EMG circuit - Line frequency interference

Thread Starter

Dritech

Joined Sep 21, 2011
828
Hi all,

I am currently experimenting with an EMG circuit. The instrumentation amplifier being used for this circuit is the AD8237.
The following snapshots show what kind of line frequency interference is being encountered with this circuit.

1) Snapshot below shows the interference level whenever I am not making contact with anything:

upload_2018-10-14_9-40-11.png

2) Snapshot below shows the interference level whenever I touch the surface of my laptop (plugged to the mains supply) - 50Hz interference:

upload_2018-10-14_9-41-35.png

3) Snapshot below shows the interference level whenever touch the floor barefooted - 50Hz interference:

upload_2018-10-14_9-44-12.png

Can someone please advise what kind of measures I can take to eliminate/reduce this interference?

I can introduce a notch filter, but since the EMG frequency is in the range of 10-300Hz, introducing a notch filter will also attenuate the EMG signal.

In the datasheet, it is mentioned that if additional filtering is required, one can introduce a filter at the inputs of the IA (snapshot below). Will this help in reducing line frequency interference and will it drastically affect the CMRR of the IA?

upload_2018-10-14_9-54-20.png

Any kind of help would be highly appreciated.
 

Picbuster

Joined Dec 2, 2013
979
Look at the layout of the pcb most important part of eliminating unwanted influences.
Shielding the I/O tracks in your multi layer PCB.
( data lines between the layers; make sure that no current can flow in shielding layers and the shielding start at AD8237 )
Never leave inputs floating.


Picbuster
 

Thread Starter

Dritech

Joined Sep 21, 2011
828
Hi @Picbuster

The PCB is any a two-layer board (top and bottom).
I may be wrong, but if the issue is with the PCB design, why is the interference happening only when making physical contact with the floor or my laptop? If it is a PCB design issue, shouldn't the interference be happening at all times?
 

Picbuster

Joined Dec 2, 2013
979
Two layers is tricky
You need to look at the ground contacts between your device and the real ground.
Is your complete (electrical) system connected to ground?
Is this a wooden house?
Are you using fluorescent light?
Try the following take a wire and touch the pc do you have the same effect?

Picbuster
 

Audioguru

Joined Dec 20, 2007
11,251
Is the EMG signal similar to an ECG signal? An ECG circuit uses an instrumentation amplifier and its common-mode interference signal is inverted and fed to the patient's leg to cancel the common mode interference. Of course the sensors use shielded cables.
 

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Hymie

Joined Mar 30, 2018
710
With signal levels in the range 50μV – 20mV, I’d give serious consideration to including a high Q notch filter in the circuit. With the circuit tuneable say from 40 – 60Hz and a variable Q factor, you could then optimise its response to eliminate mains noise from the circuit.

If you manage to overcome the noise issue – you could switch the filter out of circuit.
 
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