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by
Dennis Clark
Q. When building a homemade robot controller, how
do I separate logic and analog ground? I’ve read
that all grounds need to be tied together, even if
you are using more than one battery. If this is the case,
don’t these need to be separate?
— Mike Bonner
A. This question comes up a lot. I’m going to answer
the second part of the question first since that will
make the first part easier to understand. The biggest
confusion comes from the concept of “ground.” Because
our robots are mobile (most of the time), they really aren’t
“grounded,” which means they aren’t connected to Earth
ground. The minus (-) side of the battery is really just the
current return — the completion of the circuit. It is often
called common because all signals are referenced to this
“common” electrical level. In other words, 5 volts DC is only
5V DC in reference to the common. When more than one
battery is in a system, this common point becomes very
important. How would we know what the voltage is
between parts of the circuit if there is no agreed upon
common point? Why is this important? Here is why.
Let’s say that we have an H-bridge that gets its power
from a 12V battery. Let us also say that we have a 5V
battery powering our micro. If the bridge needs a logic
“high” of 3.5V to turn on the power to the motor, that
has to be 3.5V in reference to the common. If our batteries
do not have their returns (or grounds if you wish) tied
together, what does that 3.5V mean? I can see the
questioning look in your eyes already ... Why would the
return be different for one battery than the other? It can
be, and it can be hugely different.
In order to measure a voltage, we measure a “potential
difference” of electrons between two points. If those points
are isolated from each other, there is no current path. We
have all heard that “current follows the path of least
resistance.” This is partly true, but I won’t get into the
details of that right now. Most of the current follows the
path of least resistance, but all of it travels somewhere.
14 SERVO 05.2008
When all of the paths are high resistance — like air — you
could get any value at that measured point, even hundreds
of volts (I’ve seen that happen on an improperly grounded
power supply.) There is just no way to know what will be
seen at that control pin. Your operation will be erratic, and
most likely non-existent. With the minus (-) leads all tied
together, the common reference is defined and, if you will,
agreed upon by all the components in the circuit. This is why
you must have all batteries’ minus (-) leads tied together.
Now that I’ve told you that, I’ll contradict myself a little
bit. If you need to have positive and negative voltages in
the circuit, how would you do that? Let’s say that you need
to have plus and minus 12V in your robot. Remember that I
said that the ground really isn’t ground? That it is really the
common reference? (Can you guess what I’m going to
write?) Yup, if you need a -12V, you would tie the plus (+)
side of one battery pack to the common and take the
voltage from the minus (-) side of that pack. I’m really not
contradicting myself here — that the common point is
the reference — that battery is providing -12V with respect
to the common reference. Hopefully, that will make sense
to everyone. Okay, now that I’ve answered that part of the
question, let’s answer the first part of the question. Why
are the logic and analog (or high power circuits, as well)
kept separate, and how can they be separate if they have
to be tied together (as said in the first part of my answer)?
Well, there is separate and there is separate. We know
that all of the grounds/commons must be tied together,
but there are ways to do this that are good and ways that
are bad. First some background: The traces on a circuit
board are copper, and because copper is not a perfect
conductor, it has resistance. The longer the trace, the
higher the resistance. This makes the trace look like a
resistor to current flow. Ohm’s Law states that V = IR,
which means that there is a voltage drop (V) across any
resistor (R) through which current (I) flows. So, while our
DVM set to beeper mode tells us that any point in the
ground plane (or return path if you want) is the same, in
reality while the devices on the board are running, the
