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.

  • 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

  • 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.

  • 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.

    Kindly give your advice.

    Regards,

    Alam.

  • Hi Alam,

    It sounds like you've been doing a very good job testing your circuit. High frequency noise is often a problem in instrumentation circuits and it is very good practice to filter this out. I would recommend placing this circuit at your inputs anyway because RF interference can often be rectified in the In Amp and seen as a DC offset at the output if you don't filter it out first. Take care to put the capacitor, called Cd in the datasheet, between the + and - inputs of the amp, because otherwise component mismatch in the LPFs can reduce your CMRR. I would also like to call your attention to how this changes the differential corner frequency. Hence, if you took the advice of the datasheet and made your Cd 1uF, then you would want to put a approximately 50 ohm resistor for your LPFs to have about the same corner frequency. Of course, you probably want smaller caps and a larger resistor than this, but that's how the math would work out anyway.

    With respect to deriving the baseline noise for the circuit, what you have done makes sense. Once you have calculated this total noise level that you learned from Matt in the video, you still have two basic components of the baseline noise level. The first is 1/f noise, which is the 0.1Hz to 10Hz p-p value listed in the datasheet. That will probably be close to 0.8uV at a gain of 495, but for safety we can estimate 2uV p-p. The rest is the broadband noise. To find this, you multiply your total noise level by the square root of the bandwidth of interest, which is any signal that is not filtered out. So if you have 14nV/rtHz, then you multiply by sqrt(1590hz - 10hz) and you get an rms voltage of about 0.56uV. Now to combine these, we can convert the first number from peak to peak voltage to rms voltage by dividing by 6.6 and then adding the same way we have been: sqrt( (2uV/6.6)^2 + (.56uV)^2) = 0.64uV rms. Which is 4.2uV peak to peak if we convert back. This means that at the output, with a gain of 495, you should expect to see about a 2mV noise level, and a 20uV input spike would be visible above your noise level because it would be gained to 9.9mV.

    Sorry to write so much on the subject, but I hope this helps!

    Scott

  • Dear Scott,

    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.

    Regards,

    Alam.