the noise itself (on the 5V supply) consistent or does it vary e.g. with hard drive activity and everything?The common mode filter seemingly produced a way stronger effect in the >10kHz range than the pi filter (which doesn't appear to do anything at all?), but I have no idea if that's due to the source (statistically insignificant) or not. Also, common mode would suggest that you have bigger problems with layout or grounding; which may not even be "bad", it's just that -100dB is a very sensitive baseline to measure against, and every little bit counts!Tim

author: hgg
author_rank: Frequent Contributor
timestamp: April 11, 2015, 01:29:46 pm
content: T3sl4co1lthis is the actual circuit of the preamp:and this is the filter that gets power from USB and gives power to the preamp:The common mode choke has a value of 2x13mH and does a good job especially on the highfrequency noise.When I use the USB from the LCD screen hub, power is more stable and does not changewith HDD activity, but when I use the PC's USB directly, its worse and has some variations as welldepending on activity.I think that there is a ground loop problem. I've also ordered an isolated DC-DC converter blockthat might help. 5V USB --> Common Mode Choke --> DC-DC converter --> Filter --> Regulator.

author: radix
author_rank: Contributor
timestamp: April 11, 2015, 04:56:20 pm
content: Well, if the PSU schematic is correct, then C8 is your problem. It passes only AC. Add an LC filter instead.

author: hgg
author_rank: Frequent Contributor
timestamp: April 11, 2015, 05:04:31 pm
content: The reason is that I am testing the noise of 5V USB power line, before and after the filter.I am only interested on the ripple on the 5V rail. When I find a filtering solution that worksreasonably well I will remove C8 of course. (I cannot connect the 5V rail directly to thePC's microphone input. I connect the output from C8 to mic in, record 30 seconds and lookat the result with a software spectrum analyser.)

author: Zero999
author_rank: Super Contributor
timestamp: April 11, 2015, 08:13:09 pm
content: Using components outside their specifications doesn't help much:The LM2940T-5 is only specified down to 6V and won't so much when run off 5V, other than just draw a high quiescent current to saturating the output transistor, dropping between 100mV to 1V, depending on the temperature and current.The TL072 is only specified down to a total supply voltage of 7V (+/-3V)The N5532 is not specified below 6V (+/-3V)Eliminate the LM2940T-5 and use an op-amp which is properly specified down to 5V such as the TLC2202 or MCP602.

author: hgg
author_rank: Frequent Contributor
timestamp: April 11, 2015, 08:16:42 pm
content: Sorry. The regulator is the LP2950 with a 3V output.I could not find something similar when making the schematic...)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 11, 2015, 09:13:34 pm
content: What does the connection to the USB connector (including shield) look like?You should also have a pull-down on the regulator output to perform the noise test, otherwise you are measuring a potentially floating output.3V isn't nearly enough to run a TL072, what are you using? If you also aren't using exactly what's on the schematic, please be meticulous and mention every difference!One way to break the ground loop would be, to add a small resistor (perhaps 10-100 ohms) in series with the output connector ground. This will drop some voltage when a load is connected (for example, if this circuit were used to drive headphones instead of driving another audio input), but you can compensate for this, and for the difference in ground loop voltage, by ground-referencing the mic input and op-amp feedback nodes to this ground, rather than power ground.Tim

author: hgg
author_rank: Frequent Contributor
timestamp: April 11, 2015, 09:28:46 pm
content: Sorry for the confusion. In the schematic of the preamp I am using the LM4562 op-amp but I amtrying to filter the noise of the USB first. The preamp is not even connected and I will not power itof course from 3V.I get the 5V USB power line from a USB to 3.5" DC cable (no shield), apply the filter on that and thenafter I AC couple the output signal with a 100nF capacitor (C8) I pass it to the microphone input of mysoundcard. So actually I am recording only the ripple and noise of the 5V USB power line.I was thinking of using the 3V from the regulator as the middle biasing point on the opamp insteadof the usual resistor divider, but after I used it, I realised that LDOs are not the quietest of components...It introduced white noise in the whole audio frequency range. The two 330uF capacitors corrected that,but not completely.

author: Zero999
author_rank: Super Contributor
timestamp: April 11, 2015, 09:56:53 pm
content: Quote from: hgg on April 11, 2015, 09:28:46 pmThat's better, the LM4562 is specified down to a total supply voltage of 5V (+/-2.5V)A voltage regulator will be more noisy than a potential divider. If you're worried about the thermal noise from the resistors in the divider, then bypass one of them to the supply, rail via a capacitor which is large enough to make the cut-off well below the audio band.For example if the resistors in the potential divider are 1k, the output impedance will be 500R, so for a cut-off of 10Hz:C = 1/(2pi*Fc*R) = 1/(2pi*10*500) = 31.8*10-6FIn practise a 47µF capacitor would do.

author: hgg
author_rank: Frequent Contributor
timestamp: April 11, 2015, 10:12:36 pm
content: I will try that as well, although the main source of noise is the USB 5V rail which I am tryingto filter.Do you think that using an isolated DC-DC converter to pass the 5V through it will make itmore quite or that the switching inside the DC-DC converter will produce more noise again?

author: Zero999
author_rank: Super Contributor
timestamp: April 11, 2015, 10:43:50 pm
content: I think the switching inside the DC-DC converter will make it more noisy.

