The following is from an AC Adaptor connected to an EEG amplifier. Even though the 60Hz was notched filter and differential amplifier used, how much the harmonics were not substracted? And what exactly created the 60hz harmonics? The vertical lines are multiples of it like 120Hz, 180Hz, etc. How are they caused exactly?

 

The ripple frequency from the adapter could be the rectified 60Hz, which has a fundamental of 120Hz plus harmonics of that.
That would be difficult to filter from the signal.

Likely better to reduce the ripple noise from the AC adapter output with perhaps an LC low-pass filter along with a common-mode choke.

 

The ripple frequency from the adapter could be the rectified 60Hz, which has a fundamental of 120Hz plus harmonics of that.
That would be difficult to filter from the signal.

Likely better to reduce the ripple noise from the AC adapter output with perhaps an LC low-pass filter along with a common-mode choke.

The following is the exact AC adaptor. Why is it difficult for differential amplifier to remove the noise when it is just one vertical line spaced every 60Hz apart? It is the simplest common mode that differential amplifier should easily deduct. It's not even white noise. Just the vertical lines.

 

The following is the exact AC adaptor. Why is it difficult for differential amplifier to remove the noise when it is just one vertical line spaced every 60Hz apart? It is the simplest common mode that differential amplifier should easily deduct. It's not even white noise. Just the vertical lines.

View attachment 353477

Think about where the harmonics are coming from. Is it the signal inputs or the power bus?
https://www.globtek.com/datasheets/GTM21097_SERIES-50W.pdf


Lots of designs use batteries to eliminate PS generated power bus noise.

Show us the schematic.

 

EDIT;
Differential amplifier’s purpose is to reject common-mode signals while simultaneously amplifying signals that appear differentially across its terminals. Which I suspect is what is happening. You have got a differential signal.

But…You are not showing nor explaining how you connected the AC adapter to the EGG amplifier. Without that information we would be only guessing what exactly is going on.

 

This is the schematic. But is it not the 60Hz and harmonics are common mode. Why arent they easily removed by the differential amplifier since they are just vertical lines that appear at all channels?

 

This is the schematic. But is it not the 60Hz and harmonics are common mode. Why arent they easily removed by the differential amplifier since they are just vertical lines that appear at all channels?

View attachment 353483

What happens when it's battery powered?
Do you really understand what those vertical line mean? Do you have any real understanding of electronics and signal processing?

 

What happens when it's battery powered?
Do you really understand what those vertical line mean? Do you have any real understanding of electronics and signal processing?

When it is battery powered. There were no vertical lines. I know those vertical lines mean harmonics, but why aren't they cancelled? and what formed them? I once took a half course in electronics but moved to biology halfway. and so my idea of signal processing is only the Zilog Z80 microprocessor.

 

1) What is the amplifier connected to via its USB port?
2) Does the amplifier have any floating inputs?
3) At which point in the circuit are you detecting the 60Hz and harmonics?
4) What grounding and screening arrangements does the adapter have?

 

1) What is the amplifier connected to via its USB port?
2) Does the amplifier have any floating inputs?
3) At which point in the circuit are you detecting the 60Hz and harmonics?
4) What grounding and screening arrangements does the adapter have?

1.g.USBamp
2. How do you know if it has floating input?
3. The 60Hz harmonics is from the EEG recording
4. See adaptor in original message..it has no grounding in the plug

 

You have 60Hz harmonics not 120Hz harmonics, the biggest being the 3rd harmonic at 180Hz. That eliminates any possibility of its coming from rectified AC.

 

You have 60Hz harmonics not 120Hz harmonics, the biggest being the 3rd harmonic at 180Hz. That eliminates any possibility of its coming from rectified AC.

Why cant the 60Hz harmonics come from rectified AC? Where does the harmonics come from then? Note I used Matlab FFT to see them from raw EEG waveforms.

 

Why cant the 60Hz harmonics come from rectified AC? Where does the harmonics come from then? Note I used Matlab FFT to see them from raw EEG waveforms.

