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Old 9th January 2013, 11:55 AM   #21
qusp is offline qusp  Australia
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I think it really depends on the application and layout, just throwing low ESR caps at the reg with values chosen indiscriminately may risk oscillation, but it is far from a general rule. I have 4-5 builds where the LM317/337 are used very successfully with low ESR caps, with measurements to back it up. all have ceramic bypass + polymer caps and not a sniff of oscillation/resonance.

these are all projects by Owen (opc) and extensive measurements figure very highly in the development and all are taken with high dollar measurement gear (AP2 + more) and a skilled technician. care must be taken with layout, but these are all well executed SMD multilayer boards.

results of the projects they power are some of the best results ive seen here on the forum, several of them test the abilities of the AP2 analogue input/ADC
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Old 9th January 2013, 12:23 PM   #22
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Hi qusp, it won't necessarily cause oscillation, but it may cause nastyness in the output impedance of the reg. Attached is a sim of Hengy's circuit. I put an arbitrary esr of 0.1 ohms for the 1uF cap (probably a bit low for an electro, but quite likely high for a 1uF ceramic!).

The impedance peak is around 27 ohms at 30Khz!! It is only a sim, but unfortunately I don't have the knowledge (or probably the equipment) to do output impedance measurements on the real thing

I had to go to around 100uF output cap and 0.7 ohms before I could get rid of all traces of the resonance...

edit: of course adding some series resistance via a small value resistor is another option and is what I did with my PS with low esr electros utilized.

Tony.
Attached Images
File Type: png output_impedance.png (32.5 KB, 173 views)
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Last edited by wintermute; 9th January 2013 at 12:28 PM. Reason: add comment about adding series resistance.
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Old 9th January 2013, 01:35 PM   #23
qusp is offline qusp  Australia
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I know what you mean, but like I said, as a blanket statement its incorrect, but like adding bypass caps to anything it needs care in application.

the builds of opcs were absolutely put through the ringer at RIM on AP2, APxwhere he works (measured in reality) and no aberrations of any kind. but he will have modeled the layout, modeled the circuit and then prototyped different values.

i'll see if I can dig up the independant measurements of the PSU on its own, but here is the SE-SE version of the headphone amp being powered by it.

the schematic includes ceramic bypass + 330f polymer, then of course more ceramics and polymers of the same types in parallel to that at the load end

I think you'll agree, there is nothing of concern there....

and its the same on the LPUHP amp (see below), which is literally the cleanest 15-20W i've ever seen, which is packed with ceramic on the reg output (reference bypass) plus as there is 8 buffers in parallel per channel, it has 8 x 10uf ceramics in parallel per rail on the buffers and 3 x 1uf per rail on the instrumentation FE/VAS... all ceramics, so that would be pretty low ESR

same with his big poweramp which uses 317/337 for the lme49830 input. its all about the application
Attached Images
File Type: jpg FFT.jpg (121.5 KB, 177 views)
File Type: jpg SNR.jpg (79.4 KB, 173 views)
File Type: jpg THD distortion product ratio.jpg (95.5 KB, 170 views)
File Type: jpg THD ratio vs F 2.jpg (105.0 KB, 163 views)
File Type: jpg THD ratio vs F 3.jpg (103.6 KB, 15 views)
File Type: jpg THD ratio vs F.jpg (107.2 KB, 16 views)
File Type: jpg THD vs power.jpg (120.9 KB, 15 views)
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Old 9th January 2013, 01:38 PM   #24
qusp is offline qusp  Australia
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I particularly like the relative level measurements on the headphone amp =)

and this quote

Quote:
Originally Posted by opc
To put it into perspective, the highest harmonic here would produce about 3.12 femto-watts into a 32 ohm headphone.

Last edited by qusp; 9th January 2013 at 01:41 PM.
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Old 9th January 2013, 01:53 PM   #25
Hengy is offline Hengy  Canada
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SY, wintermute,

I have tried the circuit in in about 4 different places (10s of km away), and I always turn off and unplug as many things as possible. The first time I made the circuit, I noticed that my wireless keyboard would induce 50mVpp noise, and my cell phone would induce 200+mVpp noise when placed close!

The spikes you see in my pics are consistent between each location, so it would be logical to say that it is the smps that is making the noise, as it is common in every location.

I realize that I will have to shield my enclosure against EM and RF noise, as well as pay attention to the grounding when I finally put the circuit on perfboard.

I think that the noise that I am hearing in my headphones is caused by the smps, so I will try that first.


I might see if i can source a battery and take some measurements. That should give me a good idea of if it is in fact the smps or not creating the noise.

Thanks for your suggestions and ideas!

