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audioguru what do you think if i will use a TL071C non-inverting amplifier to set the Gain to 143 for the last stge amplification?for susre TL071C is much better than a LM741 yes?
That scope photo looks like 60 Hz pickup saturating the amplifier. That needs to be investigated before going any further with circuits after the AD620. Personally, I would divide and conquer. Take the 741 amp circuit out, altogether. Set a single gain resistor on the AD620 to something that you can measure on your scope at the AD620 output. The differential ECG signal is about 2 mV, so set the gain resistor for a good, measurable signal on the scope but no greater, at first. Connect the right leg to circuit ground, and the differential probes where they are supposed to go. If you see nothing, raise the gain by changing the gain resistor until you do see something, or never anything, whichever comes first. Record what happens.
Even before hooking up to your body, connect an artificially created mV source with known amplitude, say from a battery supplied voltage divider, to the input of the AD620 through your probes and ensure that the output is clean and and of the expected amplitude using an oscilloscope.
Edit: Something of concern to me is the power supply voltage is only +/- 3V to the AD620. I seems to me that anything attached to the body with high input impedance may easily see greater than 3 volts of 60 Hz pick-up, which will exceed the common-mode voltage range, so the AD620 cannot reject it. A increase in supply voltage may be in order.
The AD8031 can operate at very high frequencies and might oscillate in your circuit.
The TL071 has a problem called Phase Inversion that might cause trouble when its supply voltage is low.
The AD706 is a low frequency dual opamp that will work fine.
thanks a lot..and a last guestion,,i will use the AD706 for right leg driver as shown on my cct shcematic,can i also use this AD706 for the last stage of the cct which is the ampification of G=143 as a non-inverting amplifier?or i shall use a different op-amp for the amplification stage?
Use one of its opamps to drive the right leg and use its other opamp for the G=143 amplifier. Keep the parts for each circuit and pcb traces separated.
Thanks a lot ccurtis for your help..i will do as you told me step by step by checking the output of each part of the cct..about the voltage supply of the AD620 +- 3v is from the data sheet of the AD620 and says to supply the amplifier from +- 2.5 v to +- 6 v.and on the ecg shcematic in the AD620 datasheet the power supply of the whole cct is +-3v..when i check the output of the cct on the oscilloscope i could see a signal only when the power supply was 3.5v.less or above this supply the signal dissapeared..so what do you think i shall do?
..when i check the output of the cct on the oscilloscope i could see a signal only when the power supply was 3.5v.less or above this supply the signal dissapeared..so what do you think i shall do?
That signal you are seeing is not a proper ECG signal and most likely 60 Hz pick-up, maybe oscillation. Whatever it is, it is saturating an amplifier with a gain of only 7. If it dies after you raise the supply voltage (all other things in working order), that is a good thing, not a bad thing. At that point, since you are not seeing any signal, it is probably (all other things in working order) because you don't have enough gain, insufficient input, or o-scope v/div scale is not set low enough to see the signal. The AD620 cannot run on supply voltage below +/-2.3V. +/-3 volt supply is too close for comfort, as far as I am concerned, especially when you need all the common mode rejection you can get in this application. BTW, the AD620 can run on supply voltages as high as +/- 18V.
ok i will try to follow your procedure of checking the ouput of each stage starting with the AD620 and see what is the gain that required to see a signal. and i will set the power supply from +- 5V to +- 9V to see how is performing..i hope it will work like that.
and something else that i notticed is that i have to add decoupling capacitors close to power suppy inputs to reduce noise by grounding voltages..i am correct?
and something else that i notticed is that i have to add decoupling capacitors close to power suppy inputs to reduce noise by grounding voltages..i am correct?
Of course, that is standard practice. Yes, do use those. You are the eyes and ears. The scope photo is good, but in the future include the vertical v/div, timescale, state where the 0v reference is (middle graticule of scope, or wherever), and if the input coupling is AC or DC, preferably DC so any DC offset is shown. When you get a good ECG signal then maybe add that other op amp and test again, and maybe that will allow you to reduce the power supply voltage. I suspect you may very well find that you don't need to add any of that stuff after the output of AD620.
oh yes i ve seen again before this cct and i wanted to buy the AD624 but is expensive one and this project must be efficient and cheap.so i decided to build the cct using AD620.
really thanks for the information you gave me and i will try to make it work soon
oh sorry somethng last..the cables that i am using to connect my body with the amplifier are coaxial cables..and i connected the inner core with aligator clips and the other end the inner core on the amplifier input and the shield wire to the cct ground..is that correct?
oh sorry somethng last..the cables that i am using to connect my body with the amplifier are coaxial cables..and i connected the inner core with aligator clips and the other end the inner core on the amplifier input and the shield wire to the cct ground..is that correct?
That should be fine. Keep the cables as close together as possible to reduce the antenna loop area between them. As an experiment, connect the body ends of both shields to the right leg and do away with the third coax cable. Heck, try simple wires without coax and see the difference. The AD620 should substitute pretty well for the AD624. Don't get too hung-up on the exact parts. Understand the circuit and how it is supposed to work. Then can you make measurements and experiment with the parts you have to understand why it isn't working.
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