Devlog #2

Almost completed PCB Design for slider + stator (design error)

I redesigned my PCB several times to achieve the optimal layout for all the components. I ran into several issues along the way.

1) Rx electrode and Tx transmitter positioning

The Rx electrode and Tx transmitter array are sensitive capacitive devices. When positioned close to other electrical components, parasitic capacitance can be introduced, which can disrupt our weak signal and create crosstalk. It may also affect the integrity of our 8-phase signal, causing phase delays and shifts.

2) Connecting the Tx array pads to the 74HC595

Wiring the Tx array pads (under the Rx electrode) in a way so that every 8th pad was connected was a nightmare. I couldn’t directly draw traces over the copper pads as that would disrupt the ground plane, which is crucial for blocking noise from other components and external sources. For example, the human body is essentially a giant capacitor, and a human placing their hand over an unshielded Tx array would completely ruin our delicate signal. Therefore, it is crucial that the copper traces avoid cutting up as much of the ground plane as possible.

Thus, I ended up reducing the width of both the Rx electrode and Tx array to make extra room for running the copper traces to the side. I also added a small grounded copper pad between the traces and the Rx electrode just to shield it from any parasitic capacitance.

3) Design mistake

I realized just now after looking at the PCBs I ordered that I made a tiny mistake with my op-amp circuit. To the left of my Seeed MCU, I have an op-amp functioning as an ultra-high input impedance buffer. I’ve also attached an image of the schematic.

An op-amp normally takes a weak voltage signal, compares its two inputs, and multiplies the difference to create a much stronger output voltage. On my circuit, Pin 4 and Pin 1 (VOUT) of my circuit are connected to create a 1:1 voltage ratio, creating a unity-gain buffer feedback loop, so no amplification is actually happening. What DOES happen is that we’ve exposed the op-amp’s high input impedance, meaning it draws incredibly little current from our incredibly weak Rx electrode capacitor. This prevents our Rx electrode from just straight discharging our AC signal our circuit - preserving its original timing and phases. We feed the signal into pin 3 of the op-amp, allowing it to ride the 1.65 V (introducing micro-fluctuations, e.g. 1.64V-1.66V), and our op-amp outputs this signal directly to our MCU pin.

My mistake was that I connected the Rx electrode straight to pin 4 instead of pin 3.
I will see if I can fix it with some quick mods. Fingers crossed.