AD8220 Noise calculations

Dear Matt,

I've been doing a very critical desing using AD8220. But i'm afraid that the IC might fail the design requirement at the very last moment as i found difficulties. Hence need your worthy help.

Fist of all i need to explain my design requirement. I'm doing Neural Interfacing. The Neuronal spikes are very very small (about 50uV extracellular). We need to record those signal (about 1kHz). The problem is you can't draw much current from the cell. Hence we preffer 1pA. But anything less than 10pA is safe... So, 2 main desing requirements: ultra small signal (<50uV) and ultra low bias current (<10uA). I tried with AD8236. But it failled. The conventional system uses INA116 (Texus Instruments). But i don't like it becuase of it's bulky size. I want to use AD8220. I'm using the B version. so the current is okay for me (10pA). But i missed out the noise. Input noise seems okay. But output noise density is too high (90n//V). I just connected AD8220 without any gain resistor. Hence the gain should be 1. Without any input signal i found the baseline noise about 20uV. My question is: Is it correct? If so, then how i calculate the baseline noise? Or how i know by reading the datasheet? I need anyting less than 5uV baseline noise so that i can record 20uV spikes with good fedality. Any suggestion?

Alam,

Applied Neuroscience lab,

Hong Kong Polytechnic University.

Parents
    •  Analog Employees 
    on Aug 11, 2011 10:53 PM

    Hi Alam,

    Matt is currently on vacation and I'm sure he'll have more to say about this subject when he gets back if you need it, but in the mean-time maybe I can be of some assistance.

    The output voltage noise, Eno, is 90nV/rtHz on the datasheet. However, if you were to use a gain for your In Amp, that output voltage noise would be divided by that gain. (E.g. If you could design in a gain of 10, Eno would be 9nv/rtHz. For the AD8220, this gives you a calculated RTI noise of ~16.6nv/rtHz.) Matt has actually made a couple very nice videos on this subject, (follow this link to see the noise video: http://www.youtube.com/watch?v=ZaDK-Nqfp-U ) I don't know the specifics of your design, but I imagine you need some gain to work with signals that small anyway. Hopefully something like this will work for your design.

    Good luck!
    Scott

Reply
    •  Analog Employees 
    on Aug 11, 2011 10:53 PM

    Hi Alam,

    Matt is currently on vacation and I'm sure he'll have more to say about this subject when he gets back if you need it, but in the mean-time maybe I can be of some assistance.

    The output voltage noise, Eno, is 90nV/rtHz on the datasheet. However, if you were to use a gain for your In Amp, that output voltage noise would be divided by that gain. (E.g. If you could design in a gain of 10, Eno would be 9nv/rtHz. For the AD8220, this gives you a calculated RTI noise of ~16.6nv/rtHz.) Matt has actually made a couple very nice videos on this subject, (follow this link to see the noise video: http://www.youtube.com/watch?v=ZaDK-Nqfp-U ) I don't know the specifics of your design, but I imagine you need some gain to work with signals that small anyway. Hopefully something like this will work for your design.

    Good luck!
    Scott

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