Mains hum...

[Jan Panteltje]

On a sunny day (Wed, 6 Apr 2022 10:20:25 -0400) it happened Ralph Mowery
<rmowery42@charter.net> wrote in
<MPG.3cb76bb730946ade989c2a@news.eternal-september.org>:

In article <MPG.3cb77f6e2b5215d59896ac@usenet.plus.net>,
gravity@mjcoon.plus.com says...


Yes, that looks like the typical RST waveform of the heart. The heart
often beats near 60 Hz.

Although I believe my pacemaker is set to let me get a bit slower before
it starts geeing me up...



The 60 beats is just a number near the average. For people that are in
very good health and do a lot of physical activity the heart may beat
slower than the \'nornal average\' when at rest, others may beat faster if
not very active and in good physical shape. I think mine is mnore like
70 some BPM.

From what I see this is a simple decvice and maybe has only 2 leads.
The heart monitors I am familiar with has 3 leads where the internal
circuits filter out the stray electrical noise picked up by the monitor.
Big difference in a device under $ 100 and the professional devices.
Then there are the multilead devices.

I just measured 42 per minute when sitting on a chair in front of the table looking at the laptop doing nothing.
But if I really relax its lower.
So it seems I am slower than you earthlings,
I know about some doctor who could control his heart beat.

 

[Tauno Voipio]

On 6.4.22 15.45, Cursitor Doom wrote:

On Wed, 6 Apr 2022 11:58:48 +0300, Tauno Voipio
tauno.voipio@notused.fi.invalid> wrote:

My guess is that the hum is coming from your patient / target.

The capacitance of an average person is about 20 pF to the
active phase line and 200 pF to ground. You can assume that
the patient has about 10% of the line voltage through a pretty
high impedance voltage divider.

Yeah, I know what you mean but no cigar for you this time. I was able
to clean up the hum by using good old fashioned ferrite beads inside
the box at the point where the power lead comes in. Worked like a
charm. Sometimes the best solutions are the old ones...

I do not quite subscribe it.

The ferrite beads do not attenuate the 50 or 60 Hz hum
from the mains.

Do you have a reference electrode on the patient?

--

-TV

 

[Arie de Muijnck]

On 2022-04-06 14:45, Cursitor Doom wrote:

On Wed, 6 Apr 2022 11:58:48 +0300, Tauno Voipio
tauno.voipio@notused.fi.invalid> wrote:

My guess is that the hum is coming from your patient / target.

The capacitance of an average person is about 20 pF to the
active phase line and 200 pF to ground. You can assume that
the patient has about 10% of the line voltage through a pretty
high impedance voltage divider.

Yeah, I know what you mean but no cigar for you this time. I was able
to clean up the hum by using good old fashioned ferrite beads inside
the box at the point where the power lead comes in. Worked like a
charm. Sometimes the best solutions are the old ones...

Since these beads do not attenuate 50/60Hz, you probably had a problem
with mains modulated interference being rectified in the input circuits
of the amplifier. You need to add some RF filtering at the input, before
any non-linear elements like transistors or op-amps.

Arie

 

[Ricky]

On Wednesday, April 6, 2022 at 4:59:35 AM UTC-4, Tauno Voipio wrote:

My guess is that the hum is coming from your patient / target.

The capacitance of an average person is about 20 pF to the
active phase line and 200 pF to ground. You can assume that
the patient has about 10% of the line voltage through a pretty
high impedance voltage divider.

When we made ECG equipment in the early 1970\'s, the solution
was to make the input of the amplifier differential and as
high impedance as possible, including guard bootstrapping the
shield braids of the input cables. The impedance of the
connection electrodes are hardly ever identical, and this creates
voltage dividers with the amplifier input impedances. The
imbalance works directly to convert the common-mode hum into
differential input.

That would make sense if each measurement was relative to ground. These measurements only make sense relative to each other. Treating the body as a capacitor to the power line is not of much use. Noise can be introduced when the different parts of the body have different levels of noise. The displayed signals are either one electrode relative to a \"reference\" electrode, or multiple electrodes are averaged together to form a virtual reference which is the reference for each electrode.

I don\'t know for sure, but I expect the body is a relatively low impedance voltage source. I\'m willing to bet the best way for removing power line noise is a simple notch filter in the amplifier. The noise amplitude is low, so no worry about over driving the amps. Just filter it out, best with a digital filter as the design is not complex and the frequency is not subject to drift with component tolerance, temperature, etc., so high Q can be used.

