Replacing Lm741 with tl082

[afnan311]

I am trying to simulate this circuit: https://www.instructables.com/Electrocardiogram-ECG-Circuit/
But I can't use LM741. Only TL082 opamps. What changes should I make?

 

[KlausST]

Hi,

Changes? Replace the LM741 with the TL082.
Test it and see what happens.

If there are any problems, then give us a detailed description of the problem.

Klaus

 

[Audioguru]

The LM741 is a single opamp but the TL082 is a dual opamp with completely different pin numbers.
Use TL081 single opamps instead that has the same pin numbers as an LM741.

 

[danadakk]

Keep in mind, as the circuit is an IA you expect to have high CMR (Common Mode Rejection)
to get rid of signals that contaminate your signal path. Like lighting controllers, SMPS power
sup[plies, poor grounds....Your fdbk resistor accuracy most important when you are doing this.

An example -

Thats why most designers use IAs that are laser trimmed in production.

Topologies w/o high precision components, attached.

Attached R calculations for CMR.



Regards, Dana.

 

[KlausST]

Hi,

AOL difference if 50% is huge ... especially when both Opamps are on the same die (TL082)
AOL of TL082 is much higher than LM741.

On the other hand 60dB is not that bad for this application.

Klaus

 

[danadakk]

AOL difference if 50% is huge ... especially when both Opamps are on the same die (TL082)
AOL of TL082 is much higher than LM741.

I would definitely take this to a production engineer for comment.

I will contact AD on this. As a past production and test engineer I would not
bet the farm on this.

Multiple OpAmps on die to create IAs are Laser trimmed because of variations.


Regards, Dana.

 

[dick_freebird]

There's a big difference between "normal" and "worst we would like to be able to ship" IME. We always sandbagged to 3 sigma test / char data (or wider if a hard to test param) on the datasheets.

 

[KlausST]

Hi,

OP talks about simulation, not about mass production of a professional medical device.

The circuit behind the link is clearly from a hobbyist..meant for hobbyists.
The description is incomplete and so is the whole circuit. There are a lot of errors in the description.

Thus I see it rather relaxed.

Klaus

 

[danadakk]

From the LTC ap note for starters -

Regards, Dana.

 

[KlausST]

Hi,

Are you sure you talk about the OP's circuit / requirements?
Hes worried about using TL082 instead of LM741

I mean they (in the link) used two single LM741 and you talk about laser trimmed instrumentation amplifiers.
But I agree with you: They don't play in the same league.

Klaus

 

[danadakk]

He is trying to build an IA out of discretes and 3 amps with a dual amp, in an
environment with very high 50/60 cycle CM.

Yes I am sure.


Regards, Dana.

--- Updated Apr 29, 2022 ---

afnan311, here is an excellent reference for using IA's -

https://www.analog.com/media/en/training-seminars/design-handbooks/designers-guide-instrument-amps-complete.pdf

Regards, Dana.

 

[Audioguru]

He is making a simulation, not a real circuit.
Don't all separate opamps in a simulation match perfectly and there is no common-mode interference?

 

[danadakk]

He is making a simulation, not a real circuit.
Don't all separate opamps in a simulation match perfectly and there is no common-mode interference?

Yes if ones sims are with ideal components....no simulating spreads and tolerances, no Monte Carlo
sim. But then we are assuming thats all he is going to do.

OP, is this an exercise or a real design goal ?

Regards, Dana.

 

[danadakk]

Hi,

AOL difference if 50% is huge ... especially when both Opamps are on the same die (TL082)
AOL of TL082 is much higher than LM741.

On the other hand 60dB is not that bad for this application.

Klaus

The answer I got from Analog Devices is that variation between amps would
be covered by min/max values speced in datasheet for Aol and BW. The
response frankly did not come from either a Test EE or a Production EE, they are
generally speaking keepers of device characterization responsibility, as
he included "if there are any variations". You can look at dice layout handbooks
and in these generic opamps, they are not interdigitated structures, therefore
each amp experiences different thermals and/or bias and/or parasitics and/or....

Here is a 324 die, clearly not inter digitated.

The LM324 Aol spec is 25K min to typ 100K, so variation could be quite large.

This all supports why IAs are laser trimmed on production line to get the performance
needed.


Regards, Dana.