7815 noise and measurement

Guest
We are looking at noise of a 7815 voltage regulator. Missing other instrumentation our first idea was to measure AC across the 15V with a Keithley 2700 DVM.

If we connect the minus of the regulator to Earth, measured AC voltage (noise?) goes down by a factor of 10. Why? 100nF is sufficient, same if fully earthed.

My first idea was that we don't know how the 2700 measures relativ to ground/Earth, so we put in an isolation transformer, but same result.

The voltage regulator is standard by data sheet, has some large aluminum electrolytes on input and output, bridge rectifier, print transformer from 230V, protective diodes.

Any ideas are appreciated.

Klaus
 
On Thursday, 4 July 2019 17:54:07 UTC-4, buec...@gmail.com wrote:
We are looking at noise of a 7815 voltage regulator. Missing other instrumentation our first idea was to measure AC across the 15V with a Keithley 2700 DVM.

If we connect the minus of the regulator to Earth, measured AC voltage (noise?) goes down by a factor of 10. Why? 100nF is sufficient, same if fully earthed.

My first idea was that we don't know how the 2700 measures relativ to ground/Earth, so we put in an isolation transformer, but same result.

The voltage regulator is standard by data sheet, has some large aluminum electrolytes on input and output, bridge rectifier, print transformer from 230V, protective diodes.

Any ideas are appreciated.

Klaus
Click to expand...

Noise of the 7815 is supposed to be 60uV or so.

AC CMRR of the 2700 is specified as 70dB.

So to get a 600uV reading on the meter you only need a few volts AC common mode voltage.


--Spehro Pefhany
 
buec...@gmail.com wrote:
We are looking at noise of a 7815 voltage regulator. Missing other instrumentation our first idea was to measure AC across the 15V with a Keithley 2700 DVM.

If we connect the minus of the regulator to Earth, measured AC voltage (noise?) goes down by a factor of 10.
Click to expand...

** You need to explain that one cos it makes no sense.


Why? 100nF is sufficient, same if fully earthed.

** The 100nF cap is for stability, not noise supression.

FSS read the flaming data sheet.


My first idea was that we don't know how the 2700 measures relativ to ground/Earth, so we put in an isolation transformer, but same result.
Click to expand...

** I take it you isolated the supply safety earth.


The voltage regulator is standard by data sheet, has some large aluminum electrolytes on input and output, bridge rectifier, print transformer from 230V, protective diodes.

Any ideas are appreciated.
Click to expand...

** Hi Klaus,

The broad band noise and hum from a 7815 reg is very small, less them 100uV rms.

The only way to measure it with accuracy is to use a pre-amp to boost the level to something a DMM can read.

I use a microphone pre-amp with gain of 60Db ( 1000 times )and a BW of 100kHz to do the job.

Of course you have to AC couple the reg to the pre-amp first.

As with any such testing, you must use a scope to monitor the signal in order to be sure you are measuring what you think you are measuring.


.... Phil
 
On Thu, 4 Jul 2019 14:54:03 -0700 (PDT), buecherk@gmail.com wrote:

We are looking at noise of a 7815 voltage regulator. Missing other instrumentation our first idea was to measure AC across the 15V with a Keithley 2700 DVM.

If we connect the minus of the regulator to Earth, measured AC voltage (noise?) goes down by a factor of 10. Why? 100nF is sufficient, same if fully earthed.

My first idea was that we don't know how the 2700 measures relativ to ground/Earth, so we put in an isolation transformer, but same result.

The voltage regulator is standard by data sheet, has some large aluminum electrolytes on input and output, bridge rectifier, print transformer from 230V, protective diodes.

Any ideas are appreciated.

Klaus
Click to expand...

I don't quite understand all that. Can you post a schematic?


--

John Larkin Highland Technology, Inc trk

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
On 07/04/19 22:54, buecherk@gmail.com wrote:
We are looking at noise of a 7815 voltage regulator. Missing other instrumentation our first idea was to measure AC across the 15V with a Keithley 2700 DVM.

If we connect the minus of the regulator to Earth, measured AC voltage (noise?) goes down by a factor of 10. Why? 100nF is sufficient, same if fully earthed.

My first idea was that we don't know how the 2700 measures relativ to ground/Earth, so we put in an isolation transformer, but same result.

The voltage regulator is standard by data sheet, has some large aluminum electrolytes on input and output, bridge rectifier, print transformer from 230V, protective diodes.

Any ideas are appreciated.

