Are fast transient response regulators worth using?

[Nemo]

I've got an analogue circuit with a gain of a few thousand. I note that
the 78M05 and 79M05 regulators providing the +/-5V rails have a couple
of mV ripple at a few kHz (the input signal being amplified, causing
increased drain in IC's down the line) and I'm concerned it will couple
into other, more sensitive parts of the circuit. I've tried LM317 /
LM337 regs but they don't seem any better, despite a 10uF decoupler on
their ref pins, perhaps because their ripple rejection is intended for
mains frequencies rather than kHz. I've noticed that there are some
expensive regulators touted as "fast transient response" around, do
people find these significantly reduce supply ripple, i.e. coupling
between IC's in analogue circuits?

 

[Joerg]

Nemo, are you sure this ripple ain't coming from somewhere other than
the regulators and is just leaking in? Sub-optimal ground structures
come to mind.

Regulators have an inductive output characteristic, which wants to
resonate with the output capacitor. That can cause some nasty noise
peaks in the 5-100 kHz range, depending on the size of the output
capacitor and the amount of current being drawn.

LDOs can really sing the blues and go into full-blown oscillation up
there. Many of those are like the princess on the pea.

A low-ohm resistor in series with the output will de-Q the resonance,
and can sometimes help a lot.

For sensitive circuitry you're often much better off using a capacitance
multiplier. Op amps have practically infinite CMR at low frequencies,
where the cap multiplier's vague idea of an output voltage occurs, but
are really horrible up in the tens to hundreds of kHz, where a good cap
multiplier is like a brick wall.

I'll second that. Plus cap multipliers are dirt cheap.

 

[Nemo]

I've got an analogue circuit with a gain of a few thousand. I note that
the 78M05 and 79M05 regulators providing the +/-5V rails have a couple
of mV ripple at a few kHz (the input signal being amplified, causing
increased drain in IC's down the line) and I'm concerned it will couple
into other, more sensitive parts of the circuit. I've tried LM317 /
LM337 regs but they don't seem any better, despite a 10uF decoupler on
their ref pins, perhaps because their ripple rejection is intended for
mains frequencies rather than kHz. I've noticed that there are some
expensive regulators touted as "fast transient response" around, do
people find these significantly reduce supply ripple, i.e. coupling
between IC's in analogue circuits?

Thank you for the replies.

Some further background: I've already split the circuit into 3 supply
zones, each with their own C-multiplier on the + and - rails. These
C-multipliers give out about +/-9V so there's plenty of headroom for the
regulators, though Miso's comments about DMOS being better than bipolar
for LDO's are interesting anyway. The main reason for using +/-5V rails
is simply that some of the ICs aren't specified as being able to take
the +/-9V coming out of the C-multipliers.

This mid section, with a gain of ~2000 isn't as critical as the front
end but I saw the ripple and thought it could be hiding a big problem.
Your feedback has helped me calm down a lot. Also, when I did the sums,
I realised the ripple simply corresponds to about 1.5 ohms impedance in
the supply lines. My next layout will have less impedance in the power
tracks (there's already a ground plane), and I will increase decoupling
after the regulators - there's only about 30uF there right now because,
mainly, of headroom problems but I'll reassess that.

I'll try a few ohms in the output of the regs, add filtering to each
critical IC's power, and relax. I thought I was seeing some subtle and
major gotcha, but your comments have helped me get this in perspective.

Cheers!

 

[Nemo]

I'm a little confused about what the problem is, but if you are trying
to reduce power supply noise, then I'd put the cap-multipliers after
the voltage regualtors. Use some of the LM317's(337) to make
(perhaps) +/- 6 volt supplies, and filter that with the cap-
mult's.

I hadn't considered it that way round. But you're right, the drop across
the C-multipliers is a pretty constant volt or so. That's a great idea.

Is the 'ripple' synchronous with the signal you are amplifying? Or is
it random noise? Or something else?

It's most noticeable when a 2.5kHz test oscillator on the board is used.
The ripple doesn't matter then, but I was concerned it was symptomatic
of possible coupling when real signals go through. I had problems on an
earlier iteration of the board where I derived a reference voltage from
a supply rail, rather than a dedicated 25 cent reference IC, and it
picked up a ~300kHz clock from an IC I've since designed out. In effect
I was mixing analog and digital supply rails. So, having seen it once, I
am a bit wary of echoes of the signal feeding back to an earlier stage
of the circuit and causing a self-reinforcing instability. That's why I
split the circuit into 3 sections with individually filtered rails.

