ECG amplifier

[ccurtis]

**broken link removed**

You may be interested in the link above, where on the left in blue background, is a circuit which appeared in a 2000 issue of Scientific American magazine (Amateur Scientist column). It's a simpler circuit.

The leg connection is a ground lead. Even differential amplifiers need a ground connection between source and amplifier if there is a considerable voltage difference between the ground of the source and the ground of the amplifier. That difference appears as a common mode voltage which could exceed the maximum common mode voltage specs of the amplifier. Without the connection to the leg, the source ground is floating at who know's what voltage relative to the amplifier ground.

 

[audioguru]

For the attached RLD ... the 24.9k (R3 and R2) resistors are used for what and they do not in any way affect the gain right of AD620 right?

They must have exactly the same value (1% or better tolerance). They add the voltages at pin 1 and pin 8 of the IC which is the common-mode DC and low frequency AC signals from the patient to be cancelled when fed to the leg.

Secondly, can R3 and R2 can be any values ... like 290 ohms?

Of course not. They are in series and are in parallel with the gain-setting resistor. If they are 290 ohms then the gain of the instrumentation amplifier and the gain of the AD705 opamp will be much too high.

 

[gooshuk]

Sorry, my bad about the 290 ohms because my design used 1k for my RG so the gain should be around 135.

I thought R2 and R3 do not affect the gain of AD620 because in the diagram it showed G=7. When I take R2 and R3 into the calculation the AD620 gain should be around 8, and I can only get 7 if R2 and R3 are not included, so that was why I thought R2 and R3 do not affect the gain ... or maybe there is an error in my calculation.

 

[audioguru]

The datasheet shows how to calculate the gain.
A gain of 7 and a gain of 8 are almost the same so who cares. Just make the circuit as they show it.

 

[sapphire_blue]

I build my ECG amplifier with AD627 and it works fine. But I just have a quick question....do we need to use an Instrumentation amplifier or will a low-noise op-amp serve the purpose?

I know that istrumentation amplifier is used for differential signals to get high CMRR. ECG is also differential, isnt it?
Can someone clear this please?
Thanks!

 

[Sceadwian]

You may be able to use a cheaper opamp, it depends entirely on the quality of the output you need. All opamps are differential by nature.

 

[Roff]

You may be able to use a cheaper opamp, it depends entirely on the quality of the output you need. All opamps are differential by nature.

Three op amps are required to make a differential (instrumentation) amplifier, plus 7 precision resistors. It is generally easier to buy an instrumentation amplifier.

 

[Sceadwian]

Woops, I'm thinking funny =\ Sorry about that, I told you I wasn't good at the analog stuff =) I really should try a few opamp circuits to get my feet wet, cause I keep ending up having to stick them in my mouth =O

 

[Roff]

Woops, I'm thinking funny =\ Sorry about that, I told you I wasn't good at the analog stuff =) I really should try a few opamp circuits to get my feet wet, cause I keep ending up having to stick them in my mouth =O

One approach would be to not reply about subjects that are not in your area of expertise.
I try to live by that rule, and I still wind up with my foot in my mouth on occasion.

 

[keane2097]

Here is another ECG circuit that uses the right leg for cancelling the DC and very low frequency common-mode voltages:

https://www.electro-tech-online.com/attachments/ecg-circuit-png.26416/

Can anyone confirm for me that the output amplifier A3 in this circuit acts not only as a gain stage, but also as an anti-aliasing filter with a cutoff frequency of ~200kHz?

That's the only thing I can think it might be, although 200kHz seems too high for the purpose...

 

[Roff]

Can anyone confirm for me that the output amplifier A3 in this circuit acts not only as a gain stage, but also as an anti-aliasing filter with a cutoff frequency of ~200kHz?

That's the only thing I can think it might be, although 200kHz seems too high for the purpose...

The -3dB corner is determined by the 1MegΩ*1.5nF feedback network, and is at 106Hz.

 

[keane2097]

The -3dB corner is determined by the 1MegΩ*1.5nF feedback network, and is at 106Hz.

Are you sure about that? I would have thought it was the Cap and the 5kOhm resistor?

 

[audioguru]

Are you sure about that? I would have thought it was the Cap and the 5kOhm resistor?

When the capacitive reactance of the 1.5nF capacitor equals 1M at 106Hz then the gain of the opamp is reduced -3dB.

 

[keane2097]

When the capacitive reactance of the 1.5nF capacitor equals 1M at 106Hz then the gain of the opamp is reduced -3dB.

Ok cool, thanks both of you for the clarification. So A3 acts as both an amplifier and a low-pass filter correct? Boosting the signal with a gain dependant on the ratio of the two resistors while attenuating the signal above a cutoff frequency dependant on the relationship between the feedback resistor and capacitor?

Does anyone have any idea what the purpose of this filtration is in this application? Anti-aliasing maybe?

 

[Roff]

Ok cool, thanks both of you for the clarification. So A3 acts as both an amplifier and a low-pass filter correct? Boosting the signal with a gain dependant on the ratio of the two resistors while attenuating the signal above a cutoff frequency dependant on the relationship between the feedback resistor and capacitor?

Does anyone have any idea what the purpose of this filtration is in this application? Anti-aliasing maybe?

Apparently 100Hz is a standard decided upon by the American Heart Association. Not everyone agrees that 100Hz is adequate. Search "ECG bandwidth".
Any bandwidth beyond what is "adequate" will just pass unwanted noise and transients which will tend to make reading the ECG more difficult.

 

[keane2097]

Apparently 100Hz is a standard decided upon by the American Heart Association. Not everyone agrees that 100Hz is adequate. Search "ECG bandwidth".
Any bandwidth beyond what is "adequate" will just pass unwanted noise and transients which will tend to make reading the ECG more difficult.

Great stuff - thanks a lot. I've stumbled across a few very interesting websites due to that search...

 

[iqatie]

i want to detect the fetal heratbeat by using electrode per minute.Many people that i ask will say that this is crazy project. I just want to sure that it is correct that electrode cannot detect the fetal heartbeat? I want to use the instrumentation amplifier and low pass filter like the ECG circuit. Do you have any suggestion to me?

 

[audioguru]

How will you attach the electrodes to the unborn baby?
They use ultrasound or a stethoscope.

 

[duffy]

i want to detect the fetal heratbeat by using electrode per minute.Many people that i ask will say that this is crazy project. I just want to sure that it is correct that electrode cannot detect the fetal heartbeat? I want to use the instrumentation amplifier and low pass filter like the ECG circuit. Do you have any suggestion to me?

I don't think it's crazy. Yes, start with that ECG circuit, seems like it ought to work. Why do people think it's crazy?

 

[manoj443]

Can somebody please expalin me the working of this circuit?? its urgent



Can somebody please expalin me the working of this ECG amplifier circuit?? All opamps are TL074 and the bottom one which has B and C inputs is an AMP02.
Thank you....it's really urgent for me