Klaus
Click to expand...

if you want to measure 100uV level signals, you will need a
high gain preamp in front of the dvm. Needs to be in a
screened box as well. At that sort of level, nose leakage
from the dvm internals to the input terminals could have
an effect, as will any ground loops, local transmitters
etc.

Always used a diecast box for such preampss, with bnc input
and output and the internals running from batteries inside
the box. Too much noise otherwise...

Chris
 
Am 05.07.19 um 03:09 schrieb Chris:
On 07/04/19 22:54, buecherk@gmail.com wrote:
We are looking at noise of a 7815 voltage regulator. Missing other
instrumentation our first idea was to measure AC across the 15V with a
Keithley 2700 DVM.

If we connect the minus of the regulator to Earth, measured AC voltage
(noise?) goes down by a factor of 10. Why? 100nF is sufficient, same
if fully earthed.

My first idea was that we don't know how the 2700 measures relativ to
ground/Earth, so we put in an isolation transformer, but same result.

The voltage regulator is standard by data sheet, has some large
aluminum electrolytes on input and output, bridge rectifier, print
transformer from 230V, protective diodes.

Any ideas are appreciated.

Klaus

if you want to measure 100uV level signals, you will need a
high gain preamp in front of the dvm. Needs to be in a
screened box as well. At that sort of level, nose leakage
from the dvm internals to the input terminals could have
an effect, as will any ground loops, local transmitters
etc.

Always used a diecast box for such preampss, with bnc input
and output and the internals running from batteries inside
the box. Too much noise otherwise...

Chris
Click to expand...

I have measured that 2 years ago. You can assume that the 7815
is voltage-wise 3 times worse than the 7805 because it amplifies
the noisy band gap 3 times more.

<
https://www.flickr.com/photos/137684711@N07/24070698809/in/album-72157662535945536/
>

0 dB is a noise density of 1nV/sqrt(Hz), 20 dB is 10 nV/sqrt(Hz).
Most regulators are more or less equally bad.
The LT3042 / 3045 / 3094 are in a class of their own.

Load is usually 47 Ohms, circuit like proposed in the data sheet.
Moto=motorola, Fair=Fairchild etc.

Pictures left/right are noise of LEDs and Zeners.
Everything is measured using the same setup.
The preamp voltage noise is 220 pV/rtHz. (20 parallel op amps ADA4898)

regards, Gerhard
 
On Thursday, July 4, 2019 at 5:54:07 PM UTC-4, buec...@gmail.com wrote:
We are looking at noise of a 7815 voltage regulator. Missing other instrumentation our first idea was to measure AC across the 15V with a Keithley 2700 DVM.

If we connect the minus of the regulator to Earth, measured AC voltage (noise?) goes down by a factor of 10. Why? 100nF is sufficient, same if fully earthed.

My first idea was that we don't know how the 2700 measures relativ to ground/Earth, so we put in an isolation transformer, but same result.

The voltage regulator is standard by data sheet, has some large aluminum electrolytes on input and output, bridge rectifier, print transformer from 230V, protective diodes.

Any ideas are appreciated.

Klaus
Click to expand...

I'm not sure about your grounding issues. PhiL H. has a copy of
linear V reg. noise (LM317) as a function of load capacitance...
Hey I found it here,
https://electrooptical.net/static/oldsite/www/sed/ErrolDietzRegulatorNoisePeaks.pdf

George H.
 
On a sunny day (Thu, 4 Jul 2019 14:54:03 -0700 (PDT)) it happened
buecherk@gmail.com wrote in
<015f11a1-8f3e-43c8-9ea8-9d9bb6f95118@googlegroups.com>:

We are looking at noise of a 7815 voltage regulator. Missing other instrumentation our first idea was to measure AC across the
15V with a Keithley 2700 DVM.

If we connect the minus of the regulator to Earth, measured AC voltage (noise?) goes down by a factor of 10. Why? 100nF is
sufficient, same if fully earthed.

My first idea was that we don't know how the 2700 measures relativ to ground/Earth, so we put in an isolation transformer, but
same result.

The voltage regulator is standard by data sheet, has some large aluminum electrolytes on input and output, bridge rectifier,
print transformer from 230V, protective diodes.

Any ideas are appreciated.

Klaus
Click to expand...

If I understand your circuit right,
then you cnange from 15V DC out to about 1.2 V DC out,
so about 1/10 the gain
and so about 1/10 the noise
because normally the 1.2 V reference noise is multiplied 15 / 1.2.
?
 
