Project: EOGee – EOGlass Drift Reduction

In a previous article I showed how the bias current of the AD8226 amplifier was causing a 20nV current to flow through the electrodes resulting in a large voltage offset due to the impedance mismatch between the left and right electrodes. This resulted in a large drift in the signal due to changes in impedance between the user and electrodes as they moved or sweated, which often saturated the ADC. I built a test board and showed how this could be reduced by a factor of about 1000x by using the AD8220 which has a bias current of less than 10pA.

The signal drifts a lot over time with the AD8226

With the confidence that the AD8220 would improve my design, I modified the existing EOGlass2 design to use the AD8220 amplifier instead. Although the AD8226 and AD8220 are pin-compatible, the AD8220 has a minimum voltage supply of 4.5V. This meant I had to add a third voltage regulator to the design to generate a 4.5V rail from the 5V USB supply.

Unfortunately, the design is already packed and there is no space for another AP2210K voltage regulator in a SOT-23-5 package. Instead I replaced the single 3.3V AP2210K voltage regulator in the top right with two LP5907 voltage regulators, one generating 3.3V and one generating 4.5V. These come in a tiny 0.65mm square package with four pins.

EOGlass2 design with a single AP2210K voltage regulator in the top right
EOGlass2.1 design with two LP5907 regulators in the top right (unfortunately I don’t have CAD for those chips)

The LP5907 also has a better power supply rejection ratio over the entire specified frequency range up to 1MHz which is well beyond the operating frequencies of EOGee, as long as the current draw is less than 20mA which will be true as it is only powering the AD8220 which has a quiescent current of <1uA.

Comparing the PSSR vs Frequency for the two chips (the y-axes are inverted but mean the same thing)

With the design in place, I ordered and assembled a board. I then reviewed data from the two designs to compare the drift:

Comparison of EOGlass2 and EOGlass2.1 performance over the time frame of a couple of hours

We can see that with both designs the signal drifts heavily when the glasses are first put on, before settling a little after about 20 minutes. However, over the remainder of the data the EOGlass2.1 design is much more stable and sits much closer to the middle of the range. In fact, between 1.25 hours and 2 hours the signal barely drifts at all.

Overall I have had far fewer issues with the EOGlass2.1 design and have never seen it saturate the ADC and fail to produce a signal.

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