--

Rick C.

-+ Get 1,000 miles of free Supercharging
-+ Tesla referral code - https://ts.la/richard11209

 

[Tauno Voipio]

On 6.4.22 19.51, Ricky wrote:

On Wednesday, April 6, 2022 at 4:59:35 AM UTC-4, Tauno Voipio wrote:
My guess is that the hum is coming from your patient / target.

The capacitance of an average person is about 20 pF to the
active phase line and 200 pF to ground. You can assume that
the patient has about 10% of the line voltage through a pretty
high impedance voltage divider.

When we made ECG equipment in the early 1970\'s, the solution
was to make the input of the amplifier differential and as
high impedance as possible, including guard bootstrapping the
shield braids of the input cables. The impedance of the
connection electrodes are hardly ever identical, and this creates
voltage dividers with the amplifier input impedances. The
imbalance works directly to convert the common-mode hum into
differential input.

That would make sense if each measurement was relative to ground. These measurements only make sense relative to each other. Treating the body as a capacitor to the power line is not of much use. Noise can be introduced when the different parts of the body have different levels of noise. The displayed signals are either one electrode relative to a \"reference\" electrode, or multiple electrodes are averaged together to form a virtual reference which is the reference for each electrode.

I don\'t know for sure, but I expect the body is a relatively low impedance voltage source. I\'m willing to bet the best way for removing power line noise is a simple notch filter in the amplifier. The noise amplitude is low, so no worry about over driving the amps. Just filter it out, best with a digital filter as the design is not complex and the frequency is not subject to drift with component tolerance, temperature, etc., so high Q can be used.

That is true, but we do not have contacts to the low impedance
source. The electrodes can have impedances of tens of kohm,
with mismatch between the electrodes. We do not hit contact
spikes through the skin.

A notch filter is not a good idea, as much of the interesting
frequency components are in the mains frequency range. The
ECG signal is spiky by its very nature, and narrow band filters
will spoil the information looked for.

--

-TV

 

[whit3rd]

On Wednesday, April 6, 2022 at 5:45:30 AM UTC-7, Cursitor Doom wrote:

On Wed, 6 Apr 2022 11:58:48 +0300, Tauno Voipio
tauno....@notused.fi.invalid> wrote:

My guess is that the hum is coming from your patient / target.

The capacitance of an average person is about ...

Yeah, I know what you mean but no cigar for you this time. I was able
to clean up the hum by using good old fashioned ferrite beads inside
the box at the point where the power lead comes in. Worked like a
charm. Sometimes the best solutions are the old ones...

Didn\'t the original post say that the problem occurred with battery power?
Sounds like you\'ve found PART of the solution, not all of it. Ferrite
beads don\'t do much at power-line frequency.

 

[Ricky]

On Wednesday, April 6, 2022 at 2:21:53 PM UTC-4, Tauno Voipio wrote:

On 6.4.22 19.51, Ricky wrote:
On Wednesday, April 6, 2022 at 4:59:35 AM UTC-4, Tauno Voipio wrote:
My guess is that the hum is coming from your patient / target.

The capacitance of an average person is about 20 pF to the
active phase line and 200 pF to ground. You can assume that
the patient has about 10% of the line voltage through a pretty
high impedance voltage divider.

When we made ECG equipment in the early 1970\'s, the solution
was to make the input of the amplifier differential and as
high impedance as possible, including guard bootstrapping the
shield braids of the input cables. The impedance of the
connection electrodes are hardly ever identical, and this creates
voltage dividers with the amplifier input impedances. The
imbalance works directly to convert the common-mode hum into
differential input.

That would make sense if each measurement was relative to ground. These measurements only make sense relative to each other. Treating the body as a capacitor to the power line is not of much use. Noise can be introduced when the different parts of the body have different levels of noise. The displayed signals are either one electrode relative to a \"reference\" electrode, or multiple electrodes are averaged together to form a virtual reference which is the reference for each electrode.

I don\'t know for sure, but I expect the body is a relatively low impedance voltage source. I\'m willing to bet the best way for removing power line noise is a simple notch filter in the amplifier. The noise amplitude is low, so no worry about over driving the amps. Just filter it out, best with a digital filter as the design is not complex and the frequency is not subject to drift with component tolerance, temperature, etc., so high Q can be used.
That is true, but we do not have contacts to the low impedance
source. The electrodes can have impedances of tens of kohm,
with mismatch between the electrodes. We do not hit contact
spikes through the skin.

