DRL Feedback design

Alberto Gancedo

Other Parts Discussed in Thread: INA333, INA2141, OPA333, INA132, OPA2188, INA141, OPA2180, ADS1298, TINA-TI

Hello,

I am trying to develop a low power EEG measurement system based on the INA333 ECG example showed in its datasheet. As I am planning to realize the design with the INA2141 I don't know if this is the best option (as there is no model of the INA2141).

I am not 100% of the proper result plot of the 1/Beta graph and I am having troubles to plot the real open loop gain of the OPA333 in order to get the frequency compensation done.

Some pictures are attached to clarify the questions.

Am I missing something or the results are the expected?

Regards,

Alberto

over 7 years ago

0 Pete%20Semig over 7 years ago

TI__Mastermind 25795 points

Hello Alberto,

From what I understand of EEG you will require lower noise than what the INA333 or INA141 provide. A couple options include an integrated analog front end solution (ADS1298), or perhaps a hybrid instrumentation amplifier (e.g. OPA2188 or OPA2180 for input stage and INA132 for output stage). I'm not sure if the ADS1298 will meet your low power requirement, though. For the hybrid option you will require precision resistors for the feedback loops. In addition, please be aware that the OPA2188 and OPA2180 can swing to within 1.5V of the supply voltage. Therefore your common-mode input range given a 3.3V supply is 0<Vcm<1.8V.

There is also a lot of additional information in the following FAQ:

e2e.ti.com/.../2437.ecg-heart-signal-acquisition

The "Design considerations for ECG Systems" may be of particular interest.

Hope this helps!

0 Alberto Gancedo over 7 years ago in reply to Pete%20Semig

Prodigy 20 points

Hi Pete,

For sure it helps, but my main problem now is the ability to simulate the frequency response of some parts of the circuit. I understand that there are many other posibilities to design the INA, but as my main concern is to reduce the noise to the minimum, adding resistances will increase the noise of the circuit by a lot. Because of that, I was looking for an alternative device to INA2141 in order to get almost the same response in the simulation.

As you said, the ADS1298 doesn't meet the low power requirement, so it is not an option.

Is there any instrumentation amplifier with the same (or close enough) transfer function that I can trust while designing the circuit? I am planning to use the INA2141 in the PCB so I just need to design the circuit with respect that component.

Also, thanks for the link to the FAQ section, there is plenty of useful information in there.

Regards,

Alberto Gancedo

0 Pete%20Semig over 7 years ago in reply to Alberto Gancedo

TI__Mastermind 25795 points

Hello Alberto,

If you're primarily interested in the noise and transfer function, I recommend building a discrete INA141 using ideal amplifiers and adding noise sources externally. I have created a rudimentary TINA model (attached) that seems to work pretty good with respect to noise and functionality. Please understand, however, that it uses ideal op amps. Therefore all other parameters are not modeled (e.g. offset voltage, BW, common-mode voltage, output swing, etc.). Note that I only added voltage noise sources. You may want to add a current noise source. If you do this, you can find the current noise source in TINA-TI under File->Open Examples->TI Examples->Noise Sources. Finally there is a good presentation on INA noise that can be found here:

e2e.ti.com/.../3514.instrumentation-amplifier-noise

I hope this helps. Best of luck!

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