Need Help in finding a Suitable OpAMP for EEG active sensor Electrode design

Hi Chandra,

because of the absence of conducting gel, dry electrodes are considerably higher ohmic than wet electrodes. The equivalent circuit of a dry electrode is a high resistance (10k...100k ?) in parallel with a high capacitance (10n...100n ?), with highly varying values from electrode to electrode. So, in order to get a high common mode rejection, the input impedance of preamp of active electrode should be extremely high in the signal frequency range. Take an OPAmp with JFET or CMOS input stages. (This also minimizes the risk of suffering from popcorn noise.) Take an OPAmp with extremely low common mode input capacitance. Every pF counts! And avoid additional capacitances in the input filtering. Only use a current limiting resistor at the input of OPAmp. Use an input diode clamp only, if at all, in combination with low junction capacitance diodes showing ultra low leakage current.

For lowest noise take an OPAmp showing not only low input voltage noise but also low input current noise. Remember, the electrode impedances are very high at low frequencies. Look for low noise OPAmp especially at the very low frequencies! An OPAmp with low noise at 10kHz but high noise at 100Hz is not what you should search for. But look for an OPAmp with very low noise at the lowest frequencies.

And, important: Do only use batteries for the powering of electronics being in direct contact to the patient. Use safe galvanic isolation when connecting the front-end electronics to mains powered equipement !!!!!!!

Kai

Hi sir,

Thank you very much for a detailed reply. yes device is powered by battery. sir, it would be great , if you can able to filter out some specific part numbers to start with. bcz many parameters, leading to so much confusing. also in pure analog electronics design, our expertise is limited. so if you can get your help and any reference designs, it would be good. also in which configuration it needs to be wired? 

can we use an instrumentation amplifier for this purpose?  

Thank you

chandra

Hi Chendra,

this depends on how you intend to arrange the electrodes. There are several options.

The highest common mode rejection can be achieved by a pre-amp which only works as a buffer with a gain of 1. The common mode rejection via instrumentation amplifier is done in a second step, where the signals of two adjacent channels are subtracted from each other in an instrumentation amplifier. This offers the best common mode rejection, because there are no gain setting resistors with their manufacturing tolerances and drifts. Another advantage is the absence of input cable with its additional and unpredictable stray capacitance. And the output cable can be driven very low ohmically, keeping the common mode rejection high. In this case a standard OPAmp could be used in the pre-amp.

Is this what you plan?

Kai

The OPA376 is often used for such applications.

Kai

Hi kai, & tamara,

thanks for your valuable input. in datasheets of OPA376 & TLV376 its mentioned as single supply voltage.. but in specs its mentioned -+2.75 max.. can i hook up these between -2.5V for -vs and +2.5 for  +Vs? 

also in order to configure it as unity gain buffer, just hookup the V+ to dry electrode, and v- to the output. is there any power supply capacitors or any other capacitances needed in input /output. I think we may need to use ESD protection diodes for input and oputput. in that case do i need to wire the GND also? Does other components on input and output creates any disturbance on the signals..?

the Drive voltage will be the Bias output from the ADS1299 .. do i need to reference this voltage in the buffer preamplifier stage? if so how?

sorry for my so basic questions, just like to know more in details, before we proceed to test in real. Thank you

Hi sir,

Thanks you for your help, seems like with the provided information, i might able to start designing, but sadly OPA376 seems out of stock, not even available any where. can OPA387 be used, seems like it has still lower offset level. but even that is out of stock. any alternatives?

Hi Chandra,

chopper OPAmps should not be used here, because they tend to be very noisy with the very high source impedance of active electrodes. So I would not take the OPA387.

Many other OPAmps can be used instead of OPA376, but most of them are more noisy. The OPA391 might be a good candidate, because of very low input offset voltage and ultra low input capacitance. Unfortunately, the noise is higher compared to the OPA376.

Maybe Tamara can help with alternatives?

Kai

Hi Tamara & kai,

Thank you. but TLV376 seems to have higher offset around 100micro volt. will that be fine for a EEG application where it demands for lowest offset.THank you

Hi Tamara & kai,

i found OPA392 & OPA397 parts available in stock from mouser, can these parts fit in place of OPA376.THank you