Mains hum...

[Cursitor Doom]

Greetings, gentlemen,

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output. I can\'t understand where it\'s
coming from (airborne in some form I guess) and therefore how to get
rid of it. Initially I assumed the hum would disappear when the board
was mounted in a screened metal box, but it hasn\'t made much
improvement and the noise is very evident on the scope trace (see
pictures). This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU. The links below
show the noisy trace and the set-up as I\'ve currently implemented it.
I\'m dealing here with high impedance, low level signals approximating
to a dynamic mic in character I would imagine, so it\'s a PITA. Any
suggestions?


https://disk.yandex.com/i/u7pJajKXHlzv8A


https://disk.yandex.com/i/W_rLu6HinY26VQ

 

[whit3rd]

On Tuesday, April 5, 2022 at 11:05:32 AM UTC-7, Cursitor Doom wrote:

Greetings, gentlemen,

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output.
...This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU.

If it\'s not twice mains frequency, it\'s probably not acoustic. The item has a metal
box, so light sensitivity is likewise ruled out. That leaves magnetic coupling, i.e.
ground loops. Less likely, could be capacitive.

Pairing the input wires, twisting the pairs, would be one approach. Keeping the
area between the wires small, and orienting it differently, will change the coupling.
Also, find a room with no large current-draw appliances, and no knob-and-tube
wide separation of AC hot and neutral wires. In the old physics building,
sensitive experiments were done with lights-off in the library, after hours.
If there are transformers or motors nearby, unplug \'em (a 12V high intensity
light transformer is a BIG magnetic AC source).

Old-school treatment: put a twin-tee filter in the signal path to trap that pesky power frequency.
New-school treatment: FFT the signal, and zero out the power frequency: maybe look at the
harmonics, too, for other artifacts.

 

[ehsjr]

On 4/5/2022 2:05 PM, Cursitor Doom wrote:

Greetings, gentlemen,

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output. I can\'t understand where it\'s
coming from (airborne in some form I guess) and therefore how to get
rid of it. Initially I assumed the hum would disappear when the board
was mounted in a screened metal box, but it hasn\'t made much
improvement and the noise is very evident on the scope trace (see
pictures). This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU. The links below
show the noisy trace and the set-up as I\'ve currently implemented it.
I\'m dealing here with high impedance, low level signals approximating
to a dynamic mic in character I would imagine, so it\'s a PITA. Any
suggestions?


https://disk.yandex.com/i/u7pJajKXHlzv8A


https://disk.yandex.com/i/W_rLu6HinY26VQ

What happens when the leads are properly terminated?
In use, they are attached to sensor pads that are
stuck near the collar bone (1) and on the left and right
side of the belly

Ed

 

[John Larkin]

On Tue, 05 Apr 2022 19:05:23 +0100, Cursitor Doom <cd@notformail.com>
wrote:

Greetings, gentlemen,

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output. I can\'t understand where it\'s
coming from (airborne in some form I guess) and therefore how to get
rid of it. Initially I assumed the hum would disappear when the board
was mounted in a screened metal box, but it hasn\'t made much
improvement and the noise is very evident on the scope trace (see
pictures). This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU. The links below
show the noisy trace and the set-up as I\'ve currently implemented it.
I\'m dealing here with high impedance, low level signals approximating
to a dynamic mic in character I would imagine, so it\'s a PITA. Any
suggestions?


https://disk.yandex.com/i/u7pJajKXHlzv8A


https://disk.yandex.com/i/W_rLu6HinY26VQ

Yandex. Russian. Makes sense.

--

If a man will begin with certainties, he shall end with doubts,
but if he will be content to begin with doubts he shall end in certainties.
Francis Bacon

 

[Ricky]

On Tuesday, April 5, 2022 at 2:05:32 PM UTC-4, Cursitor Doom wrote:

Greetings, gentlemen,

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output. I can\'t understand where it\'s
coming from (airborne in some form I guess) and therefore how to get
rid of it. Initially I assumed the hum would disappear when the board
was mounted in a screened metal box, but it hasn\'t made much
improvement and the noise is very evident on the scope trace (see
pictures). This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU. The links below
show the noisy trace and the set-up as I\'ve currently implemented it.
I\'m dealing here with high impedance, low level signals approximating
to a dynamic mic in character I would imagine, so it\'s a PITA. Any
suggestions?


https://disk.yandex.com/i/u7pJajKXHlzv8A


https://disk.yandex.com/i/W_rLu6HinY26VQ

I see the signal, it is clearly not sinusoidal. So you will need to do more than just trap the fundamental, but also deal with harmonics. Since the fundamental frequency drifts, it won\'t be possible to use an extremely high Q filter. what frequency range are you interested in? Is it possible to perform post capture processing to remove the background noise?