The body is still low impedance with respect to the amplifier inputs. They use paste and a large contact area to minimize skin resistance. Given the high input impedance of the amp, the mismatch in impedance is of little consequence.

A notch filter is not a good idea, as much of the interesting
frequency components are in the mains frequency range. The
ECG signal is spiky by its very nature, and narrow band filters
will spoil the information looked for.

You seem to be confusing a notch (band reject) with a narrow band-pass filter. An adequately narrow band reject filter would have virtually no impact on the signal of interest. It only need be wide enough to accommodate the normal variations in mains frequency.

--

Rick C.

+- Get 1,000 miles of free Supercharging
+- Tesla referral code - https://ts.la/richard11209

 

[Ricky]

On Wednesday, April 6, 2022 at 4:42:48 PM UTC-4, whit3rd wrote:

On Wednesday, April 6, 2022 at 5:45:30 AM UTC-7, Cursitor Doom wrote:
On Wed, 6 Apr 2022 11:58:48 +0300, Tauno Voipio
tauno....@notused.fi.invalid> wrote:

My guess is that the hum is coming from your patient / target.

The capacitance of an average person is about ...
Yeah, I know what you mean but no cigar for you this time. I was able
to clean up the hum by using good old fashioned ferrite beads inside
the box at the point where the power lead comes in. Worked like a
charm. Sometimes the best solutions are the old ones...
Didn\'t the original post say that the problem occurred with battery power?
Sounds like you\'ve found PART of the solution, not all of it. Ferrite
beads don\'t do much at power-line frequency.

I\'d like to see what the output looks like with various inputs, all contacts shorted, resistors between contacts, etc. If the problem really has disappeared, I expect it will return and not by removing the ferrite beads.

--

Rick C.

++ Get 1,000 miles of free Supercharging
++ Tesla referral code - https://ts.la/richard11209

 

[Cursitor Doom]

On Wed, 6 Apr 2022 16:06:28 -0700 (PDT), Ricky
<gnuarm.deletethisbit@gmail.com> wrote:

On Wednesday, April 6, 2022 at 2:21:53 PM UTC-4, Tauno Voipio wrote:
On 6.4.22 19.51, Ricky wrote:
On Wednesday, April 6, 2022 at 4:59:35 AM UTC-4, Tauno Voipio wrote:
My guess is that the hum is coming from your patient / target.

The capacitance of an average person is about 20 pF to the
active phase line and 200 pF to ground. You can assume that
the patient has about 10% of the line voltage through a pretty
high impedance voltage divider.

When we made ECG equipment in the early 1970\'s, the solution
was to make the input of the amplifier differential and as
high impedance as possible, including guard bootstrapping the
shield braids of the input cables. The impedance of the
connection electrodes are hardly ever identical, and this creates
voltage dividers with the amplifier input impedances. The
imbalance works directly to convert the common-mode hum into
differential input.

That would make sense if each measurement was relative to ground. These measurements only make sense relative to each other. Treating the body as a capacitor to the power line is not of much use. Noise can be introduced when the different parts of the body have different levels of noise. The displayed signals are either one electrode relative to a \"reference\" electrode, or multiple electrodes are averaged together to form a virtual reference which is the reference for each electrode.

I don\'t know for sure, but I expect the body is a relatively low impedance voltage source. I\'m willing to bet the best way for removing power line noise is a simple notch filter in the amplifier. The noise amplitude is low, so no worry about over driving the amps. Just filter it out, best with a digital filter as the design is not complex and the frequency is not subject to drift with component tolerance, temperature, etc., so high Q can be used.
That is true, but we do not have contacts to the low impedance
source. The electrodes can have impedances of tens of kohm,
with mismatch between the electrodes. We do not hit contact
spikes through the skin.

The body is still low impedance with respect to the amplifier inputs. They use paste and a large contact area to minimize skin resistance. Given the high input impedance of the amp, the mismatch in impedance is of little consequence.


A notch filter is not a good idea, as much of the interesting
frequency components are in the mains frequency range. The
ECG signal is spiky by its very nature, and narrow band filters
will spoil the information looked for.


You seem to be confusing a notch (band reject) with a narrow band-pass filter. An adequately narrow band reject filter would have virtually no impact on the signal of interest. It only need be wide enough to accommodate the normal variations in mains frequency.

Leaving aside that the problem has now been resolved, I would have
thought Tauno was right in what he said. The wanted and unwanted
components of the waveform are just too close together to target one
without degrading the other to some meaningful extent.

