Connecting Components and Circuits

Electronic components are connected to create circuits. Electronic circuits are connected to create electronic equipment. Often, different pieces of equipment are connected to achieve the desired result.

An audio amplifier. This shows the combination of a microphone, two small signal amplifiers and a power amplifier connected to make a complete, useful circuit.

Problems with connections:

Impedance mismatch

You have already learned the importance of input and output impedance when connecting circuits together. Please review Impedance Matching in DC Circuits .

Shielding

Shielded Cable

When different electronic components, circuits or equipment are connected, they convey information from one to another. This information, whether analog or digital, is in the form of voltage or current levels, usually voltage. As you have already learned, voltage measurements are relative; one voltage has no meaning unless it is compared to another voltage. When one circuit conveys information to another circuit in the form of voltage levels, there must be a common reference to compare those voltage levels to. This reference voltage is called "ground".

You have already learned the concept of ground as a reference voltage. When connecting circuits it is important to protect the integrity of ground. Improper grounding of circuits, at best, corrupts the information being passed between circuits (this corruption is called noise). At worst, improperly grounded circuits can be dangerous.

Ground Loops

Problems with ground integrity are called a ground loops. Ground loops occur when one part of a circuit or system has a good quality connection to ground but another part of the circuit, that should be connected to ground, is not. There may be an open circuit on merely some unexpected resistance between the point that should be ground and the actual ground. Either way, the ground integrity is lost. When you have a ground loop, current that should be going directly to ground is passing through some resistance. This results in a voltage differential that should not exist.

Two circuits connected to a good ground

The above circuit represents two different circuits connected to the same ground. If the ground is good, there is complete isolation between the two circuits. The 2 mA in the left circuit does not affect the right circuit, neither does the 1 mA in the right circuit effect the left circuit.

Two circuits with a ground loop.

In the above illustration, there is 1 ohm of resistance between the common connection, that is thought to be ground, and the actual ground. 1.99 mA flows through the unexpected resistance resulting in 1.99 mV at the incorrect ground. This results in all voltages in the right circuit to be 1.99 mV higher than expected. This may not sound like much, however, if the circuit to the right is an audio amplifier, and the current in the circuit to the left has a significant AC component, there will be considerable noise in the amplifier.

Hum

The above example can result in the most common symptom of a ground loop. It is not uncommon for circuits that are connected to household power to have significant AC currents, with a frequency of 60Hz, flowing into ground. If there is a ground loop, the unexpected voltage will have a 60Hz AC component. If the right circuit is an audio circuit, that 60Hz component will be heard as a humming in the background. On a television, it will be seen as horizontal stripes where the picture is brighter or dimmer than the rest of the screen (on analog TVs these stripes will slowly climb up the screen).

Lately, ground loops have become a common occurrence in the home because people are connecting computers to televisions as part of home theater systems. The problem is that, by design, most television equipment is not grounded through the wall plug. Typically, your television, cable box and DVD player are grounded through the TV cable. This cable is usually grounded to a cold water pipe some distance away from the equipment. Computers are usually grounded directly through the wall plug. This connection will have less resistance to ground that the long path of the TV cable. As soon as you add a computer to your home theater, you have pieces of equipment connected to each other, that are connected to different ground points. This introduces a ground loop and results in audio hum and brightness bars in the video.

There are three approaches to eliminate this ground loop: One is to use a two-prong adapter where the computer plugs into the wall. This may eliminate the ground loop but also eliminates the safety factor of a grounded computer case. Another is to put a ground isolator in the TV cable before it reaches the splitter that splits the signal between the cable box, DVD recorder, etc. This can be costly and may interfere with two-way cable signals. The third and most appropriate solution is to solidly connect the computer case to the grounded shield of the TV cable. This can be done by physically bolting the splitter to the computer case. If no splitter is being used, you can bolt a grounding block to the computer case. The cable will connect through the grounding block, thus connecting the cable ground to the computer case ground.

Some low power personal computers use external power supplies with no ground connection. These are well suited to connect to home entertainments centers because they don't introduce ground loops.

Low-level audio cables, such as are used for microphones, are often of the shielded twisted pair type. If the grounded shield is grounded at both ends, you have two grounds that are not the same ground. This, of course,is a ground loop. Such cables are often grounded only at one end (indicated by some marking such as an arrow). Cables may also have a switch on the connector to disconnect the ground at one end.