author: hgg
author_rank: Frequent Contributor
timestamp: April 11, 2015, 11:57:04 pm
content: ...it seems that cleaning the USB power line is not so easy after all...

author: David_AVD
author_rank: Super Contributor
timestamp: April 12, 2015, 04:44:53 am
content: Why not use an R/C filter (resistor in series and cap to ground) to derive the mic supply? Maybe try 1K and 100uF to start with.

author: hgg
author_rank: Frequent Contributor
timestamp: April 12, 2015, 09:04:12 am
content: I've just tried it. It had no effect.Is there any standard guide or reference on filtering such kind of noisy lines as the 5V USB?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 12, 2015, 09:41:34 am
content: Then the noise isn't from +5V, it's from USB_GND.Tim

author: hgg
author_rank: Frequent Contributor
timestamp: April 12, 2015, 10:07:14 am
content: Quote from: T3sl4co1l on April 12, 2015, 09:41:34 amCan you please explain the reasoning behind that conclusion?(The USB ground is the same as the mains ground and I can measure 33.5mVDC potentialfrom mains earth wall socket nearest to the PC)Thank you.

author: David_AVD
author_rank: Super Contributor
timestamp: April 12, 2015, 10:31:28 am
content: Sounds like the layout needs improving then?

author: Zero999
author_rank: Super Contributor
timestamp: April 12, 2015, 10:34:19 am
content: Try filtering both the positive and negative rails to the microphone and use a differential amplifier?

author: hgg
author_rank: Frequent Contributor
timestamp: April 12, 2015, 11:41:25 am
content: Hmmm, sounds like a good idea. I will try it.

author: Rupunzell
author_rank: Frequent Contributor
timestamp: April 12, 2015, 04:42:27 pm
content: Put a current probe on the USB ground wire and conductors that power the circuit.BerniceQuote from: T3sl4co1l on April 12, 2015, 09:41:34 am

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 12, 2015, 09:11:09 pm
content: Quote from: hgg on April 12, 2015, 10:07:14 am"GND" is just a wire, it can have any potential on it. Just because you measure a DC potential (a frequency of 0Hz, obviously not the frequency that is interfering with your circuit!) near zero, doesn't mean it's "ground" in some cosmic absolute sense! Indeed, you seem to have proven that it is not: 33mV is hardly zero, even as DC goes!Indeed, it's most likely picking up voltage drops across internal circuitry, before leaving the computer via the USB connector. All that switching noise and stuff then loops back through your circuit, and you've got noise.Tim

author: hgg
author_rank: Frequent Contributor
timestamp: April 13, 2015, 07:47:40 am
content: I see. So there is definitely a ground loop problem.What are the best solutions for that?Will an isolated DC-DC converter solve the problem ?http://www.mouser.com/ds/2/281/kdc_meu1-267600.pdfIt might introduce some switching noise but maybe that will be more easily filtered(?)Thank you.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 13, 2015, 08:00:56 am
content: I posted this earlier:Quote from: T3sl4co1l on April 11, 2015, 09:13:34 pmTim

author: hgg
author_rank: Frequent Contributor
timestamp: April 13, 2015, 08:04:34 am
content: You are right, ok I will try that.Thanks again!

author: hgg
author_rank: Frequent Contributor
timestamp: April 24, 2015, 08:52:58 am
content: It seems that there is a solution to the USB power noise problem.iUSBPowerhttp://ifi-audio.com/portfolio-view/micro-iusbpower/A 9V battery has 0.6uV noise and they claim 0.1uV !I have not succeeded to lower USB noise any further and may be this is why... :"To create a USB power supply that is ultra-quiet is an engineering feat in its own right."Below is the PCB, nicely labeled:All PCBs should be labeled like that I think. Neat.I don't see how they solve the ground isolation they claim.There is a switch for the ground management.Maybe they are swapping the ground from the USB to the DC regulated 9V ground (?)I think they are cheating and have not actually managed to filter the 5V USB power,since they are using an external regulated clean 9V power supply. Maybe they areusing the 9V to power only the "Ultra Low Noise Super Regulator" chip.Anyway, nice design but there is a small problem. It costs $230...I think I will stick with the 9V battery.0.6uV of noise, for free.George.

author: wraper
author_rank: Unknown
timestamp: April 24, 2015, 09:21:38 am
content: Quote from: hgg on April 24, 2015, 08:52:58 amProblems I see on this pcb are bullshit claims on the silkscreen and marketing wank. Also they made "impedance matched high speed data path"... on what seems to be 2 layer PCB considering track width.Also:QuoteHell yeah,BULLSHIT. And what with USB signal integrity may I ask?

author: hgg
author_rank: Frequent Contributor
timestamp: April 24, 2015, 09:33:54 am
content: I also thought that something was not exactly right...

author: wraper
author_rank: Unknown
timestamp: April 24, 2015, 09:35:59 am
content: They do have even more ridiculous product in it's claimshttp://ifi-audio.com/portfolio-view/accessory-ipurifier/QuoteLike if there is something else other than USB data packets in the USB cable.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 24, 2015, 12:21:24 pm
content: Looks to be audiophool wank... pay it no mind.Nothing wrong with the impedance matched traces, though I have to wonder: are they too close together? USB spec includes both common mode and differential impedances.Tim