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,

I just finish testing AD8220B IA one more time. I put 100 ohm for 495 gain. Then short the input terminals (inverting + non-inverting + ref). Then check the output signal with oscillosecope. I found that the baseline voltage noise is about 10mV. So, the actual noise should be around 20uV (10mv/495)? But if i also found that most of this noise is in high frequencies (>101kHz). So, if i put a single order passive low pass filter this noise level goes down to 2mV! (actual noise should be 2mv/495 = 4uV)! Is it correct? My filter R and C are: 1k and 0.1uF. So, the cut-off is around 2 kHz.

Now, i'm not sure if it is the real case or not. If it is then i would like to know where sould i put this lowpass filter? Can i put it in the input terminal as mentioned in datasheet? Because i was planning to put a high pass second-order filter in the output terminal. I don't like to put band-pass there due to some other limitations.

Regards,

Alam.

• Dear Scott,

I just finish testing AD8220B IA one more time. I put 100 ohm for 495 gain. Then short the input terminals (inverting + non-inverting + ref). Then check the output signal with oscillosecope. I found that the baseline voltage noise is about 10mV. So, the actual noise should be around 20uV (10mv/495)? But if i also found that most of this noise is in high frequencies (>101kHz). So, if i put a single order passive low pass filter this noise level goes down to 2mV! (actual noise should be 2mv/495 = 4uV)! Is it correct? My filter R and C are: 1k and 0.1uF. So, the cut-off is around 2 kHz.

Now, i'm not sure if it is the real case or not. If it is then i would like to know where sould i put this lowpass filter? Can i put it in the input terminal as mentioned in datasheet? Because i was planning to put a high pass second-order filter in the output terminal. I don't like to put band-pass there due to some other limitations.