Chris wrote:


if you want to measure 100uV level signals, you will need a
high gain preamp in front of the dvm. Needs to be in a
screened box as well.
Click to expand...

** Correct.


At that sort of level, nose leakage
from the dvm internals to the input terminals could have
an effect, as will any ground loops, local transmitters
etc.
Click to expand...

** Irrelevant, once the signal has been boosted to the 100mV level.


Always used a diecast box for such preampss,
Click to expand...

** Any metal box will do, tin plate or aluminium are fine.


with bnc input
and output and the internals running from batteries inside
the box. Too much noise otherwise...
Click to expand...

** Nope, my gain box has a balanced XLR input and RCA output with remote PSU.

A simple adaptor converts the input to BNC so I can use a scope probe.

The output of a 3 terminal reg is VERY low impedance so e-field pickup is not an issue.



..... Phil
 
On Fri, 5 Jul 2019 04:22:22 +0200, Gerhard Hoffmann <dk4xp@arcor.de>
wrote:

Am 05.07.19 um 03:09 schrieb Chris:
On 07/04/19 22:54, buecherk@gmail.com wrote:
We are looking at noise of a 7815 voltage regulator. Missing other
instrumentation our first idea was to measure AC across the 15V with a
Keithley 2700 DVM.

If we connect the minus of the regulator to Earth, measured AC voltage
(noise?) goes down by a factor of 10. Why? 100nF is sufficient, same
if fully earthed.

My first idea was that we don't know how the 2700 measures relativ to
ground/Earth, so we put in an isolation transformer, but same result.

The voltage regulator is standard by data sheet, has some large
aluminum electrolytes on input and output, bridge rectifier, print
transformer from 230V, protective diodes.

Any ideas are appreciated.

Klaus

if you want to measure 100uV level signals, you will need a
high gain preamp in front of the dvm. Needs to be in a
screened box as well. At that sort of level, nose leakage
from the dvm internals to the input terminals could have
an effect, as will any ground loops, local transmitters
etc.

Always used a diecast box for such preampss, with bnc input
and output and the internals running from batteries inside
the box. Too much noise otherwise...

Chris


I have measured that 2 years ago. You can assume that the 7815
is voltage-wise 3 times worse than the 7805 because it amplifies
the noisy band gap 3 times more.


https://www.flickr.com/photos/137684711@N07/24070698809/in/album-72157662535945536/


0 dB is a noise density of 1nV/sqrt(Hz), 20 dB is 10 nV/sqrt(Hz).
Most regulators are more or less equally bad.
The LT3042 / 3045 / 3094 are in a class of their own.

Load is usually 47 Ohms, circuit like proposed in the data sheet.
Moto=motorola, Fair=Fairchild etc.

Pictures left/right are noise of LEDs and Zeners.
Everything is measured using the same setup.
The preamp voltage noise is 220 pV/rtHz. (20 parallel op amps ADA4898)

regards, Gerhard
Click to expand...

I like your pic of the SFP guts:

https://www.flickr.com/photos/137684711@N07/42256986280/in/album-72157662535945536/

SFPs are amazing. I'm trying to find a non-telecom use for them.
Imagine a 1 MHz to 5 GHz bidirectional fiber link for under $20 per
end. Pity they are AC coupled, but they have to be for telecom.


--

John Larkin Highland Technology, Inc trk

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
On Fri, 05 Jul 2019 05:17:43 GMT, Jan Panteltje
<pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Thu, 4 Jul 2019 14:54:03 -0700 (PDT)) it happened
buecherk@gmail.com wrote in
015f11a1-8f3e-43c8-9ea8-9d9bb6f95118@googlegroups.com>:

We are looking at noise of a 7815 voltage regulator. Missing other instrumentation our first idea was to measure AC across the
15V with a Keithley 2700 DVM.

If we connect the minus of the regulator to Earth, measured AC voltage (noise?) goes down by a factor of 10. Why? 100nF is
sufficient, same if fully earthed.

My first idea was that we don't know how the 2700 measures relativ to ground/Earth, so we put in an isolation transformer, but
same result.

The voltage regulator is standard by data sheet, has some large aluminum electrolytes on input and output, bridge rectifier,
print transformer from 230V, protective diodes.

Any ideas are appreciated.

Klaus

If I understand your circuit right,
then you cnange from 15V DC out to about 1.2 V DC out,
so about 1/10 the gain
and so about 1/10 the noise
because normally the 1.2 V reference noise is multiplied 15 / 1.2.
?
Click to expand...

