High common mode voltages can often be a top concern when designing in a particularly noisy or harsh environment as they can cause irreparable damage to a signal chain or significantly impact a system's accuracy. However, sometimes high common mode voltages are present by design and the overall system needs to be able to accommodate these high voltages for everything to cohesively work together. Whether intentional or not, the components along the signal chain need to be able to withstand and operate effectively in these conditions and TI’s multiplexer portfolio offers many options to handle a myriad of situations where this can be a possibility while also offering the performance needed for any application. In this report, several high common mode voltage applications will be explored which will include analog I/O modules for programmable logic controllers, industrial protocols such as CAN or RS485, and battery cell monitoring.
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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:
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.
For multiplexer applications that may be exposed to high common mode voltages, there are several multiplexer parameters that need to be considered for these situations.
The most evident parameter that one must consider is the operating supply range of the multiplexer. During nominal operation, the expected signals propagating through the multiplexer can usually be fairly low. However, if there exists common mode voltages that can potentially be upwards of ±36 V in some cases, then a higher voltage capable multiplexer would be needed to be able to survive and operate in these instances. While it may seem that this would necessitate a higher supply voltage, which certainly can be the remedy, there is a way to still utilize a lower supply rail while being able to survive an event where a high common mode voltage is present. Below illustrates how external diodes and current limiting resistors (typically in the 1k to 10k range) can be implemented to suppress damage to the multiplexer while allowing time for the fault to subside and resume nominal operation:
Lastly, if there still needs to be operation during a high common mode voltage event and there is no access to high supplies in the system, beyond the supply multiplexers can be used instead which offer low voltage supply rails, but can pass and tolerate voltages on the multiplexer inputs beyond those supply rails.