AD620 ECG problem

[Fluffyboii]

This got out of hand so I had to ask here. I am aware I am being to dependent on this forum and it is not something I enjoy. I tried solving this problem multiple times with different approaches and failed.

The problem is no matter how much gain I apply and how I filter the signal I can not get anything identifiable that looks like a hear beat on the oscilloscope. Even in the highest sensitivity which is something like 10mV, it is just flat noise with no ECG like component when I connect the props to my arms and leg. I started with AD620 datasheet circuit

And went to something as simple as this after nothing I did work.

When bottom circuit is tested with a signal generator I can see that It works and I have about 500 times gain. I put a low pass filter to output to set 3db point about 50Hz. Even if I amplify the output 100 times more to a total 5000 with 2 more op amp stages I get nothing. I tried 3 different breadboards, different ICs and checked the connections to props and confirmed that cables weren't open or broken in places. Even with the simplest circuit I should see some fluctuations in voltage with that much gain but instead I usually get 50Hz line noise or something that is about 5-10Hz that is not a heartbeat. Using the top circuit causes op amp to saturate and hit one of the rails even though "ground" is set to the body. Nothing I tried works. At this point I believe the connections are not working because we are not using the cream thing that is supposed to be used with securing the connection to skin.

I can not proceed with the project without getting any signal. Any ideas what may be holding me back.

 

[rjenkinsgb]

Unless it's a battery oscilloscope and you also connect the ground from that to an electrode to keep it at body potential, any oscilloscope ground is going to prevent the amp circuit tracking body ground voltage.

The amp circuit you have would need an isolation circuit of some sort at its output, such as that linear opto you used in another thread, to avoid the scope or eventual display affecting the minuscule input voltages.

Or everything including the display has to be self-contained and battery powered?

 

[Fluffyboii]

Unless it's a battery oscilloscope and you also connect the ground from that to an electrode to keep it at body potential, any oscilloscope ground is going to prevent the amp circuit tracking body ground voltage.

The amp circuit you have would need an isolation circuit of some sort at its output, such as that linear opto you used in another thread, to avoid the scope or eventual display affecting the minuscule input voltages.

Or everything including the display has to be self-contained and battery powered?

That makes a lot of sense. I have my small oscillscope and by luck I had a battery case that fits 6 AA batteries that actually makes it run off grid. Would this work. View attachment 139856

 

[Fluffyboii]

I tried using 2 9V batteries to get +/-9V and when I set the gain to something large like 250 it just rams into one of the rails, often the negative one. I set the oscilloscope to DC so that it wont distort the low frequency signal but it is still dead flat. I can not see any movement.

 

[rjenkinsgb]

At this point I believe the connections are not working because we are not using the cream thing that is supposed to be used with securing the connection to skin.

That may be a problem.
Any water based skin cream, or even soap and water should work.

Another possible problem, if the electrodes are anything other than stainless steel or conductive plastic - "battery" type effects, when a conductive liquid (or sweat) comes in contact with any reactive metal?

 

[Fluffyboii]

That may be a problem.
Any water based skin cream, or even soap and water should work.

Another possible problem, if the electrodes are anything other than stainless steel or conductive plastic - "battery" type effects, when a conductive liquid (or sweat) comes in contact with any reactive metal?

It could be. It was about at the middle of the rails at some point something like -1V with adequate gain but it still did not show any response to heart beat. I took the wires and made them touch directly instead of using the electrodes like the diagram shows, with again no improvement.

 

[audioguru]

Your simple circuit has its inputs too close together on the patient.
Also its gain is much too high causing it to amplify its own input offset voltage.
On the ECG tests I have had, the probes were stuck to me maybe with conductive adhesive?

 

[crutschow]

the inputs of the AD620 are floating and missing a bias voltage near 0VDC.

Isn't that what the AD705 op amp does?

 

[Lightium]

maybe electrolytic gel?

 

[danadak]

If you use G = 1000, tie the two inputs together, with Vos ~ = 125 uV
you will get ~125 mV out. The inputs do have an internal bias source
so that should be OK to tie together.

@ G = 1000 you have ~ 120 db CMR so thats excellent.



Regards, Dana.

 

[danadak]

I tried using 2 9V batteries to get +/-9V and when I set the gain to something large like 250 it just rams into one of the rails, often the negative one. I set the oscilloscope to DC so that it wont distort the low frequency signal but it is still dead flat. I can not see any movement.

Thats a clue because that should not happen. If you tie the inputs together, set G = 1000,
you should only see ~ 125 - 150 mV on output, per specs.

Is there a wiring error generating a large input signal ?


Regards, Dana.

 

[Fluffyboii]

Thats a clue because that should not happen. If you tie the inputs together, set G = 1000,
you should only see ~ 125 - 150 mV on output, per specs.

Is there a wiring error generating a large input signal ?


Regards, Dana.

I think it is something to do with the electrodes. As you said when two inputs are tied together it is around 0V so there is not much off set from the amplifier itself. But when I touch the electrodes off set appears. I do not have the proper gel for them but maybe I should try different substances as rjenkinsgb said. I have some quality shielded audio cable maybe I should use it for electrode connections. I doubt it would help since I was using very short cables to begin with. I will try setting the gain to 1000 and see what it does. It is achieved with 50 ohm Rgain resistor it seems.

