The Art of Noise Management
By Kenneth K. Whiteman
Many times I’ve been asked ‘why my product is acting weird? The signals are noisy and unstable.’
The truth is noise management is not something to be taken lightly. Somewhat unpredictable, I have solved problems by simply rotating a sensor, or moving a receiver a couple inches. Sometimes standing in front of the unit is sufficient to alter the signal. The culprit, more times than not is ground noise. So here are a few tips from my bag of experiences.
Rule #1: NEVER run AC wires in the same conduit/pathway with DC wires.
Rule #2: NEVER run Signal wires and AC wires together.
Ok, now that we’ve got that out of the way…
Let’s talk about AC Power systems, primarily 120VAC circuits.
Typically, Hot is Black, but may be Red. Neutral is White, and Ground is always Green. When the 110VAC enters your building thru the Tap on the outside, it’s important to insure that the Green and the White wires are securely connected together on the grounding bus bar. Yes, I said Green and White. This is the ONLY place where the two different circuits/wires should be connected together.
Very likely there is also a thick copper rod pushed deep into the earth and a green wire attached. This is to assure a good earth ground. All of these connections need to be good solid/tight connections, as they can be a source for noise.
Routing AC power from the main breaker box.
This is important, so listen up!
The Black (and/or Red) wires that run from the breaker are the hot side. It will be accompanied by a White wire and a Green wire, both of which should be attached to the grounding bus bar. From this point on, never again shall the White and the Green connect. The White is referred to as the return, and the Green is the Safety Ground.
The White wire will carry the return current for the circuit, and the Green will serve as a shunting circuit should a short occur between the Hot and oh, say the chassis, or if the white wire should become disconnected.
Connecting to switches, sockets, etc.
Here again good tight connections are essential. I have found in the past that inserting the stripped end of the wires into the convenient little holes on the back of most switches and sockets are a good source for noise. Don’t use them. Wrap the wire around the connection screw and secure with the screw. Yes, I know it’s legal to use the little holes, but then again, we’re trying to manage noise, so…
Be sure to attach a green wire from the switch or socket grounding frame to the green wire, and insure it’s a good solid connection. You may well see one set of wire coming in and one set of wires leaving. This is the Daisy-chain method, and again, good solid connections are a must here as well.
Connecting to DC controlled circuits.
When combining AC and DC circuits, plan carefully to route each in such a way as to minimize proximity to each other. If you must cross them, try to cross them at a 90 degree angle.
Routing DC power from the DC power supply.
Here, pretty much the same principals apply. Good solid connections, avoid proximity to AC circuits, etc.
There are several conventions concerning wire color, Red is positive, Black is negative/ground. Another convention is that White is hot/positive, and Black is negative/ground. There are other methods as well. Whichever method you chose, it’s wise to get it right and remain consistant.
When planning for you DC power requirements, supply filtering is an area that should be reviewed carefully. Place any external filtering components as close to the power supply as is feasible. In some cases it may be necessary to provide additional filtering very near other devices attached to the same power source.
DC Wiring Methods:
Ok, let’s get specific on the DC wiring!
Depending on the load of each component/sub system attached to the DC power source should dictate the number and size of the wires used. A good rule of thumb is set the value of the load to 70% of the amperage limit of the conductor/wire.
Fusing each path/circuit is also a good idea in case something does go wrong, you don’t end up damaging other circuits when the power supply is stressed beyond its limits. The fuse will stop this from happening by constraining the fault to only the circuit feeding it. Always fuse the Hot or Positive side, never the Ground or Negative side.
When attaching to the power supply, use a method known as the “Star” connection, meaning that all the wires connect to a single point. One method of implementation is a terminal strip. When selecting a location to attach each individual circuit manage them in such a way as to put the higher load circuits closest to the source feed end of the terminal strip. Also, feeding the terminal strip in the center as opposed to one end is a better way to go.
DC Ground/Negative Circuits:
Now for the negative(ground) side, the best scenario is to take a large gage Green wire to a chassis mounted ground lug and secure it well. This is important because this is where you are going to establish the three (3) types of ground circuits. They are as follows:
This is the most critical of the 3 ground types. You must be very judicial in making certain that you do not create a ground loop. What is a ground loop you ask? It can best be described as a single point that has multiple paths to ground. The problem here is that the length of these paths are different from one another, and you also create a condition that can and very often does develop a charge between the paths much like a capacitor. Now you have an elevated value for ground, and almost certainly a noise generator. In this context, keep it simply. One wire to ground for each circuit. Again we will want to utilize the “Star” method back to the power supply. Here you will want to try to keep the analog ground as segregated as possible from everything else. If your signals are running on shielded cable, be sure to terminate the shield drain as close to the connection as possible, yet still remain serviceable. Only terminate one end of the shielded cable. On the opposite end, trim off the braid and insulate with heatshrink tubing or some other insulating material.
This can be the single largest contributor of noise in a DC system. Consider this; your microprocessor is running an oscillator with frequencies in the megahertz range, flip-flops are flopping, ring counters are ringing…it’s just a noisy environment! Remember, typically analog circuits are high input impedances so that their being attached to a signal has almost no effect on the signal it’s connected to. That means almost no input current, and so we are monitoring voltage changes, and guess what noise is? Ok, don’t get discouraged. Good decoupling and filtering practices is the key to success. Run a little bit bigger wire gauge and in this case, it’s ok to pick up several points along the way. Do get carried away however, 1 path is likely only looking for trouble. When you are placing digital ground wires, try to avoid sharp bends as these tend to transmit noise as opposed to a slow/sloping curve.
Ok, this the most forgiving of the 3 types. The power supply should do a pretty good job of filtering the DC thru the use of large capacitors and inductors. Any additional filtering you can provide won’t hurt. Be sure to use sufficient gauge wire to carry the return current back to the power supply grounding point.
In all 3 of the above types of grounding as well as the power wiring, carefully think thru the routing, considering the currents that will be traveling thru the conductors.