1 Differential, Single-Ended, and Common Mode Signals

Differential signals are standard for many different communications protocols including LVDS, CAN, USB, RS485/422, HDMI, and so on and incorporates two complementary signals (differential pair) that are used to transmit data.

While single ended communication protocols such as I2C or SPI are often more ubiquitous due to their simplicity and are referenced to a single, stationary potential, differential signaling offers numerous advantages over single ended communication schemes and several are as follows:

• EMI (electromagnetic interference) and Common Mode Noise Resistance

• EMI and Common Mode Noise can be introduced from a variety of sources that can be coupled to the data transmission signals and can distort what the receiver is reading. In protocols with differential signaling, this effect is mitigated significantly as the noise is emitted onto both signals meaning that each inverted and non-inverted signal will constructively cancel each other out once they reach the receiver:

• Ground Shift Immunity

• In addition, many differential signal protocols do not share any common ground or return current path, so ground reference shifts and ground noise acquired along the path are of no concern when utilizing such differential signaling protocols. However, for protocols that do call for common termination (such as the CAN interface as an example), there needs to be more care taken to reduce ground offsets as much as possible.

• Reduced Power Requirements and Higher Frequency Operation

• Differential signaling often calls for voltage signals to be in the hundreds of millivolts or just up to 1 V or 2 V and can greatly simplify power consumption needs of an application. In turn, this also allows these differential signals to operate at higher frequencies due to the much lower radiated emissions these signals have from the smaller voltages used (in addition to the resistance from external EMI imposed by using a differential pair).

Common mode signals can be simply defined as the voltage that is common between the input terminals of a device and are often an unwanted element in most systems. For instance, take the visual representation as shown in Figure 1-3.

It is evident that when a common mode voltage is introduced to a system, this will shift the reference point of the affected components to be outside of the anticipated operating range by a given VCM (as shown above from both a graphical and equivalent circuit perspective). While common mode signals can be attributed to EMI, ground shifts, miswirings, coupling, or even lightning strikes, they can be influential enough to the system at large that it can significantly affect measurement accuracy or even permanently damage devices by causing them to exceed absolute maximum operating ratings. Hence, it is essential that the design is defined by the environment the system will be used in and components selected that can tolerate such conditions.