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?
Applied Neuroscience lab,
Hong Kong Polytechnic University.
Thank you so much for your excelent support. After sevaral testing i've finalized my desing. I'm glad to write that I'm very happy with AD8220B IA for my biological applications. My final desing gives only 4uV baseline noise. So, i can pickup 20uV signal very easily. I used sallen-key filter tropoligy for 5kHz Low-pass signal. It would also help me to work as anti aliazing high frequencies for the Data acqusition card that i'm using.
I'm just little confused about 2 other thinks and need to clear my concept about it. Since i don't expect the bilogical tissue to produce high current, i use very low bias current IA. So, i'm using AD8220B version. Am i right?
Another think is since the input souce impedence is very high (>1Mohm), i'm not sure what value of resistance i should use in the input terminal. Should i still make use of 5pF of terminal capacitance to cancel out RFI noise? Or just no need?
For 2 pole Lowpass sallen key filter amplifier (11 Gain) that should match with AD8220B which Op-amp i should use? I preffer to use: 1 OP-amp/ic since it fascilitate me to desing the tracks. Moreover, by this way i can use SOT23-5 IC!
Thank you again Scott for your kind help.