 

[garyr]

I must the only person around who doesn't know what a cap multipler is. How
about a schematic?

 

[Joerg]

Did ya try google?
http://en.wikipedia.org/wiki/Capacitance_multiplier

Strong word of caution here: Do not use the opamp variant to feed a
voltage rail that has bypass capacitances on it. Opamps generally do not
like capacitive loads and might sing the blues. Or worst case ... phut
.... *POOF*

 

[Joerg]

Or else use higher beta transistors that let jack up the resistor and
use ceramics. Two pole multipliers are better.

Cheers

Phil Hobbs

And while at it, use low noise transistors with a low 1/f knee if that
matters. The BCX70K comes to mind.

 

[Joerg]

Hmm, I didn't see the opamp variation... What's the purpose of that?
I thought the cap multiplier was a bjt transistor circiut.

Well, in theory the opamp works. In reality, not so well if there's
capacitance on the output.

 

[[email protected]]

That's my favorite bipolar transistor, beta graded yet. 2N7002 is my
favorite mosfet; it will switch 50 volts in under 1 ns.

The 2N7002's threshold is too high for most of my uses. I've recently been
using the Diodes DMG1012T (cheaper and smaller than the IRLML2402). As far as
bipolars go, I rarely use anything other than the MMBT3904/6.

 

[[email protected]]

For 3.3 volt gate drive, gumdrop apps, we like FDV301, sot-23 mosfet.
It's more expensive than 2N7002, 5 cents instead of 2 cents.

The BCX70J is beta graded from 250 to 460 at 2 mA, which can be handy.


What do you pay for the DMG1012T? Its Rds_on is really low.

Under a nickel (I'd have to look it up to get more accurate). DigiKey has
them for $.052. It's a nice part. I started using it in an LED driver
current source.

 

[Joerg]

I must be slow today. But the opamp circuit makes no sense to me.
It's a simple buffered low pass but... "What's the purpose of R2?"
It's either working against the opamp output impedance to feed some of
the noisy input signal to the output. Or it's postive feedback
working against the output impedance of whatever is sourcing the input
voltage Vi. Or I'm missing something?

It is used to pull up the output. Most opamps can't get very close to
the positive rail so you have to "help it along". However, the circuit
isn't very good anyhow because no matter how small you make R2 there
will be a voltage drop. But the input voltage at IN+ is Vi, so it'll
never reach it. The only way this could work is if IN+ is drawn slightly
down from Vi, enough to allow the opamp to regulate.

(Let's forget any capacitance on the output for the time being.)

That's almost what a guy at an institute at my university said. Shortly
before a pop-pop-pop was heard, followed by profound cussing

 

[John Devereux]

On Feb 15, 10:16 pm, John Larkin

[...]

But no resistor from the input to output! :^)

I'll have to order some BCX70K's... No through hole that I could see.

George H.

Note that there's some ripple feed-through in the basic c-mult. That's
sort of an Early-voltage thing, worse because Vce is so low. I think
the ripple would be lower if Vce were allowed to go up some.

I really like the polymer electrolytic caps for low-noise power rail
stuff.

I found that Y5V and X7R capacitors are significantly microphonic when
used in precision "reference" circuits like your ones. So that 10uF
ceramic rings alarm bells for me. NPO were OK for smaller values, and
tants for larger ones. Never used the polymer electrolytics.

 

[Joerg]

John Larkin wrote:

[...]

and some sundry c-mult stuff

ftp://jjlarkin.lmi.net/C-mult_BCX70.JPG

I want that Fun Generator and the dunes

 

[Rich Grise]

John Larkin wrote:

[...]

and some sundry c-mult stuff

ftp://jjlarkin.lmi.net/C-mult_BCX70.JPG

I want that Fun Generator and the dunes

Make your own fun gen.

The red thingies on the right aren't dunes, they are our company logo.
That is an abstraction of Mt Tamalpias and the setting moon, as seen
from the Bay Bridge toll plaza.

And if you turn it 90 degrees, it's alien soft-core porn. ;-)

Cheers!
Rich

 

[m II]

"m II" wrote in message
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Hash: SHA1

Forgery reported to forgers provider. [email protected]

Please read the headers until this is resolved.


mike
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[m II]

"John Larkin" wrote in message

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John, why the FTP protocol on your pictures? Doesn't HTML work well when
the thing is located at a web page anyway?

It's an FTP site, not a web page.

And what's all that PGP SIGNED hash stuff about?