It looks like your leads are made to attach to the patches placed on a person. I would have thought to use coax wire, but maybe there\'s no point since the person it\'s attached to is not able to be shielded. Are your probes connected to a subject when you see this signal? Have you tried attaching them to a subject? It may be that the high impedance input is seeing a high noise signal which would essentially be grounded through the patient if the leads were connected... just a thought. Also look at the signal without the input leads at all.

BTW, why do you obscure the readout data in the scope capture? I count 4.3 divisions per cycle of the waveform. I can\'t think of a multiplier that would make that a power line frequency anyplace I know of.

--

Rick C.

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

 

[Phil Allison]

Cursitor Doom wrote:
=================

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output. I can\'t understand where it\'s
coming from (airborne in some form I guess)

** 50/60 Hz electric fields are everywhere inside wired buildings.
That is why you shield things.

This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU.

** But you have a DSO connected and that is supply grounded.
That is your problem creating a lot of common mode hum.

The links below
show the noisy trace and the set-up as I\'ve currently implemented it.
I\'m dealing here with high impedance, low level signals approximating
to a dynamic mic in character I would imagine,

** Dynamic mics are low impedance and well shielded.
ECGs are very special devices.

...... Phil

 

[Phil Allison]

Ricky the IDIOT puked:

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

> I see the signal, it is clearly not sinusoidal.

** That is the fucking heart beat you moron !!!
The 50/60 Hz hum is the superimposed fine wiggling.

Rest of this retarded wanker\'s absurd drivel snipped.


..... Phil

 

[Cursitor Doom]

On Tue, 5 Apr 2022 12:15:19 -0700 (PDT), Ricky
<gnuarm.deletethisbit@gmail.com> wrote:

On Tuesday, April 5, 2022 at 2:05:32 PM UTC-4, Cursitor Doom wrote:
Greetings, gentlemen,

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output. I can\'t understand where it\'s
coming from (airborne in some form I guess) and therefore how to get
rid of it. Initially I assumed the hum would disappear when the board
was mounted in a screened metal box, but it hasn\'t made much
improvement and the noise is very evident on the scope trace (see
pictures). This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU. The links below
show the noisy trace and the set-up as I\'ve currently implemented it.
I\'m dealing here with high impedance, low level signals approximating
to a dynamic mic in character I would imagine, so it\'s a PITA. Any
suggestions?


https://disk.yandex.com/i/u7pJajKXHlzv8A


https://disk.yandex.com/i/W_rLu6HinY26VQ

I see the signal, it is clearly not sinusoidal. So you will need to do more than just trap the fundamental, but also deal with harmonics. Since the fundamental frequency drifts, it won\'t be possible to use an extremely high Q filter. what frequency range are you interested in? Is it possible to perform post capture processing to remove the background noise?

It looks like your leads are made to attach to the patches placed on a person. I would have thought to use coax wire, but maybe there\'s no point since the person it\'s attached to is not able to be shielded. Are your probes connected to a subject when you see this signal? Have you tried attaching them to a subject? It may be that the high impedance input is seeing a high noise signal which would essentially be grounded through the patient if the leads were connected... just a thought. Also look at the signal without the input leads at all.

BTW, why do you obscure the readout data in the scope capture? I count 4.3 divisions per cycle of the waveform. I can\'t think of a multiplier that would make that a power line frequency anyplace I know of.

Er, yeah, that\'s why I obscured the time/div., to avoid confusion for
those in the US. Well, that was the plan, anyway. The period is 20mS
which equals 50Hz. I\'m in Yurp and that\'s the mains frequency here.

 

[Cursitor Doom]

On Tue, 5 Apr 2022 11:51:10 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Tuesday, April 5, 2022 at 11:05:32 AM UTC-7, Cursitor Doom wrote:
Greetings, gentlemen,

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output.
...This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU.


