[docs] Added page about ground loops and fixes.

docs/_data/index.yaml

@@ -19,6 +19,8 @@ sections:
         url: /troubleshooting
       - title: Specifications
         url: /specifications
+      - title: Ground Loops
+        url: /ground-loops
   - title: Tutorials
     docs:
       - title: Hoverboard Guide

docs/getting-started.md

@@ -68,9 +68,13 @@ Connect the encoder(s) to J4. The A,B phases are required, and the Z (index puls
 <div class="alert">
  Always think safety before powering up the ODrive if motors are attached. Consider what might happen if the motor spins as soon as power is applied.
 </div>
+
 * Unlike some devices, the ODrive does not recieve power over the USB port so the 24/56 volt power input is required even just to communicate with it using USB. It is ok to power up the ODrive before or after connecting the USB cable.
+
 * To power up the ODrive, connect the power source to the DC terminals. Make sure to pay attention to the polarity. Try to connect the power source first and then turn it on to avoid inrush current. If this can't be avoided then a small spark is normal. This is caused by the capacitors charging up.
 
+* Make sure to avoid a ground loop! See the [ground loop page](ground-loops.md) for details.
+
 ## Downloading and Installing Tools
 Most instructions in this guide refer to a utility called `odrivetool`, so you should install that first.
 

docs/ground-loops.md

@@ -0,0 +1,15 @@
+# Ground Loops
+
+For electrical devices to communicate, most of the time they require a common ground connection. Best practice is to connect the grounds back to a single point, called a "star ground". If there are multiple paths to ground, a "ground loop" is formed. Ground loops and wire inductance can cause issues for high current electronics like ODrive. As an example of what can go wrong, look at the diagram below.
+
+![Ground Loop with inductance](ground_loop_bad.png)
+
+The issue is the inductance of the power wires between the ODrive and power supply. The inductance and the high current drawn by the ODrive causes V_1 to not be the same as V_2. If the voltage caused by the wire inductance and current is high enough, the 0-5V gpio signals can swing much higher or lower than the normal 0-5V range. This causes a current to flow through the ODrive GPIO pins. 
+
+To fix this, the ground loop must be broken. This can be achieved by isolating the power supplies (no common V-) and connecting a signal ground between the RPi and ODrive. An example of this is a single ODrive connected to a battery and a device like a RPi connected to a mains power supply or different battery. If more than one ODrive is in use and they share a power supply, you have a ground loop again.
+
+The easiest way to fix this is to isolate the data connection between the RPi and ODrive(s). The diagram below illustrates where the isolator should go.
+
+![Ground Loop fixed by isolator](ground_loop_fix.png)
+
+By isolating the data connection (whether it is GPIO, USB, or UART), the ground loop is broken. Isolation can be achieved by using a USB isolator or a signal isolator for GPIO connections.