John


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I guess IE 8 has become so accommodating I didn't even realize there was an
ftp protocol happening. Not many people use that bsolete method of file
transport anymore.

The hash stuff is about security with this person forging my posts I need to
identify my posts securely so others do not confuse the two of us.

This person has annoyed me since 2004 and I have been tracking his posts and
capturing the stupidest statements this troll makes in order to get him away
fro the world. He won't beat me in an argument again. I hate being trolled
and taught him a lesson by posting this in 2005 and 2006 for the world to
see his stupidity.

mike
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[m II]

"m II" wrote in message
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It's an FTP site, not a web page.

And what's all that PGP SIGNED hash stuff about?

The troll Josepi has been using my 'm II' posting name and posting
racist garbage in a few groups. He also psost pictures og himself killing
babies and small women.
He follows me around and acts like a
nine year old. Hell, he probably IS a nine year old.

I've been using a cry each night as a means of protection
against the forgeries. Each signature matches only the message preceding
it. Tampering in the message body screws up the calculation done by the
algorithm. No key, no body cares. If people don't like it they can
killfilter me.

Until his internet providers get off their butts and do something with
the countless forwardings I've sent them, I may even tell my mother.
It's not fair for this guy to do the same thing to me that I did to him.

Know anyone who does networking user tracking as a hobby?

I have been tracking this guy since 2004 and recording everything he posts.

These are the posting headers of the perp: Please note he may use 1000 other
names and be you own sister.

Do not trust anybody.
Harassment only works one way in my world.

Now let's see if everybody is one my side!

===============================================



Reply-To: "m II" <X-Complaints-To: [email protected]>
From: "m II" <[email protected]>
Newsgroups: sci.electronics.design
References: <[email protected]>
<[email protected]> <[email protected]>
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<[email protected]>
<bb19a3db-4407-442e-a8a7-2a0e26058742@y31g2000prd.googlegroups.com>
<[email protected]>
<a11ae348-ce88-4556-bcee-0cfbf161cff3@a11g2000pro.googlegroups.com>
<[email protected]>
<[email protected]>
<[email protected]> <[email protected]>
In-Reply-To: <[email protected]>
Subject: Re: Are fast transient response regulators worth using?
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Message-ID: <[email protected]>
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NNTP-Posting-Date: Thu, 17 Feb 2011 03:37:22 UTC
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[m II]

"John Larkin" wrote in message

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The troll Josepi has been using my 'm II' posting name and posting
racist garbage in a few groups. He follows me around and acts like a
nine year old. Hell, he probably IS a nine year old.

I've been using PGP as an electronic signature as a means of protection
against the forgeries. Each signature matches only the message preceding
it. Tampering in the message body screws up the calculation done by the
algorithm. No key, no valid signature.

Nobody is going to check all that crypto stuff. Why not change your
nym?

John

---------------------

I have been using this nickname since the early 2000. I have a reputation
for tracking this troll and putting him in his place publicly for many years
now.

I am not about to give all that up.

Besides he could just find me again the first time I post my lists.


mike

 

[m II]

"m II" wrote in message
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Nobody is going to check all that crypto stuff. Why not change your
nym?

His anonymous remailer allows aliases. A new posting name would make no
difference.

The message IDs puts him at

http://www.hushmail.com/

Which has a British Columbia, Canada phone number.

I'll see what a call to them tomorrow brings. Canadian laws prohibit
causing harm by forgery.



mike





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Why are you forging my name jerkwad?

Why do you make up lies about me, dipshit?

Why do you accuse everybody that disagrees with you to be the same person,
POS?

Why do you act like the scum of the earth and then cry like a baby when
somebody copies your actions, asswipe?

Why do you try to act like a know-it-all and then act liek a complete
imbecile on most subject matters?

Why do you call people names for giving help online and then use that
information in other groups?

Why don't you grow up?

How long do your think you will get away with being a troll in dozens of
groups using many different aliases and newsreaders and providers and still
whine when you get caught in your disgusting actions?

How long do you think it will be before somebody comes to knock on your
door?

How long before your employment is in jeopardy for misuse of the
institutions computers for harassment?


Get used to it. You been practicing this crap for a long time and now it's
my turn, fucking crybaby.

http://groups.google.ca/groups/sear...group=&as_usubject=&as_uauthors=m+II&safe=off



mike

 

[Rich Grise]

Nobody is going to check all that crypto stuff. Why not change your
nym?

He's just starved for attention, and crowing about how somebody is
stalking him gives him a feeling of self-importance. I got tired of
his sniveling, so have filtered him.

Cheers!
Rich