If it\'s not twice mains frequency, it\'s probably not acoustic. The item has a metal
box, so light sensitivity is likewise ruled out. That leaves magnetic coupling, i.e.
ground loops. Less likely, could be capacitive.

I had a similar problem before with another such die-cast box housing
a very small signal amp. On that occasion I was able to kill the hum
stone dead by clamping the box to an earthed metal sink. But that
hasn\'t worked this time for some reason. :-/

Pairing the input wires, twisting the pairs, would be one approach. Keeping the
area between the wires small, and orienting it differently, will change the coupling.
Also, find a room with no large current-draw appliances, and no knob-and-tube
wide separation of AC hot and neutral wires. In the old physics building,
sensitive experiments were done with lights-off in the library, after hours.
If there are transformers or motors nearby, unplug \'em (a 12V high intensity
light transformer is a BIG magnetic AC source).

Old-school treatment: put a twin-tee filter in the signal path to trap that pesky power frequency.
New-school treatment: FFT the signal, and zero out the power frequency: maybe look at the
harmonics, too, for other artifacts.

Some good suggestions all in all; cheers.

 

[Phil Allison]

On Wednesday, April 6, 2022 at 9:34:41 AM UTC+10, Ralph Mowery wrote:

In article <cbab2f4d-9771-481b...@googlegroups.com>,
palli...@gmail.com says...

Ricky the IDIOT puked:

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

I see the signal, it is clearly not sinusoidal.

** That is the fucking heart beat you moron !!!
The 50/60 Hz hum is the superimposed fine wiggling.

Rest of this retarded wanker\'s absurd drivel snipped.



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

** ?????

More like 60 bpm.


....... Phil

 

[Ricky]

On Tuesday, April 5, 2022 at 7:39:17 PM UTC-4, Cursitor Doom wrote:

On Tue, 5 Apr 2022 12:15:19 -0700 (PDT), Ricky
gnuarm.del...@gmail.com> wrote:

On Tuesday, April 5, 2022 at 2:05:32 PM UTC-4, Cursitor Doom wrote:
Greetings, gentlemen,

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output. I can\'t understand where it\'s
coming from (airborne in some form I guess) and therefore how to get
rid of it. Initially I assumed the hum would disappear when the board
was mounted in a screened metal box, but it hasn\'t made much
improvement and the noise is very evident on the scope trace (see
pictures). This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU. The links below
show the noisy trace and the set-up as I\'ve currently implemented it.
I\'m dealing here with high impedance, low level signals approximating
to a dynamic mic in character I would imagine, so it\'s a PITA. Any
suggestions?


https://disk.yandex.com/i/u7pJajKXHlzv8A


https://disk.yandex.com/i/W_rLu6HinY26VQ

I see the signal, it is clearly not sinusoidal. So you will need to do more than just trap the fundamental, but also deal with harmonics. Since the fundamental frequency drifts, it won\'t be possible to use an extremely high Q filter. what frequency range are you interested in? Is it possible to perform post capture processing to remove the background noise?

It looks like your leads are made to attach to the patches placed on a person. I would have thought to use coax wire, but maybe there\'s no point since the person it\'s attached to is not able to be shielded. Are your probes connected to a subject when you see this signal? Have you tried attaching them to a subject? It may be that the high impedance input is seeing a high noise signal which would essentially be grounded through the patient if the leads were connected... just a thought. Also look at the signal without the input leads at all.

BTW, why do you obscure the readout data in the scope capture? I count 4..3 divisions per cycle of the waveform. I can\'t think of a multiplier that would make that a power line frequency anyplace I know of.
Er, yeah, that\'s why I obscured the time/div., to avoid confusion for
those in the US. Well, that was the plan, anyway. The period is 20mS
which equals 50Hz. I\'m in Yurp and that\'s the mains frequency here.

How does hiding info \"avoid confusion\". I think we all here are aware that the world is not on 60 Hz.

So this is an actual heart beat with the tiny 50 Hz noise? That doesn\'t look so bad. I think a notch filter will clean that up very well. That can be implemented easily in the digital domain if you are capturing the signal using an ADC.

--

Rick C.

