AD8221 at 3.3V

Hi everyone, I'm choosing a low noise in-amp for an EEG project. I've found that AD8221 has the best noise characteristics for my set up.

Unfortunately it requires at least +-2.3V, I am running at 3.3V.

I've tested an AD620 which is very similar to the AD8221. It does run at 3.3V just fine. I am not worried about output voltage swing because I am amplifying a very small signal only by 10 times. I would like to know if I will run into any major problems by running this op amp at 3.3V. Specifically I am interested in:

- crashes, instabilities

- signal distortions

thanks.

  • Clarification: I am running at 3.3V, not +-3.3V, that means  +-1.65V

  • 0
    •  Analog Employees 
    on Dec 3, 2012 8:05 PM

    Hi, Kurt.

    It's good to know that AD620 had worked with that supply voltage. However, we cannot guarantee it's performance since you applied a lower voltage than the specified minimum supply of +-2.3V. We don't also assure that AD8221 will work properly since the supply you want to apply, +-1.65V, is lower than the specified value in the datasheet.

       

    As alternative to this, ADI offers a lot of in amps that can operate with supplies as low as +-1.1V. You may want to consider AD627, AD8226 or AD8227. These are recommended for low power medical instrumentation like EEG and the supplies are within your requirement. They have rail-to-rail outputs so they can swing close to the supply. The voltage noise (RTI) values are less than or equal the value of AD8221. The noise densities are quite higher than AD8221's 8nV/rtHz.

    If you need a dual in amp, AD8426 is a good option based on your requirement.

    Hope this helps. Let us know if you have anymore questions.

    Regards,

    Anna

  • Does the common voltage of these two amplifiers need to be at half supply? I am thinking of adding a step up regulator (properly filtered) to drive the amplifier at 0-5V, and keep everything else identical (that means the signal is centered at 1.5V and goes from 0V to 3V)

  • PS: I am only sampling up to 59hz. The boost converter I found (MAX619) has a 100mv voltage ripple at 500khz. I presume this won't influence things and that I can just add a cap to the output to smooth out the ripple. Current consumption of the amplification circuitry should not vary very much in time.

    Is it a very bad idea to use a boost switching regulator to power the instrumentation amplifier?

  • 0
    •  Analog Employees 
    on Dec 4, 2012 8:13 PM

    Hi, Kurt.

    Will the regulator be the supply of AD8221? Will you use 5V or 3V or +-1.65V? As alternative to the regulator you found, you may want to try ADI's regulators depending on the voltage and you can find it here.

    If you'll use 0-3V supply for AD8221, your input voltage will be 1.9V. It will be the minimum and maximum input voltage you can apply to the part because the AD8221's specified input voltage range is -Vs +1.9V to +Vs-1.1V when operating at voltage spans of 10V or less. Also with this supply, the lowest expected output would be 1.1V and the highest will only be 1.8V.

    The common-mode voltage will depend on the supply voltage you'll use. you may refer to Fig. 8 and Fig. 9 of the datasheet.

    To guarantee the proper performance of AD8221, you have to provide a negative supply to it. Another way that you can try in order to satisfy your supply voltage requirement, you may try the rail-to-rail in amps, such as AD8226 or AD8227. It can swing very close to your supply.

    Hope this helps. Let us know if you have anymore questions.

    Regards,

    Anna