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OPA388: Chopping amplifiers for biopotential measurements

Alex Karnaukhov
Prodigy 95 points
Part Number: OPA388
Other Parts Discussed in Thread: OPA376, OPA141, OPA140

Hi all,

Are your chopping amplifiers, such as OPA388, are well-suited for biopotential measurements (as EEG,ECG...) ? 

Now I`m using conventional JFET-input amplifier with 500nV p-p noise in 0.1-10Hz band, it works normally, but I`m trying to upgrade it to OPA388. 

But with OPA388 I have a strange issue: noise level become very high (about 9uV p-p in 0.1-10Hz band, roughly white) when I touch inputs (shorted thru 10k resistors) whith my hand (look at point marked on attached sheme). Without touching this point noise level is almost close to datasheet values.

With conventional JFET op amp this "touching" leads only to DC offset, but noise level remains the same (about 500nV). But with OPA388 there is BOTH offset change and great noise increasing.

I can understand DC offset changing due to EMI rectification because, may be, my body serve as "antenna" for surrounding EMI sources. But why noise level grows? Or, may be, this noise comes from chopping transients? 

  • 0
    Guru 140200 points

    Hi Alex,

    don't you have any low pass filtering at the inputs of OPAmps?? Low pass filtering and input protection circuitry is mandatory in such a circuit!

    Kai

  • 0 in reply to kai klaas69
    Prodigy 95 points

    Low-pass filter (i.e. capacitor on non-inverting pin) is bad thing in biopotential measurement: input capacitance should be as low as possible and whenever possible equal at all channels. In other case there would be overal CMRR degradation. Consider 100pF input capacitance - for 50Hz it would mean only 32MOhm input impedance (it is not very good for biopotential measurement), and if input impedance of reference and channel differs only by 10kOm it leads to 70dB CMRR, not good.
    If external input capacitor is absolute requirement for chopper op amps for high-impedance sources - than it means them just not suited for biopotential measurements (and "medical instrumentation" tag in datasheet misleads me)...

    What about input protection - this is simplified scheme just for this forum. But with conventional JFET op amps we had not a single case of op amp failure caused by ESD even without external protection (we make EEG amplifiers since 2010). And first of all the problem is in noise. I`m sure, protection circuit wouldn`t help with noise.

    UPD. Now I see, that 200pF input capacitance concerned as acceptable solution for EMI-filtering purposes in many papers about biopotential measurement, but not more. I`ll try it, but it looks like a serious drawback...

    And still it is not clear: what is the source of such noise.

  • 0
    TI__Guru 60610 points
    Alex,
    Chopper amplifiers are NOT best suited for most applications with relatively high input impedance, like EEG or ECG, because of its higher current noise coming from the chopping action of the front-end switches. The higher current noise gets converted across the input resistors into higher voltage noise causing the problem you see. Instead, please take a look at using a non-chopper CMOS amplifier like OPA376.
  • 0 in reply to Alex Karnaukhov
    Guru 140200 points
    Hi Alex,

    your thread reminds me of this thread where also a too high source impedance made trouble:

    e2e.ti.com/.../767997

    Regarding the input filtering: I apologize, I didn't want to offend you. I only was surprised to see no protection diodes and filtering caps. A low pass filter made of 47k and 47pF has a corner frequency of 70kHz and can help quite a lot. And an additional low leakage silicon diode like the BAV199, for instance, can provide a very useful protection.

    Kai
  • 0 in reply to Marek Lis
    Prodigy 95 points

     Thank you for clarification. Still I have some doubts: can I be sure, that my noise have only chopper-specific nature? Because I`m looking at OPA140 (or OPA141, can`t understand what is significant difference between them) as conventional alternative, but it has similar EMIRR characteristics as OPA388... 

     Yes, I read this topic... Thing, which is strange for me in my case is that there is almost no extra noise (at least inside 0-500Hz band) without connection to body... So only high input impedance does not looks like sufficient condition for noise increasing...

    As for 47kOm at inputs - it is not good solution too... 47kOm equals to 27nV/sqHz noise floor... It`s just no need to use OPA388 in such case) Biopotential measurment (if you want to obtain best quality) is not easy and full of trade-offs)

  • 0 in reply to Alex Karnaukhov
    TI__Mastermind 49455 points

    HI Alex,

    ECG/EEG applications typically require very low noise in the low frequency range, or very low 1/f /flicker noise.

    Chopper amplifiers offer no 1/f noise, however, the input bias current is higher on chopper amplifiers when compared to other traditional CMOS linear amplifiers.

    Chopper amplifiers may be used on ECG applications (there is a TI Precision Design, TIPD116, that may be of interest), however, as I understand things, the type of ECG probe plays a role: some ECG/EEG probes are more sensitive to leakage currents and may require a very high input impedance. As Marek has mentioned, the OPA376 is a very good fit for these ECG/EEG applications, since its bias current is only 0.2pA (typical), and it offers very low 1/f noise (10nV/Sqrt(Hz) @ 50Hz.

    The main difference between the OPA140 and OPA141 is in the offset spec.  The OPA140 typical offset is +/-30uV and +/-0.35uV/C offset drift (typical).  The OPA141 offset is +/-1mV and its drift is +/-10uV/C (typical).

    Thank you and Regards,

    Luis