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OPA129: Using for EEG circuit

Dilshan Ganepola
Prodigy 40 points

Part Number: OPA129

We are interested capturing EEG signals ( using ADS1299 ). Its said in the datasheet the input impedance is around 1Gohms. For our specific task we need to have more higher input impedance. Could we achieve it by adding a high impedance opamp like OPA129 in voltage follower configuration for differential analog input pins.

Thank you.

  • 0
    TI__Guru* 93105 points

    Hello Dilshan,

    Certainly, an ultra high input impedance Op amp could be used in front of the ADS1299 MUX inputs to increase the impedance the EEG sensor sees. The input impedance of JFET and CMOS input Op amps connected as a unity-gain buffer can be many decades higher than 1 Gigahoms. Since with a buffer the non-inverting input loads the sensor, the impedance is the extremely high.

    The OPA129 is a high-voltage JFET input op amp and would certainly provide extremely high input impedance. However, I see from the ADS1299 datasheet that it analog circuitry operates from +5 V, and in that case a low-voltage CMOS Op amp may be a better match from a power supply standpoint.

    Another consideration in a low-level signal application such as EEG is the Op amp internally generated noise. Do review the Op amp noise specifications and take that into account. Here's a link to one of TI's low-voltage, CMOS input Op amps that is worth considering:

    http://www.ti.com/lit/ds/symlink/opa376.pdf

    You will find that in order to preserve the ultra-high input impedance of the Op amp that PC board cleanliness must be fully considered as part of the assembly and handling process.

    Regards, Thomas

    Precision Amplifiers Applications Engineering 

  • 0 in reply to Thomas Kuehl
    Prodigy 40 points
    Dear Thomas,
    Appreciate your detailed explanation.
    OPA129
    (f = 10Hz) 85 - nV/√Hz
    (f = 100Hz) 28 - nV/√Hz
    CMRR 118db

    OPA376
    Input voltage noise (f = 0.1 Hz to 10 Hz )0.8 μVPP
    CMRR 90db

    Just to clarify, for low frequency range ( <50htz) wouldn't OPA129 perform better in term of noise ?
    As you said OPA129 needs +/- 15v, which is a problem as main circuitry operates in +/- 5v.

    I tried to find input impedance of OPA376, but it was not there in the datasheet. Could you help me to find input impedance of OPA376 to conclude on OPA129 / OPA 376.

    To explain you more specifically about our requirement on high input impedance , could you please contact me privately / ganepola.dilshan@gmail.com .
    Regards
  • 0 in reply to Dilshan Ganepola
    TI__Guru* 93105 points

    Hello Dilshan,

    I was out of the office for the past week and apologize for the delayed response.

    When you list the noise for the OPA129 where the units are nV/√Hz, is in reference to noise spectral density at the two frequency points of 10 Hz and 100 Hz. It is derived from the noise power NO, which is the noise power per unit bandwidth (W/Hz). Where as the OPA376 as listed above has a different noise measure refereed to as "input voltage noise" having the unit of uVpp. Therefore, their noise cannot be directly compared based on the differences in their meaning and units.

    A more direct comparison would be to compare the Input Voltage Noise Spectral Density for the OPA129 and OPA376 at 10 Hz and 100 Hz. You list for the OPA129:

    (f = 10 Hz)     85 nV/√Hz
    (f = 100 Hz)   28 nV/√Hz

    And for the OPA376, from Figure 5. Input Voltage Noise Spectral Density:

    (f = 10 Hz)     18 nV/√Hz
    (f = 100 Hz)    9  nV/√Hz

    That is a meaningful comparison for the EEG frequencies that you intend to monitor. The Input Current Noise Spectral Density is so low for the two devices it will not be a significant factor in the total noise.

    The OPA376 is a CMOS Op amp and the input impedance is extremely high. Once the resistive portion of the input impedance gets into the hundreds or thousands on Gigaohms, or Teraohms, it becomes very difficult to measure accurately. What the OPA376 datasheet is the Input Capacitance, differential and common-mode, and that is what is listed in the Electrical Characteristic table. Basically, the OPA376 inputs look like a capacitance to the circuit they load. The OPA129 Electrical Characteristics table list the resistive and capacitive values under Input Impedance.

    You mention that there is +/-5 V power in your EEG circuit. Do note that the OPA376 has a maximum supply of +5.5 V (+/-2.75 V). Therefore, it can be directly powered by a +/-5 V supply. There are other TI low noise, higher voltage CMOS amplifiers that could be considered.

    Regards, Thomas

    Precision Amplifiers Applications Engineering