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

 

[Jan Panteltje]

On a sunny day (Tue, 05 Apr 2022 19:05:23 +0100) it happened Cursitor Doom
<cd@notformail.com> wrote in <o70p4h94qqbdftsoe1el8suv1gmklc9np2@4ax.com>:

Greetings, gentlemen,

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output. I can\'t understand where it\'s
coming from (airborne in some form I guess) and therefore how to get
rid of it. Initially I assumed the hum would disappear when the board
was mounted in a screened metal box, but it hasn\'t made much
improvement and the noise is very evident on the scope trace (see
pictures). This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU. The links below
show the noisy trace and the set-up as I\'ve currently implemented it.
I\'m dealing here with high impedance, low level signals approximating
to a dynamic mic in character I would imagine, so it\'s a PITA. Any
suggestions?

One way is perhaps to pick up mains with some wire
and then 180 degrees inverse it with some opamp
and then subtract that from the output?

This is sometimes done in ham radio to cancel noise from wallwarts in the house etc
small antenna indoors to subtract from the real big antenna outside,

 

[Ricky]

On Wednesday, April 6, 2022 at 12:20:15 AM UTC-4, Jan Panteltje wrote:

On a sunny day (Tue, 05 Apr 2022 19:05:23 +0100) it happened Cursitor Doom
c...@notformail.com> wrote in <o70p4h94qqbdftsoe...@4ax.com>:
Greetings, gentlemen,

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output. I can\'t understand where it\'s
coming from (airborne in some form I guess) and therefore how to get
rid of it. Initially I assumed the hum would disappear when the board
was mounted in a screened metal box, but it hasn\'t made much
improvement and the noise is very evident on the scope trace (see
pictures). This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU. The links below
show the noisy trace and the set-up as I\'ve currently implemented it.
I\'m dealing here with high impedance, low level signals approximating
to a dynamic mic in character I would imagine, so it\'s a PITA. Any
suggestions?
One way is perhaps to pick up mains with some wire
and then 180 degrees inverse it with some opamp
and then subtract that from the output?

This is sometimes done in ham radio to cancel noise from wallwarts in the house etc
small antenna indoors to subtract from the real big antenna outside,

If the pickup is by the wires themselves, the details of the arrangement would modify the details of the pickup, so a static counter signal would not be effective.

Much better to just narrow band block that frequency. I\'m not sure what impact that might have on the signal. I believe an EKG is analyzed by eye, so that the sort of distortion caused by most filtering would not be noticeable. It\'s easy enough to digitize the signal and run a few tests.

--

Rick C.

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

 

[Martin Brown]

On 06/04/2022 00:39, Cursitor Doom wrote:

On Tue, 5 Apr 2022 12:15:19 -0700 (PDT), Ricky
gnuarm.deletethisbit@gmail.com> wrote:

On Tuesday, April 5, 2022 at 2:05:32 PM UTC-4, Cursitor Doom wrote:
Greetings, gentlemen,

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output. I can\'t understand where it\'s
coming from (airborne in some form I guess) and therefore how to get
rid of it. Initially I assumed the hum would disappear when the board
was mounted in a screened metal box, but it hasn\'t made much
improvement and the noise is very evident on the scope trace (see
pictures). This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU. The links below
show the noisy trace and the set-up as I\'ve currently implemented it.
I\'m dealing here with high impedance, low level signals approximating
to a dynamic mic in character I would imagine, so it\'s a PITA. Any
suggestions?


https://disk.yandex.com/i/u7pJajKXHlzv8A


https://disk.yandex.com/i/W_rLu6HinY26VQ

I see the signal, it is clearly not sinusoidal. So you will need to do more than just trap the fundamental, but also deal with harmonics. Since the fundamental frequency drifts, it won\'t be possible to use an extremely high Q filter. what frequency range are you interested in? Is it possible to perform post capture processing to remove the background noise?

It looks like your leads are made to attach to the patches placed on a person. I would have thought to use coax wire, but maybe there\'s no point since the person it\'s attached to is not able to be shielded. Are your probes connected to a subject when you see this signal? Have you tried attaching them to a subject? It may be that the high impedance input is seeing a high noise signal which would essentially be grounded through the patient if the leads were connected... just a thought. Also look at the signal without the input leads at all.

BTW, why do you obscure the readout data in the scope capture? I count 4.3 divisions per cycle of the waveform. I can\'t think of a multiplier that would make that a power line frequency anyplace I know of.

Er, yeah, that\'s why I obscured the time/div., to avoid confusion for
those in the US. Well, that was the plan, anyway. The period is 20mS
which equals 50Hz. I\'m in Yurp and that\'s the mains frequency here.