Hengy
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Old 9th January 2013, 03:34 PM   #26
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The LM317 bandwidth is too low to do anything about the switching noise from the SMPS. Use 10uH on the input to the regulator. You can also deploy the circuit from Wenzel Associates "Finesse Voltage Regulator Noise" -- it works -- but adds a bit of complexity.

LM317's aren't fond of low ESR caps on the output -- use 100uF aluminum electrolytic. This isn't a low dropout regulator where the output cap is more critical, but a low ESR tantalum will cause an impedance peak and if the Q is high enough it will oscillate.
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Old 9th January 2013, 09:18 PM   #27
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Hi qusp, not questioning the results of the connected equipment (and I am a complete novice compared to opc!!), but the effect is real It may not of course cause any problems with particular circuits.

I think that this paper http://www.calex.com/pdf/3power_impedance.pdf is a good one to talk about the effects.

What I found with my sims was that with the LM317, the bigger the output cap the lower the series resistance required to dampen the resonance. I ended up going with 1000uF and 0.3 ohms added series resistance. This resulted in the (simmed) lowest output impedance up to quite high frequencies (well outside the audio band).

As I understand it having a higher output impedance at certain frequencies will result in more modulation of the PS at those said frequencies. If the PSRR of the circuit being driven is good enough this will probably not matter. However I like to remove the possibility in the first place

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Old 9th January 2013, 10:26 PM   #28
gmorita is offline gmorita  United States
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Default Simple RLC PS filter

An RLC filter at the output of the power adapter can reduce the fundamental switching noise and its harmonics to a fairly low level. The attached PSpice schematic and simulation result show a two section RLC filter designed to attenuated switching noise above 50KHz.

L1, C1, L2, and C3 comprise the main filter components.
C2, R3, C4, and R7 are chosen to damp the filter response to minimize peaking at the resonant frequency.
R2, R4, R5, R6, and R8 represent the parasitic ESR of the capacitors and inductors
C5 is non critical and was chosen to present the AC adapter with a low AC impedance as a load.
R1 is meant to represent the DC load on the power supply.

C2 and C5 should be high quality, low ESR aluminum electrolytics
C1 should be a low ESR solid polymer aluminum or tantalum capacitor. Alternatively, C1 can be five 10uF ceramic capacitors in parallel.
C3 and C4 should be film or ceramic capacitors.

If ceramic capacitors are used, they must have a voltage rating of at least 2X the operating voltage and must be X5R or X7R dielectric. DO NOT use cheap Z5U or Y5V capacitors as the actual capacitance under bias can be as low as 15% of the marked value. Murata has a nice on-line tool to compare the voltage vs capacitance curves of its capacitors.

L1 should have a current rating of 2X the expected load current and can be a toroidal inductor. L2 can be a ferrite bead with an impedance of 33 ohms @ 10MHz or 330 ohms @ 100MHz and rated for 2X the load current.

Assuming that the switching frequency of the AC adapter is 50KHz, the fundamental switching noise will be attenuated by about 60dB or a factor of 1000.

Another factor that's not been addressed is the possibility that at least some of the noise you are seeing is common mode. Have you connected your scope probe and ground clip to the same point on the "power supply"? You may be surprised to see many 10's of millivolts of high frequency noise. Most cheap AC power adapters do very little to eliminate the HF common mode noise. About all you ever see is the common mode choke that looks like a blob on the DC power cord.

Good luck!
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File Type: jpg RLC Filter.JPG (63.4 KB, 85 views)
File Type: jpg RLC Filter_Response.JPG (137.4 KB, 80 views)
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Old 10th January 2013, 12:02 AM   #29
qusp is offline qusp  Australia
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OK Wintermute: yeah neither am I denying the effect (and I never did), ive known the effect you are talking about, until recently LDOs were effected by it quite a bit and because they often rely on the capacitance for any dynamic loading it adds complication, care has to be taken. so I would call it a potential for effect, its there waiting in the dark for the unsuspecting.

I can understand the compulsion to get rid of any and all potential effects, but myself until the effect actually..has..an effect..on the circuit...i'm good . for circuits with lower PSRR, I will usually use another regulator rather than adding a pole and lowering the dynamic ability of a regulator already a bit lacking in dynamic ability.

BTW if the last image I posted looks a bit funky, bare in mind it starts THD+N vs a W so noise tends to dominate =) and by that I mean visually, because at the top of the mountain its still only 0.05% THD+N @ 10W

these regs get enough bad press as it is, they arent great regs but can be used to good effect and my god are they handy! the effect you mention is there, just be aware of that.
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Old 10th January 2013, 12:42 AM   #30
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Just to illustrate -- 47uF tantalum cap on the output will cause instability (dashed line of Zout is with 100uF aluminum electrolytic):

Click the image to open in full size.

and a 1uF polyester will cause even more instability:

Click the image to open in full size.

These aren't simulations.
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