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 139856Unless 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 may be a problem.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.
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.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?
Isn't that what the AD705 op amp does?the inputs of the AD620 are floating and missing a bias voltage near 0VDC.
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.
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.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.
The circuit from the datasheet shows the opamp that is not powered.Isn't that what the AD705 op amp does?
Isn't that what the AD705 op amp does?
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.Your simple circuit has its inputs too close together on the patient.
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.The circuit from the datasheet shows the opamp that is not powered.
maybe electrolytic gel?
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.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 on there seems interesting.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
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