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ECG system, signal distorted please help

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integralx2

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I have built an ECG system. What I am not sure is why the signal is still distorted. Now i think its the wiring. I am using just basic breadboard wiring that came with breadboard. Maybe its not shield enough ? What i am struggling still to understand is why do we connect an electrode to the lower leg? What does it provide ? The instrumental amplifier i am using is an AD620. I have included the signal i get where it says oscilloscope connection. PLEASE ANY HELP WOULD BE GREAT.

THe amplifier on the most left side is a TL071 then going to the right its the AD620 then another TL071 then last amplifier is a LM339. The system is correctly setup, and this design does work! (just mine does not).


**broken link removed**


**broken link removed**
 
The leg electrode is actively driven to a voltage which is supposed to bias the (subject's body) right and left arm electrodes right in the middle of the instrumentation amplifier's common-mode range. What is inside the box labeled "patient isolation"?
 
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The waveform looks about right more or less. I'm not sure where all the distortion is coming from. Maybe you need to use shielded wires to connect the electrodes on the patient to the circuit. If you're interfacing this circuit with a PIC, the waveform doesn't have to be perfect and distortion free. Just check the ouput of the comparator and set the hysteresis so you don't get multiple triggers per heartbeat.
 
THe issue i am having now, is when i connect all 3 leads i get no signal,but if i disconnect lets say my left arm, i get somesort of result on screen. What can be causing this
 
You can't make such a high gain circuit on a bradboard that has anenna wires connecting everthing together and most of them picking up mains hum. Make a compact pcb and maybe put it in a metal box connected to 0V.
Are wires the between the patient's conductive pads and the electronics shielded wires?
 
i got conductive pads on the patient (me , lol).I have antenna wire (breadboard wire i assume you mean) running from the breadboard to alligator clips that clip onto the conductive pads. If i need shielded wire, where do i buy it and what kind ? I bought some pads online that are used in the hospital. Is there anyway i can use a breadboard for this ? I cant really afford using a pcb. Can i build my own pcb ? where do they sell the boards ?
 
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I think ordinary shielded audio cables should be used to connect the conductive pads at the patient to the shielded circuit pcb.

Breadboards are used by kids in school to demonstrate DC circuits. Maybe you could use a breadboard for an ECG system if it is in the middle of nowhere (a desert?) so there are no radio stations and no mains interference nearby.
 
I think ordinary shielded audio cables should be used to connect the conductive pads at the patient to the shielded circuit pcb.

Breadboards are used by kids in school to demonstrate DC circuits. Maybe you could use a breadboard for an ECG system if it is in the middle of nowhere (a desert?) so there are no radio stations and no mains interference nearby.

So i should just use audio cables, and cut the endings and put onto alligators clips and connect to the conductive pads ?
 
The shield of the audio cable can connect to ground, 0V or the output to the right leg at the circuit board. The shield must be insulated and not connected to anything at the patient.
Doctors and hospitals do not use alligator clips. They use conductive pads that have a jack and their cables have a matching plug.
 
I got a quick question, i connected an LED at the output of the AD620, shouldnt it blink with each heart beat ?
Its datasheet says it can drive a 2k ohm resistor when it has a 30V supply. An LED will short its output (then the trace will look like a square-wave) but it should light if has the correct polarity and if the voltages are high enough.

The AD620 can drive the series current-limiting resistor which is the base resistor for a transistor then the transistor can drive an LED without affecting the trace.
 
Ahh k that makes sense. Now another issue( not sure if that is an issue or not) on the TL071 amplifier, when i look at the output of it it says like 11V DC (pin 6 on datasheet). Isnt that wrong? Isn't that to high ? I am using about 12.5 V DC to power the AD620 and both TL071 (not 9V).
 
on the TL071 amplifier, when i look at the output of it it says like 11V DC (pin 6 on datasheet). Isnt that wrong? Isn't that to high ? I am using about 12.5 V DC to power the AD620 and both TL071 (not 9V).
Is your supply plus and minus 12.5V? Or only +12.5V with no negative supply?

The output of your TL071 is nearly saturated as high as it can go. It should be near 0V with some mains hum and small DC variations on it.
Maybe one input to the AD620 is not connected.
 
I thought (this might sound stupid) if an amplifier says +12.5V on this side and -12.5V on that side , then i run positive from battery to +12.5V side and te negative part of battery to -12.5V side. Am i correct or i dont understand something
 
A 12.5V dual polarity supply needs positive 12.5V and negative 12.5V (a total of 25V). You only have positive 12.5V (and 0V) and do not have negative 12.5V so your circuit will not work.
 
The issue i am currently having is right now , is if i only connect 2 electrodes i get a + voltage at the output of the 2nd TL071, but if i dont its negative ( my LED lit up when positive). So i connected to the LM339. I put an led on the - side input of the lm3339 and used a pot and i can seee there is voltage there. The output of the TL071 i connected to the LM339 at the + side. THen at the output i put an led on it, but it does not light up. I dont understand why there is no voltage there. I very the pot , and still nothing happens.

LM339 Datasheet pdf - Low Power Low Offset Voltage Quad Comparator - National Semiconductor

I am using a texas insruments LM3339. So on the pin i have +5V to Vcc, and pin 12 is gnded. Then i have pin 6 being fed to the varying pot, and pin 7 is fed from the output of the tl071. So that makes pin1 my output. Thats how i connected it.


I am doing this still on breadboard, but it should work if i get led to turn on at the output of the 2nd tl071 and its being fed into comparator (lm3339)


also , when i have all 3 connected, and if i press down on one of them , then the light turns on at the output of the second TL071.
 
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Doctors and hospitals do not use alligator clips. They use conductive pads that have a jack and their cables have a matching plug.

It depends on the ECG machine. I've used many machines with these electrodes. The tab connector is a crocodile clip. **broken link removed**


OP one very basic thing that may give some improvement along with adequately shielded cables that are kept as short as feasibly possible is skin prep. A good rub of the electrode sites with isopropyl alcohol will lower skin impedance and lessen interference artifact. Adequate skin prep was the first thing drilled into me in college.
 
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