Op Amps: Instrumentation Amplifier

[@jlev505]

Great video. I just got a bunch of Analog Devices 524B instrumentation amplifiers. I’m going to play around with them using audio…. Hopefully it sounds as awesome as they look. Nice ceramic 16 DIP with a pretty little gold plate on the top of the ceramic body. Thanks for the video. 👍

[@Sctronic209]

Would this work for a heart monitor input? Plus some isolation.

[@ElectronicswithProfessorFiore]

Yes, instrumentation amplifiers are designed for applications such as that. Measurement devices often require high input impedance to minimize loading, high gain to deal with small signals, wide bandwidth (both low frequency and high frequency, depending on what is being measured), and high common mode rejection to cancel out noise.

[@GTS00000]

Great, what spice frontend are you using?

[@ElectronicswithProfessorFiore]

TINA-TI. Free from ti.com, for Windows.

[@nickcaruso]

One question occurs to me: are there any applications where this would be a poor choice, not from a cost perspective, but applications where this would perform more poorly than another kind of amplifier?

[@ElectronicswithProfessorFiore]

Not quite sure what you're asking here, but if you need a high CMRR with high gain/bandwidth and a high Zin, this is the way to go. If you don't need those things, then yes, this could be overkill (i.e., greater cost and board space with no real performance benefit).

[@nickcaruso]

@@ElectronicswithProfessorFiore rephrased: if cost and space were no object, would this amplifier always be the right choice?

[@ElectronicswithProfessorFiore]

@@nickcaruso Cost and space are always issues in real world engineering. What would be the point of using this topology if its unique features were not taken advantage of? It sounds like you're venturing away from engineering and into marketing, as in "this is the best circuit". As I tell my students in lecture, there is no such thing as "best" in the absolute sense; there is only optimal fit for the circumstances. It doesn't matter if you're talking about cameras or bikes or computers or amplifier circuits. There is no universal best because applications/needs differ.

[@windward2818]

In practical instrumentation design an all integrated solution will have the best CMRR. As a comparison to your circuit you can use an Analog Devices AD8220 which has a JFET front end. Often overlooked is the fact that the input to the instrumentation amplifier is an external connection exposed to ESD and the output usually goes to an A/D, data acquisition system, or an oscilloscope so you need some sort of line driver to drive a coax cable even if not terminated. The output also has to be ESD protected. Many times if you are driving an A/D input you will want a low pass filter to remove aliasing, but you can't usually drive an A/D input sample and hold with a filter high output impedance, so you need a buffer amp anyway. So, if you want to drive a scope input with a three foot coax you would want a buffer amp like a TI THS3120 Current Feedback Op Amp (this is an older non RTR output device so you will loose some output headroom with +/- 15V supplies) with a 50 ohm series source resistor (49.9 Ohm). An optional cable driver part would be the older LT1206 which is available in the SOIC8 package. Both the LT1206 and THS3120 are unity gain stable non-inverting. So, you could have a switch to set the gain for 1 without scope 50 Ohm termination, or to a gain of 2 (slightly more peaking) with a 50 Ohm scope termination. To protect the output from ESD you could use a bidirectional TVS, and perhaps a small output capacitor like 100pF NP0 Ceramic. The supply rails have to be capable of driving very high momentary current. So probably a 47uF Tantalum and 1nF ceramic directly at the supply pins of the current mode line driver op amp. Designing instrumentation is not easy especially if you want good DC accuracy along with high bandwidth.

[@ElectronicswithProfessorFiore]

I agree, and I talk about some of these issues and integrated solutions in my free OER text book (links in Description). To be clear, these videos are designed for general engineering education purposes, and are not intended to show exhaustive designs or "best in class" solutions. They are intended to complement the text and be of reasonable length. That's one reason why I tend to use common, easily (and inexpensively) sourced parts, so that students and hobbyists can breadboard and experiment with these circuits on their own.

[@MellexLabs]

This looks like the beginnings of a high precision millivolt meter. With a 10Meg resistor divider i could probably measure into the millivolt precision on many voltage ranges... thanks for your lessons i thoroughly enjoy your teaching

[@ElectronicswithProfessorFiore]

Yep. You'd want precision op amps to minimize the offsets, of course.

[@krakamak]

I am trying to understand the difference the introduction of R2 makes from a more practical point of view. Is that correct, that by adding that resistor, the final CMRR becomes the added value of the two stages' CMRRs?

[@ElectronicswithProfessorFiore]

R2 creates a cross-coupling between the two sides, increasing performance. I cover this in some greater detail in the text (free download-see links in video description).

[@krakamak]

@@ElectronicswithProfessorFiore thanks, I'll check it out!

[@VandalIO]

I was looking for this

[@VandalIO]

Awesome !