 

[audioguru]

Isn't that what the AD705 op amp does?

The circuit from the datasheet shows the opamp that is not powered.

Isn't that what the AD705 op amp does?

 

[Fluffyboii]

With some water based skin cream on electrodes I was able to get some pulses about 1Hz that looks like what I wanted but it quickly went to positive rail as usual later. Switching the connections of left and right hand caused it to go negative. So there is indeed a voltage build up on electrodes that is causing the amplifier to saturate. I tried shorting the inputs for a while in hope that maybe the potantial difference would settle out but as soon as I separate them voltage goes to rails. It is not caused by the of set of the device but the electrodes. I wish I could just use bypass capacitors to filter it out.

Oscilloscope drains my cheap AA batteries to fast btw. I have a lead acid battery and I first tried using a buck converter to get it to 9V and plug that into the oscilloscope but it did not work for some reason. Maybe I should use a linear converter with the lead acid battery.

 

[Fluffyboii]

Your simple circuit has its inputs too close together on the patient.

I was using electrodes on arms instead of putting them on the chest as the simpler circuit suggests. I tried that as well. Off set is not high enough to saturate at 1000 gain.

The circuit from the datasheet shows the opamp that is not powered.

What does that mean? That op amp also needs to be powered and it is just not shown on the schematic right? I used TL072 to replace it and use the second op amp for second gain stage in my first try.

 

[Nigel Goodwin]

maybe electrolytic gel?

Better to get some proper stick-on electrodes, he's not going to get any sensible readings trying to hold contacts on to his slippery skin.

My heart monitor (standard Ant+ gym type, to connect to the gym machines) which I use to connect to my walking GPS unit uses a chest belt with conductive rubber contacts on the inside, and two press studs on the outside where the electronics unit plugs on. You simply moisten the rubber strips, fit the belt round your chest (with the rubber strips across the centre of your chest) and click the transmitter in place. The act of clicking the transmitter in place causes it to switch-on.

Obviously, for a proper ECG stick-on electrodes are used - and I was covered in them

First lot - the Ambulance crew covered me in electrodes, and took me to Hospital.

Second lot - the team at A&E covered me with a second set - what was wrong with just plugging in to the existing ones?.

Third lot - got send to a Hospital ward, where they applied a third set - what was wrong with using either of the existing two sets?.

And if they are going to apply new ones, why not remove the old ones?.

Perhaps the OP might find this useful?

AD8232 Heart Rate Monitor Hookup Guide - SparkFun Learn

 

[danadak]

Page 16 of datasheet has some specific notes about source Z imbalance and CMR issues
and Vref grounding and cable C and.....


Page 6-21 .....https://www.analog.com/media/en/training-seminars/design-handbooks/designers-guide-instrument-amps-complete.pdf

https://www.analog.com/en/design-center/landing-pages/002/apm/ecg-solution-2011.html

https://www.analog.com/en/applications/markets/healthcare-pavilion-home/vital-signs-measurement/electrocardiogram-ecg-measurement.html

https://www.analog.com/en/analog-dialogue/articles/ecg-front-end-design-simplified.html

Regards, Dana.

 

[audioguru]

Please post your schematic with all resistor values and power supply voltages.
Is your plus and minus power supply completely isolated from earth like batteries are?
Is the patient also completely isolated from earth?
Is the output amplifier also completely isolated from earth?

Are the probes on the arms or on the chest?

 

[Fluffyboii]

Please post your schematic with all resistor values and power supply voltages.
Is your plus and minus power supply completely isolated from earth like batteries are?
Is the patient also completely isolated from earth?
Is the output amplifier also completely isolated from earth?

Are the probes on the arms or on the chest?

Circuit was the second one from the first post. Plus and minus supply were 9V batteries. Oscilloscope was also being powered from separate batteries. I was sitting on plastic chair and was not touching the ground when trying it out. So I can say everything were separated from ground and each other. Probes were on arms and leg. I also tried touching croc wires directly over my heart as shown on the schematic. Lack of proper electrodes may be the problem but I think that simple circuit may be simply not cutting it. I need to build the first one. I think adding DC blocking after it with lower gain to not saturate to rails then getting additional gain can solve the issue.

 

[Fluffyboii]

Page 16 of datasheet has some specific notes about source Z imbalance and CMR issues
and Vref grounding and cable C and.....


Page 6-21 .....https://www.analog.com/media/en/training-seminars/design-handbooks/designers-guide-instrument-amps-complete.pdf

Circuit on there seems interesting.

I am not sure how those values are determined but I think I need low gain at the diff amp and that 4.7uF cap and 1Meg resistor for 0.03Hz high pass filter then amplify it about 100-150 times like the first circuit at the first post. Are those diodes regular silicon ones limiting the input voltage to +-0.7V. It then centers the signal around 2.5V for the ADC since it only works with positive voltages up to 5V which is excellent since Arduino is the same. There is like 5 order filter after it to filter anything more than 160Hz but I think that is bit extreme for my purpose. Not sure how the leg drive op amp gain is determined though. Maybe I should just solder everything if that will make it work better.