Front end filtering: ESD, CMRR, RFI
The resistor-capacitor low-pass filter components on the circuit board augment the digital sinc filter inside the chip at frequencies above 1MHz.
Electrode impedances in clinical practice can typically vary between a few kOhm through upwards of 50kOhm and this in combination with the filter capacitor will cause CMRR ratios to suffer because a capacitor has an impedance of 1/2/Pi/f/C which is in the same order of magnitude as the electrode impedance variability. The industry standard mitigation strategies are to keep the capacitor values low such as 100pF and to use tight tolerance capacitors such that no imbalance is created between leads. On the other hand the capacitor must be large enough to suppress frequencies above 1MHz so we don't pick up radio signals and alias them into our EEG above the sinc filter, putting us to perhaps 1nF.
The engineering dilemma then becomes to determine what is more important: CMMR or RFI suppression. Because I use short EEG leads and do not sit in a room full of electrical equipment or live close to any AM radio stations, I strongly prefer the 100pF capacitor. Other people may not feel the same so here is opportunity to share experience for discussion.
The OpenBCI folks have of course looked into this in some detail:
https://docs.google.com/spreadsheets/d/1IHy0lOcYJDAzqx9n2essxVWIHo0QjdH9r8cdquoV4pY/edit#gid=0
Looking at the schematic I see the filter caps are CKCL44X7R1H102M085AA and the "102" contained therein seems to indicate 100pF. Can someone please post the data sheet to this part?
Could someone please also post the data sheet for the ESD protection clamp TPD4E1806 so we can see the specs for possible constructive comment? I see this connects directly to the input pins whereas it can sometimes be better to have this behind a resistor to limit inrush current if someone touches electrodes to a metal surface (of course then you still need another resistor behind it same as now). Note that anyone who wishes to add a resistor in front can easily do so in line with each electrode lead.
I'm very much looking forward to using this device with BioEra
Stepan Novotill
Electrode impedances in clinical practice can typically vary between a few kOhm through upwards of 50kOhm and this in combination with the filter capacitor will cause CMRR ratios to suffer because a capacitor has an impedance of 1/2/Pi/f/C which is in the same order of magnitude as the electrode impedance variability. The industry standard mitigation strategies are to keep the capacitor values low such as 100pF and to use tight tolerance capacitors such that no imbalance is created between leads. On the other hand the capacitor must be large enough to suppress frequencies above 1MHz so we don't pick up radio signals and alias them into our EEG above the sinc filter, putting us to perhaps 1nF.
The engineering dilemma then becomes to determine what is more important: CMMR or RFI suppression. Because I use short EEG leads and do not sit in a room full of electrical equipment or live close to any AM radio stations, I strongly prefer the 100pF capacitor. Other people may not feel the same so here is opportunity to share experience for discussion.
The OpenBCI folks have of course looked into this in some detail:
https://docs.google.com/spreadsheets/d/1IHy0lOcYJDAzqx9n2essxVWIHo0QjdH9r8cdquoV4pY/edit#gid=0
Looking at the schematic I see the filter caps are CKCL44X7R1H102M085AA and the "102" contained therein seems to indicate 100pF. Can someone please post the data sheet to this part?
Could someone please also post the data sheet for the ESD protection clamp TPD4E1806 so we can see the specs for possible constructive comment? I see this connects directly to the input pins whereas it can sometimes be better to have this behind a resistor to limit inrush current if someone touches electrodes to a metal surface (of course then you still need another resistor behind it same as now). Note that anyone who wishes to add a resistor in front can easily do so in line with each electrode lead.
I'm very much looking forward to using this device with BioEra
Stepan Novotill
Comments
In the interest of addressing languaging concerns raised above, I want to be clear that no judgement about the design can be made unless additional information about materials and methods is provided and that the drop in measured CMRR is in all likelyhood appropriate, correct and fully expected given the measurement methods and materials used.
A thought about the method: Did you bandpass filter the digital output stream before calculating CMRR, in order to eliminate out of band thermal and other noise generated in the test setup? I wonder if the test results might then be more meaningful with regard to capacitor values.
Stepan
the case in clinical use then I believe you'll see a huge difference in
amplitude between the two channels due to the voltage divider effect of
the 1nF capacitors and especially so in the Beta range. Amplitude comparisons between channels are a most
important part of QEEG and therefore 100pF capacitors may be optimum. CMRR and
RF interference may well turn out to be secondary concerns. I'd be
curious what the clinical users find.
I want to know what for is the RC filter after the ESD protection? It is related with Sigma-Delta modulation, isn't it?
How should I calculate it?
Thanks!