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
  • Dear Scott,

    Thanks a lot for your worthy reply and help. I've seen Matt's video about the noise. As it says the noise has 3 contributors: Resistor noise, Input current noise and voltage noise. The output voltage noise is divided by the gain as you mentioned. I'm using 100 Gain. So the output noise should be .9nV/srtHz? Another problem is: i'm using high impedence imput (electrodes). Typically around 2M ohm. But for testing, if i don't use any input resistance, then there should not be any resitor noise contribution. Isn't it? So, we omit it. And what about the current noise? It shoiuld also be zero?

    Hence, even if i comeup with some noise factor, say 11nV/srtHz in total; how it means about baseline noise? I just need to know how this setup gives me low baseline noise to pickup very small signal (say 1uV)?

    Best regards,

    Alam.

Reply
  • Dear Scott,

    Thanks a lot for your worthy reply and help. I've seen Matt's video about the noise. As it says the noise has 3 contributors: Resistor noise, Input current noise and voltage noise. The output voltage noise is divided by the gain as you mentioned. I'm using 100 Gain. So the output noise should be .9nV/srtHz? Another problem is: i'm using high impedence imput (electrodes). Typically around 2M ohm. But for testing, if i don't use any input resistance, then there should not be any resitor noise contribution. Isn't it? So, we omit it. And what about the current noise? It shoiuld also be zero?

    Hence, even if i comeup with some noise factor, say 11nV/srtHz in total; how it means about baseline noise? I just need to know how this setup gives me low baseline noise to pickup very small signal (say 1uV)?

    Best regards,

    Alam.

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