 

[Phil Allison]

Ricky wrote:
==========

> An adequately narrow band reject filter would have virtually no impact on the signal of interest.

** Shame you have never looked at the phase and amplitude curve of such a filter.


> It only need be wide enough to accommodate the normal variations in mains frequency.

** About 0.1Hz is all the AC supply varies by.


....... Phil

 

[piglet]

On 06/04/2022 9:42 pm, whit3rd wrote:

On Wednesday, April 6, 2022 at 5:45:30 AM UTC-7, Cursitor Doom wrote:
On Wed, 6 Apr 2022 11:58:48 +0300, Tauno Voipio
tauno....@notused.fi.invalid> wrote:

My guess is that the hum is coming from your patient / target.

The capacitance of an average person is about ...

Yeah, I know what you mean but no cigar for you this time. I was able
to clean up the hum by using good old fashioned ferrite beads inside
the box at the point where the power lead comes in. Worked like a
charm. Sometimes the best solutions are the old ones...

Didn\'t the original post say that the problem occurred with battery power?
Sounds like you\'ve found PART of the solution, not all of it. Ferrite
beads don\'t do much at power-line frequency.

I think that when OP tried battery power he was still examining the
output on a mains connected scope? Interesting would be what is seen
with a battery powered scope and the whole system free of any connection
to mains ground.

No sane amount of ferrite beads will have effect at 50Hz, much more
likely some tens/hundreds of kHz from a mains SMPS in the vicinity is
getting in. The ECG amp will probably be heavily low pass filtered
somewhere along the chain so the HF interference won\'t be seen at the
output but it\'s 100Hz modulation rate will be?

piglet

 

[Phil Allison]

erichp...@hotmail.com wrote:
=======================

My guess is that the hum is coming from your patient / target.

The capacitance of an average person is about ...

I think that when OP tried battery power he was still examining the
output on a mains connected scope? Interesting would be what is seen
with a battery powered scope and the whole system free of any connection
to mains ground.

** That is my contention too, posted 2 days ago.

The ECG amp will probably be heavily low pass filtered
somewhere along the chain so the HF interference won\'t be seen at the
output but it\'s 100Hz modulation rate will be?

** Huh ? 100Hz ?

Think is is just simple capacitive coupled injection of the AC supply voltage wave.
The bane of many poorly shielded electric guitars .


....... Phil

 

[piglet]

On 07/04/2022 09:19, Phil Allison wrote:

erichp...@hotmail.com wrote:
=======================

My guess is that the hum is coming from your patient / target.

The capacitance of an average person is about ...

I think that when OP tried battery power he was still examining the
output on a mains connected scope? Interesting would be what is seen
with a battery powered scope and the whole system free of any connection
to mains ground.

** That is my contention too, posted 2 days ago.

The ECG amp will probably be heavily low pass filtered
somewhere along the chain so the HF interference won\'t be seen at the
output but it\'s 100Hz modulation rate will be?

** Huh ? 100Hz ?

Think is is just simple capacitive coupled injection of the AC supply voltage wave.
The bane of many poorly shielded electric guitars .


...... Phil

Yes, but later on the OP said he solved the problem with ferrite beads.
He hasn\'t said how many beads he used but it seems unlikely he used
enough ferrite to attenuate mains frequencies directly so the theory now
is that he instead attenuated some higher frequency pickup that was
itself power line modulated?

piglet

 

[Phil Allison]

erichp...@hotmail.com wrote:
========================

My guess is that the hum is coming from your patient / target.

The capacitance of an average person is about ...

I think that when OP tried battery power he was still examining the
output on a mains connected scope? Interesting would be what is seen
with a battery powered scope and the whole system free of any connection
to mains ground.

** That is my contention too, posted 2 days ago.

The ECG amp will probably be heavily low pass filtered
somewhere along the chain so the HF interference won\'t be seen at the
output but it\'s 100Hz modulation rate will be?

** Huh ? 100Hz ?

Think is is just simple capacitive coupled injection of the AC supply voltage wave.
The bane of many poorly shielded electric guitars .



Yes, but later on the OP said he solved the problem with ferrite beads.

** Should have tried Hippy beads or Rosary beads.

> He hasn\'t said how many beads he used ...

** Nor how much weed he smoke daily.

but it seems unlikely he used
enough ferrite to attenuate mains frequencies directly so the theory now
is that he instead attenuated some higher frequency pickup that was
itself power line modulated?

