biopotential amplifier - help needed

[raviram87]

hello people...

We are working on a project to control certain home appliances such as a tv using eye movements. The first step of this project involves the extraction of signals caused by the human body due to eye movements.

since these signals are extremely small, (250 to 1000uV) we had to amplify them. We designed an instrumentation amplifier of gain around 1000 using LM741 op-amps. However still we couldn observe the signals on the CRO in our lab. all that we could see was noise.

We tried to purchase instrumentation amplifier such as AD620 and Ad622. Unfortunately they are not available to us.

What possibly could have gone wrong? and pls offer suggestions on how our task can be accomplished.

 

[Nigel Goodwin]

First off, don't use a 741, it's a very old, very low spec opamp - and probably the noisiest one you could have used.

 

[raviram87]

i agree with you sir. had a very bad experience with it today... there was nothin on the CRO other than noise....

we tried our level best to purchase ad620.. that was not available... we are still trying...

in the mean time wat can be the alternate methods we can try?

 

[Nigel Goodwin]

Use better quality modern opamps and build an instrumentation amplifier with them.

 

[raviram87]

in this project we are lloking to amplify extremely low magnitude signals... say of the order of few uV...

what category of op-amps is operatable with such low voltage signals???

 

[ericgibbs]

in this project we are lloking to amplify extremely low magnitude signals... say of the order of few uV...

what category of op-amps is operatable with such low voltage signals???

hi,
Look at www.analog.com website for amplifiers and applications.

 

[raviram87]

thank you for your suggestions sir...

Can you people also help me with electrodes? to record the potentials comin out of the body we have to place certain electrodes at various locations on the face...

for this we are using normal ECG electrodes.... is this ok?

and we placed these electrodes on our skin and tried to c the signals on the CRO... but again only noise was visible... is this due to the fact that the signals being extremely small in amplitude get dominated by the noises and hence only noise is visible? or is it due to any other fault?

if we use the amplifiers (say typically with low input offset voltages) will we be able to observe the waveorm?

 

[ericgibbs]

hi,
I Googled for ecg amplifier designs there is alot of information.

**broken link removed**

I would suggest that you design and build a standard ecg amplifier of known quality and develop that for your project.
The micro volt levels for ecg require similar 1000 gain amps and filters.

 

[KMoffett]

Google: EOG amplifier circuit

Lots of circuits, papers and supporting info.

Ken

 

[audioguru]

An instrumentation amp IC has low noise, low input offset voltage and extremely well matched resistors inside.
That is why they are used for the circuit you are trying to make.

 

[mneary]

Where are you, that ad620 is not available to you? Is it export restricted, too expensive, shipping problem, what?

Please put your location (at least your country or continent) into your profile? With location information, maybe one of your countrymen can help solve your purchasing problem.

 

[audioguru]

Where are you, that ad620 is not available to you? Is it export restricted, too expensive, shipping problem, what?

I think it is not available on Venus or Mars.

 

[raviram87]

i am from bangalore in india.... its not available in the college in which we study... thers is a particular place in our city where we get all kinds of ICs.. we couldn find it there too... i contacted one of analog's dealers in bangalore... but they say its difficult for them to get the order if its not in bulk... :-(

as u rightly put it, if any of you can help me with this it will be much useful to us....

 

[audioguru]

Why don't people in India ask about which parts to use in a forum in India??

Here, we discuss the best and the latest parts and circuits from all over the civillized world. They are available at many places over here.

 

[raviram87]

hey people out there...

i tried my level best to find ad622 and ina126p here in few places... its just not available.... just one shop had it here.... he is quoting a very high price (in indian rupees), more than what is said in the companys website...

can u suggest me an alternative to these two instrumentation amplifiers? we were looking for these ICs because we have to amplify extremely low voltage signals... the LM741 was not working out... all that we could see was just noise.... :-(

what kind of an alternative must we use?

 

[Leftyretro]

hey people out there...

i tried my level best to find ad622 and ina126p here in few places... its just not available.... just one shop had it here.... he is quoting a very high price (in indian rupees), more than what is said in the companys website...

can u suggest me an alternative to these two instrumentation amplifiers? we were looking for these ICs because we have to amplify extremely low voltage signals... the LM741 was not working out... all that we could see was just noise.... :-(

what kind of an alternative must we use?

An Instrumentation op amp can be made, wired up from standard op amps like you did with your 741 circuit.

Just research for the best avalible op amp that you can find locally. It should not be hard to improve on your 741 experiance. I'm sure you can find a modern low noise op amp if you study the specs and seek out the best avalible to you.

https://en.wikipedia.org/wiki/Operational_amplifier_applications

Lefty

 

[audioguru]

You might be able to make your own instrumentation amplifier if you can get modern low noise opamps and very high accuracy resistors.

What do other electronics students do when they can't get modern parts in your country?

 

[littletransistor]

TS: have you tried Farnell India?

Also, try using the AD620 - it's cheaper, and even the INA126 works well too. And make sure you have electrodes to properly receive biopotentials - basic wires isn't going to cut it.

 

[Hero999]

Why don't people in India ask about which parts to use in a forum in India??

Here, we discuss the best and the latest parts and circuits from all over the civillized world. They are available at many places over here.

I'm normally not a sticker for political correctness but implying that India is not a civillised country is not on.

 

[audioguru]

I'm normally not a sticker for political correctness but implying that India is not a civillised country is not on.

Sorry, I should have called India a 3rd-world country or a developing country or an undeveloped country like in Wikipedia. They also use terms like high poverty rate and uncontrolled high birthrate. India is mentioned many times.