Because rectified unsmoothed AC is 120Hz.
As you have 60Hz harmonics, they must originate from the mains, not your power supply.

 

Because rectified unsmoothed AC is 120Hz.
As you have 60Hz harmonics, they must originate from the mains, not your power supply.

Ok if they originate in the main. Still why aren't the harmonics canceled by the differential amplifier since both channel 1 and channel 2 have the same common mode harmonics?

 

A diff amplifier is not a comb filter. It only amplifies a signal which appears differentially across its input terminals. Which somehow you must have but don’t want to share the details.
The “schematic” that you posted is only a block diagram. You don’t show where or to what were those inputs connected to during the data capture, yet you become upset at members because they aren’t mind readers.

I am done dealing with arrogant members and putting you on my ignore list. Do the same for me.

 

Ok if they originate in the main. Still why aren't the harmonics canceled by the differential amplifier since both channel 1 and channel 2 have the same common mode harmonics?

Either the common mode rejection isn't as good as it should be, or the signal is differential.
Different impedences at the two inputs would be enough to cause different amounts of pickup at the two inputs, then you have a differential signal.
Why would you think that precisely the same signal would be picked up on different parts of the head?

 

A diff amplifier is not a comb filter. It only amplifies a signal which appears differentially across its input terminals. Which somehow you must have but don’t want to share the details.
The “schematic” that you posted is only a block diagram. You don’t show where or to what were those inputs connected to during the data capture, yet you become upset at members because they aren’t mind readers.

I am done dealing with arrogant members and putting you on my ignore list. Do the same for me.

Well. I am surprised by this reply. This is true for all bioamplifiers. If you use AC adaptor, you get harmonics regardless if the inputs are floating or connected. I was asking why the harmonics still appear when they are common mode and all differential amplifiers should filter them. The harmonics show up at FFT.

 

Either the common mode rejection isn't as good as it should be, or the signal is differential.
Different impedences at the two inputs would be enough to cause different amounts of pickup at the two inputs, then you have a differential signal.
Why would you think that precisely the same signal would be picked up on different parts of the head?

Note even if the electrodes are floating or connected to head (see above message too). Same 60Hz harmonics. I tried these in 3 brands of EEG like Brainmaster, CWE BMA-2000 and the gtec g.USBamp. Same results if all electrodes are floating or connected, the 60Hz harmonics still show up even when channel 1 is deducted to channel 2. The AC 60Hz harmonics should be present in all channels without passing thru inputs but via the chip or ic power supply, isn't it?

But wait, why isn't the power supply voltage also substracted since they are also common mode.. perhaps the clue is here.

 

See adaptor in original message..it has no grounding in the plug

If the mains input to the adapter has no grounding and the adapter input and output wiring is unscreened then there could be considerable mains supply noise in the vicinity of the amplifier. Since the amplifier input impedance is specified as >100 MegOhm the amplifier will be very sensitive to radiated mains interference if its inputs are not coupled to a much lower impedance. Even head impedance may not be low enough to reduce input noise to a non-noticeable level.
Different lengths and/or positioning of the input electrode wires will prevent complete interference cancellation by a differential amplifier.

 

If the mains input to the adapter has no grounding and the adapter input and output wiring is unscreened then there could be considerable mains supply noise in the vicinity of the amplifier. Since the amplifier input impedance is specified as >100 MegOhm the amplifier will be very sensitive to radiated mains interference if its inputs are not coupled to a much lower impedance. Even head impedance may not be low enough to reduce input noise to a non-noticeable level.
Different lengths and/or positioning of the input electrode wires will prevent complete interference cancellation by a differential amplifier.

Oh. The adaptor has grounding but in my country, we never use any grounding so I just plugged the 2 pins to 220V A.C. in the outlet. See related thread about groundless A.C. Continous Fault Current passing through Earth ground | All About Circuits

Why is that if the amplifier input impedance is specified as >100 MegOhm the amplifier will be very sensitive to radiated mains interference if its inputs are not coupled to a much lower impedance?? Please elaborate. Thank you.