...which is serving as the ground reference voltage for the signals. Your power supply ground looks like -2.5V (or -3V in the original circuit) to the amplifiers.

 

ok thank you......please show the bandpass filter design and its working

 

I've been testing out various parts of the circuit on their own since I still don't have the overall circuit performing correctly...

I've been having a look at the integrator part of the circuit and I've built it stand-alone to test it. This is the setup I have:



Now all the theory I've come across suggests this should behave as a low-pass filter, and indeed this is the behavior I see when I test the circuit, however in the overall schematic of the ECG Amplifier it seems to be suggested that it should behave as a high-pass filter to eliminate baseline wander...

Can someone please explain how the setups differ?

Thanks...

 

The integrator is a triangle-wave generator when it has a comparator at its input that has feedback from the output of the integrator.

When the comparator senses that the output of the integrator is low enough then its output is low into the input of the integrator which makes the integrator make a rising ramp at its output. When the comparator senses that the output of the intergrator is high enough then its output goes low and the integrator ramps down.

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Uncle $crooge

 

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

Attached Thumbnails

 

 

Make - Volume 11

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.

 

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.

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.

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Uncle $crooge

 

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.

 

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.

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Uncle $crooge

 

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!

 

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.

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

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Ron

 

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

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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.

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Ron

 

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...