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INA828 vs INA826 EMG

Mike Decaf
Prodigy 20 points
Other Parts Discussed in Thread: INA828, INA826

Hello,

My main question refers to amplifier selection, and I have a few other as well. I'm choosing an amplifier for EMG processing. 

(1) The ADA828 is more expensive than the ADA826. However, the 826 has better CMRR at high frequency.  Is there any reason why I would want the 828 over the 826? I understand the 828 has better input noise and VOS, but I think that CMRR is the most important aspect of an EMG circuit. Is this assumption correct? 

(2) I read the powerpoint on ECG design from TI (see attached). It mentioned AC coupling of inputs, but the conclusion was not indicated. The AC-cuopled waveform appeared distorted when compared to the "traditional" looking ECG waveform. This would indicate that AC coupling is frowned upon, but why does this occur?

(3) Say I'm using a system that is an Instrumentation amplifier followed by a series of band pass filters, then an ADC. Is there a downside to using as much of the instrumentation amplifiers gain as possible to achieve high CMRR? 

5430.ECG_Fundamentals_condensed.ppt

  • 0
    TI__Guru 71516 points

    Hello Mike,

    You state the AD828 in the SUBJ and the ADA828 in the text, and both of these are Analog Device parts, therefore, we cannot comment on the usage.  If you wish to do a design with a similar/better TI device, please look at the THS4222 or the LMH6642 and we would be glad to assist with any design questions you might have.

    Regards,

    ~Leonard  

     

  • 0 in reply to LeonardEllis
    Prodigy 20 points

    Ahhh whoops. I've been looking at instrumentation amps from all companies. I meant the INA828 and INA826.

    Is it possible to change the title?

  • 0 in reply to Mike Decaf
    TI__Guru 71516 points
    done!
    And I redirected this to the Precision Amps group so they can reply.
    ~Leonard
  • 0 in reply to LeonardEllis
    TI__Mastermind 23050 points

    Hi Mike,

    Here is a spec comparison of the INA826 and INA828:

    You can see the INA828 outperforms the INA826 in most regards.

    I am not an expert in EMG amplifiers so I am not exactly sure how the signals compare to an ECG, but I believe the principles are very similar and you are still dealing with fairly low frequency signals, and consequently you would likely be more concerned about the low frequency CMRR. The ECG presentation demonstrates how even very small tolerance variations in ac coupling capacitors can heavily degrade your low frequency CMRR due to an imbalance in RC time constants. This is why the servo method with an integrator feeding back to the reference pin is used, as it avoids the need for ac coupling capacitors. However it has to correct for the gained up dc offset and if there is not sufficient headroom for it to do so it will not produce the intended ac coupling effect. 

    Adding gain limits the usefulness of the servo method mentioned above. It is true that CMRR improves with gain, but keep in mind you will also be gaining up the noise of the device along with any externally coupled noise and any offset errors. Generally speaking it is best to place your gain in the front end to avoid gaining up the noise and offsets of downstream components, but sometimes it works better to limit your bandwidth before applying gain.