# ECG monitor design

Discussion in 'The Projects Forum' started by AndrejMiller, Feb 17, 2010.

1. ### AndrejMiller Thread Starter New Member

Feb 16, 2010
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Hi everybody!
I have to design ECG monitor for my study and I have the same circuit like KEANE2097 so i will attach it to post one more time for comfort.

1)So, how i have understood somehow 1-2mV ECG RA and LA differential signal have been centred to 2.5V and then go to A1, A2, A3 and A4 amplifiers. If is it so could somebody explain me where does this happen in circuit? In INA326? Then i do not understand how.

2)i counted the amplification coefficient for A3 part and made calculation for F0 frequency (which should be 100 Hz from picture) by my own and i got cut off frequency F=43.73Hz. By cut off frequency i think frequency when K=0.707Kmax (how i understand this frequency should characterise filter) . Where is mistake?

i attached my calculation and ina326 data-sheet

i would be grateful for any advise !

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Apr 20, 2004
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3. ### MikeML AAC Fanatic!

Oct 2, 2009
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The Right Arm and Left Arm electrodes are centered in the common-mode range of the INA by the two 40K resistors. Note that the voltage at the center tap between these resistors is buffered, low-pass filtered, and is used to drive the Leg electrode. This effectively "biases" the patient's body so that it sits at the magic voltage which puts the Right and Left electrodes square in the center of the common-mode range.

The output LPF is very simple. Gain at low frequencies is 46db, corner frequency at about 100Hz.

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4. ### AndrejMiller Thread Starter New Member

Feb 16, 2010
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[/QUOTE]Welcome to the forums.

That was an old thread you posted in - http://forum.allaboutcircuits.com/sh...ad.php?t=19555 - so now you have your own. [/QUOTE]

Thank you beenthere for your welcome and MikeML for your answer.

Do you wanted to tell that 2.5V voltage shift of EKG signal came from A2 amplifier?
and i didn't clearly understand what you mean by "...which puts the Right and Left electrodes square in the center of the common-mode range."

Please be patient to me because i am just starter

5. ### MikeML AAC Fanatic!

Oct 2, 2009
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"SQUARE" is an idiom. I meant that the patient's body is connected directly to the output of the bias opamp. The Leg electrode is actively driven to 2.5V. This puts the other two electrodes at a voltage which is centered in the common-mode range of the instrumentation amp.

6. ### AndrejMiller Thread Starter New Member

Feb 16, 2010
6
0
Thank you for replay MikeML. But still there is questions in my head.

If A2 amp amplify differential signal from "common-mode range signals (let think it is 125mV AC signal) and 2.5V(DC)" about 20 times it leads A2 to go into saturation in first stage (correct me if i wrong please). So Vin+ and Vin- will be drive with 2.5V(DC). Then, when INA amp will take middle arithmetical of 2 electrods and secondly give it to A2. A2 now will amplify about 20 times difference of 125mV(AC)+2.5V(DC) and 2.5V(DC) and it will be 2.5V(AC). If i understand correct this will take off the common mode range but what about 2.5V (DC) which we need in further stages?

Looks like i lost something with my understanding of those processes.
Please explain me where i am wrong.

Thank you in advance!

Last edited: Mar 28, 2010