** I can see what looks like 50Hz in that slow ECG trace.

Try counting the wiggles.



...... Phil

 

[piglet]

On 07/04/2022 09:44, Phil Allison wrote:

erichp...@hotmail.com wrote:
========================


My guess is that the hum is coming from your patient / target.

The capacitance of an average person is about ...

I think that when OP tried battery power he was still examining the
output on a mains connected scope? Interesting would be what is seen
with a battery powered scope and the whole system free of any connection
to mains ground.

** That is my contention too, posted 2 days ago.

The ECG amp will probably be heavily low pass filtered
somewhere along the chain so the HF interference won\'t be seen at the
output but it\'s 100Hz modulation rate will be?

** Huh ? 100Hz ?

Think is is just simple capacitive coupled injection of the AC supply voltage wave.
The bane of many poorly shielded electric guitars .



Yes, but later on the OP said he solved the problem with ferrite beads.

** Should have tried Hippy beads or Rosary beads.

He hasn\'t said how many beads he used ...

** Nor how much weed he smoke daily.

but it seems unlikely he used
enough ferrite to attenuate mains frequencies directly so the theory now
is that he instead attenuated some higher frequency pickup that was
itself power line modulated?

** I can see what looks like 50Hz in that slow ECG trace.

Try counting the wiggles.



..... Phil

With the timebase unhelpfully blurred out I didn\'t before count the
wiggles to see if 50Hz or 100Hz but tried just now using the heart rate
as a guide and agree with you - it looks like it is 50Hz direct mains
pickup. Thanks Phil.

piglet

 

[Phil Allison]

erichp...@hotmail.com wrote:
========================

** I can see what looks like 50Hz in that slow ECG trace.

Try counting the wiggles.

With the timebase unhelpfully blurred out I didn\'t before count the
wiggles to see if 50Hz or 100Hz but tried just now using the heart rate
as a guide and agree with you - it looks like it is 50Hz direct mains
pickup.

** From the OP\'s first post.

\" This *is* mains interference as it works out at exactly
mains frequency. \"



....... Phil

 

[server]

Jan Pan Dipshit <pNaonStpealmtje@yahoo.com> wrote in
news:t2kb5a$kd8$1@dont-email.me:

I just measured 42 per minute when sitting on a chair in front of
the table looking at the laptop doing nothing.

I\'ll bet that you sport flawed measurement capability then, because
that rate claim is bullshit.

But if I really
relax its lower.

And you are still unable to obtain a correct reading.

Got a BP cuff? What does it say?

*I* say that you are a liar or simply have no clue how to read your
pulse.


> So it seems I am slower than you earthlings,

Right... the utter retard Jan Pan Dipshit thinks he\'s from
elsewhere.

> I know about some doctor who could control his heart beat.

So could our man Flint, but you lack the skill to even take a
proper reading, so you controlling what you cannot even read is
unlikely.

 

[server]

Phil Allison <pallison49@gmail.com> wrote in
news:e9c27709-1c68-47d3-b8db-45e594bd59ccn@googlegroups.com:

Ricky wrote:
==========

An adequately narrow band reject filter would have virtually no
impact on the signal of interest.

** Shame you have never looked at the phase and amplitude curve
of such a filter.


It only need be wide enough to accommodate the normal variations
in mains frequency.

** About 0.1Hz is all the AC supply varies by.

Yep... practically wordlwide the electrical grids sport near zero
variance with regard to frequency.

 

[server]

Phil Allison <pallison49@gmail.com> wrote in
news:02662e66-5f7e-4d87-a583-86f0abbcf9a9n@googlegroups.com:

** Huh ? 100Hz ?

Think is is just simple capacitive coupled injection of the AC
supply voltage wave. The bane of many poorly shielded electric
guitars .

Yes, and it typically sounds off at twice the line frequency.

So, 60Hz gets 120Hz noise and 50Hz gets 100Hz noise.

 

[Tauno Voipio]

On 7.4.22 2.52, Phil Allison wrote:

Ricky wrote:
==========

An adequately narrow band reject filter would have virtually no impact on the signal of interest.

** Shame you have never looked at the phase and amplitude curve of such a filter.


It only need be wide enough to accommodate the normal variations in mains frequency.

** About 0.1Hz is all the AC supply varies by.


...... Phil

Right!

A narrow notch rings as badly as a narrow peak when fed with
a spiky signal.

We did try all kinds of filtering already half a century ago,
and the basic mathematics, physics and electronics about the
filter has not changed.

--

-TV