Distinctly unhelpful since at least one poster mistook the heartbeat
waveform rather than the wiggles for the mains hum. I can\'t imagine why.

One distinct possibility is that the ECG is actually filtering out
*60Hz* mains rather than 50Hz. Most such devices have a notch reject
filter for mains with a moderately high Q so that they will give good
rejection for the spot mains frequency +/- 0.05 Hz.

Try it out in a field away from local mains and on battery power and see
if it is clean then. Some of your problem could be coming from the bench
psu.

Most of it I expect comes from the flying leads capturing magnetic flux
so plat them together leaving just enough free lead to connect it up.

Any high gain amplifier will amplify unwanted mains hum but in these
systems most of it should be common mode and rejected by the front end.

Finger on the input test works well enough for most high gain audio
amplifiers if a signal generator is not to hand.

--
Regards,
Martin Brown

 

[Tauno Voipio]

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.

--

-TV


On 5.4.22 21.05, Cursitor Doom wrote:

Greetings, gentlemen,

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output. I can\'t understand where it\'s
coming from (airborne in some form I guess) and therefore how to get
rid of it. Initially I assumed the hum would disappear when the board
was mounted in a screened metal box, but it hasn\'t made much
improvement and the noise is very evident on the scope trace (see
pictures). This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU. The links below
show the noisy trace and the set-up as I\'ve currently implemented it.
I\'m dealing here with high impedance, low level signals approximating
to a dynamic mic in character I would imagine, so it\'s a PITA. Any
suggestions?


https://disk.yandex.com/i/u7pJajKXHlzv8A


https://disk.yandex.com/i/W_rLu6HinY26VQ

 

[Phil Allison]

Tauno Voipio wrote:
=================

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

** Spot fucking on.

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.

** Yep, has the OP never handled a live audio line ?

Bluuuurp, bluuup.....

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.

** As does grounding the diff amp metal enclosure.
If the whole kaboodle is *floating* - the diff disappears.

FYI:

The OP is a notorious idiot and is \"dreaming\" since his posted scope image is good.



....... Phil

 

[Mike Coon]

In article <MPG.3cb69c19eedb9964989c29@news.eternal-september.org>,
rmowery42@charter.net says...

In article <cbab2f4d-9771-481b-8577-a2badf6dfa02n@googlegroups.com>,
pallison49@gmail.com says...

Ricky the IDIOT puked:

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

I see the signal, it is clearly not sinusoidal.

** That is the fucking heart beat you moron !!!
The 50/60 Hz hum is the superimposed fine wiggling.

Rest of this retarded wanker\'s absurd drivel snipped.





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...

 

[Cursitor Doom]

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...

 

[Cursitor Doom]

On Wed, 06 Apr 2022 04:19:25 GMT, Jan Panteltje
<pNaonStpealmtje@yahoo.com> wrote:

On a sunny day (Tue, 05 Apr 2022 19:05:23 +0100) it happened Cursitor Doom
cd@notformail.com> wrote in <o70p4h94qqbdftsoe1el8suv1gmklc9np2@4ax.com>:

Greetings, gentlemen,

I\'ve got one of those ECG boards off Ebay and am having a problem with
mains hum appearing on the output. I can\'t understand where it\'s
coming from (airborne in some form I guess) and therefore how to get
rid of it. Initially I assumed the hum would disappear when the board
was mounted in a screened metal box, but it hasn\'t made much
improvement and the noise is very evident on the scope trace (see
pictures). This *is* mains interference as it works out at exactly
mains frequency. It\'s even there if I attach the DC input leads to a
9V battery instead so it\'s not coming from the PSU. The links below
show the noisy trace and the set-up as I\'ve currently implemented it.
I\'m dealing here with high impedance, low level signals approximating
to a dynamic mic in character I would imagine, so it\'s a PITA. Any
suggestions?

One way is perhaps to pick up mains with some wire
and then 180 degrees inverse it with some opamp
and then subtract that from the output?

Ingenious, Jan; ingenious. I\'d have loved to have tried that idea out
if I\'d had a bit more time to spare. In the end, ferrite beads did the
trick. Very analogue!

This is sometimes done in ham radio to cancel noise from wallwarts in the house etc
small antenna indoors to subtract from the real big antenna outside,

 

[Ralph Mowery]

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.