The problem is not well enough defined to allow us to be helpful. No
circuit, no numbers. He may have massive amounts of ambient RF or
AC-line common-mode noise. Is the measured noise microvolts or volts?


--

John Larkin Highland Technology, Inc trk

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
On 07/05/19 06:48, Phil Allison wrote:
Chris wrote:



if you want to measure 100uV level signals, you will need a
high gain preamp in front of the dvm. Needs to be in a
screened box as well.


** Correct.


At that sort of level, nose leakage
from the dvm internals to the input terminals could have
an effect, as will any ground loops, local transmitters
etc.


** Irrelevant, once the signal has been boosted to the 100mV level.


Always used a diecast box for such preampss,


** Any metal box will do, tin plate or aluminium are fine.


with bnc input
and output and the internals running from batteries inside
the box. Too much noise otherwise...


** Nope, my gain box has a balanced XLR input and RCA output with remote PSU.

A simple adaptor converts the input to BNC so I can use a scope probe.

The output of a 3 terminal reg is VERY low impedance so e-field pickup is not an issue.



.... Phil
Click to expand...

Must have got away with that lightly :). This was in London,
1970's, low noise audio for input and mixing stages. Even the
Radford DMS2 boxes used bnc and internal batteries. Must have
been a transmitter close by, s rf seemed to get into everything
unless well screened...

Chris
 
Am 05.07.19 um 17:38 schrieb John Larkin:

I like your pic of the SFP guts:

https://www.flickr.com/photos/137684711@N07/42256986280/in/album-72157662535945536/

SFPs are amazing. I'm trying to find a non-telecom use for them.
Imagine a 1 MHz to 5 GHz bidirectional fiber link for under $20 per
end. Pity they are AC coupled, but they have to be for telecom.
Click to expand...

:) These are the slightly bigger brothers, XFP. At that time
we were dreaming of a selling price of 250$.

It has a DFB laser which draws much more current than a VCXSL.
A VCXSL bias works with a resistor instead of choke(s).
The flex cable to the laser was a 3D electromagnetics simulation
nightmare, but we managed to get pretty optical eyes.
The laser is somewhat swimming, slightly moving with the fiber
connector. Same for the ROSA.

DC does not work because of 1/f noise. It has a reason that
telecom chips spec the noise only above 12 KHz.


cheers, Gerhard
 
On Fri, 5 Jul 2019 20:48:04 +0200, Gerhard Hoffmann <dk4xp@arcor.de>
wrote:

Am 05.07.19 um 17:38 schrieb John Larkin:


I like your pic of the SFP guts:

https://www.flickr.com/photos/137684711@N07/42256986280/in/album-72157662535945536/

SFPs are amazing. I'm trying to find a non-telecom use for them.
Imagine a 1 MHz to 5 GHz bidirectional fiber link for under $20 per
end. Pity they are AC coupled, but they have to be for telecom.

:) These are the slightly bigger brothers, XFP. At that time
we were dreaming of a selling price of 250$.

It has a DFB laser which draws much more current than a VCXSL.
A VCXSL bias works with a resistor instead of choke(s).
The flex cable to the laser was a 3D electromagnetics simulation
nightmare, but we managed to get pretty optical eyes.
The laser is somewhat swimming, slightly moving with the fiber
connector. Same for the ROSA.

DC does not work because of 1/f noise. It has a reason that
telecom chips spec the noise only above 12 KHz.


cheers, Gerhard
Click to expand...

DC is also unsuitable for telecom because the received optical power
can very enormously, so the receiver needs AGC. Telecom data is DC
balanced, 8B10B or some such, so AC couples just fine.

We tested a bunch of SFPs. They are AC coupled in and out, but there
seem to be no standards for the time constants. Some work down to 1
MHz, some down to 10. We didn't investigate receiver AGC time
constants; I'm not sure how we could do that.

I could FM or OOK to restore DC coupling, but then I'd get a clock
worth of jitter. All that Shannon stuff.


--

John Larkin Highland Technology, Inc trk

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
 
On Friday, July 5, 2019 at 8:38:54 AM UTC-7, John Larkin wrote:
SFPs are amazing. I'm trying to find a non-telecom use for them.
Imagine a 1 MHz to 5 GHz bidirectional fiber link for under $20 per
end. Pity they are AC coupled, but they have to be for telecom.
Click to expand...

Scope probe, with the first MHz sent over a subcarrier?

I agree with Gerhard in that you probably wouldn't want to leave DC up
to the SFP link even if it was officially supported.

-- john, KE5FX
 
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