Exploring the ThinkPulse Active Electrodes: Experimental Findings and Observations

The ThinkPulse Active electrodes are pretty interesting especially when combined with the Ultracortex "Mark IV" EEG Headset. The combination makes this cool technology accessible to a wider audience. I wasn't able to find clear and concise documentation about those electrodes beyond Ultracortex's setup instructions. I conducted a series of experiments to characterize the device’s behavior.

Experimental Setup

The ThinkPulse features three pins: black, red, and white. The black pin is the negative power supply which I set to -2.5V, and the red pin the positive supply which I set to +2.5V. I don't know the full-range of this voltage supply and I wouldn't risk damaging the electrode by trying anything larger than this. The white pin outputs the signal.

With this setup, I connected the device to a signal generator, testing various input amplitudes and frequencies to assess the performance.

Key Findings

1. Gain Characteristics: The ThinkPulse exhibits a gain of 0 dB, indicating that the output signal maintains the same amplitude as the input signal. This result aligns with the available online data, which suggests the device does not provide amplification. As such, the main benefit is the very low output impedance, that helps drive the signal over the long cables between the electrode and the main amplifier. 

2. Frequency Response: As expected, the output begins to lag behind the input as the frequency increases. However, this lag remains within acceptable limits up to frequencies around 30 kHz.

3. Amplitude Response and Distortion: I observed no visible distortion in the output signal at amplitudes up to 3.4 V peak-to-peak (Vpp). This is promising, particularly for low-voltage signals like those used in EEG applications, which typically fall within the millivolt range. However typically there will be still a need for ground or bias electrode because as soon as the input signal exceeds the ±2V range, significant distortion occurs.

4. DC coupled: Yes, it is.

I hope you find this information useful for your ThinkPulse Active Electrode exploration.

References

The "Dry and Flexible Active Electrode for EEG Measurement Sensor Documentation" was invaluable.

Another key resource is this video along with the OpenBCI Cyton and Daisy board schematics.

The video has one inaccurate instruction though. It says to connect the active electrodes to the "top row" in Cyton, but they need to be connected to the bottom row instead. The bottom row corresponds to the N(egative) inputs of the amplifiers. As per wjcroft here, "On the Cyton this is the default and the plus pins are connected together and available on the SRB2 pin (closest to the board.) The minus pins then are placed on the scalp. And the SRB2 connected to an ear lobe." Later on the video, we also see that earlobe is connected to SRB2 (bottom row).

This also matches the OpenBCI documentation that says: "turn BIAS and SRB1 to NO and OFF". It also says "Disconnect the BIAS earclip from the Cyton board." I'm not sure about this. On the video it shows something different and it would make sense to have a Ground connection. But, due to low impedance of active electrodes, it might be that BIAS doesn't add any significant value. I can also see the point of tying the bias electrode to the ground, either by configuration, or by placing the electrode to a ground pin. The most accurate configuration, though, for me, would be to use N e.g. 8 active electrodes as regular channels and one active electrode for Reference (SRB2) and one ear clip to connect to Ground (or active bias). I don't see why a (non-active, high impedance) reference electrode wouldn't pick lots of noise and that would affect every other channel because it's used as differential reference.

It should be quick and easy to test these configurations and see what gives less noise.