Does Noise Margin in a CMOS Inverter Affect Performance?

May 19, 2020 Cadence PCB Solutions

Close-up of CMOS inverter electronic component 

Margins are in place within every field of science and electronics. These margins or limits can be safety-oriented or function governed. In either case, margins are necessary to promote overall functionality, performance, and safety. Exceeding device margins or limits typically results in catastrophic failure.

However, if a device or component can stay within its acceptable margins, then functionality, performance, and lifecycle all increase. If a device or component is to stay within its acceptable margins, one must first understand what those limits are. Case in point, a colleague of mine could not understand why his fuse in series with a capacitor repeatedly failed.

Upon further review, the culprit was the mislabeling of the amperage (margin) of the recommended fuse. As it turns out, the board stated 20 Amps but the recommended amperage was 40 amps. He made this discovery by researching the schematics. Margins and adherence to them play an essential part in functionality, performance, and durability. This includes noise margins in CMOS Inverters.

Noise Margins and CMOS Characteristics

In the field of electrical engineering, the maximum voltage amplitude of the external signal you can algebraically add to the noise-free worst-case input level without causing the output voltage to deviate from the allowable logic voltage level is called the noise margin. In the field of communications system engineering, we usually measure the noise margin in decibels (dB). 

Moreover, we define the noise margin as the ratio at which the signal surpasses the minimally acceptable amount.  In regards to a digital circuit, the noise margin is the amount at which the signal surmounts the threshold necessary to generate a "1" or a "0". 

CMOS stands for Complementary Metal-Oxide-Semiconductor. Its fabrication process consists of the use of complementary and symmetrical pairs of p-type and n-type MOSFETs for logic functions. The technology is in use in the construction of IC (Integrated Circuit) chips, microcontrollers, CMOS BIOS, microprocessors, memory chips, and other digital logic circuits.

We can also find the use of CMOS technology in analog circuits like data converters, RF circuits, highly-integrated transceivers (communications), and image sensors. Overall, the two essential characteristics of CMOS devices are low static power consumption and high noise immunity.

Because one of the MOSFET pair is always off, the series combination only draws substantial power momentarily while switching states (on and off). As a result, CMOS devices generally produce less heat than other forms of logic, for example, TTL, which typically has a standing current even if it isn't changing states. 

The Characteristics of CMOS Inverters

CMOS technology integrates into chip logic and VLSI chips with ease. Furthermore, they function at higher speeds while maintaining the characteristics of very little power loss. Moreover, a CMOS inverter provides excellent logic buffering features, since its noise margins in both high and low are equally significant.

Now, let's take a closer look at how CMOS inverters work as well as their characteristics. Firstly, a CMOS inverter contains a PMOS (p-type) and an NMOS (n-type) transistor that connects to the drain and gate terminals. Also, it incorporates a supply voltage (VDD) at the PMOS source terminal and a ground connection at the NMOS source terminal. Finally, it has a VIN connection to the gate terminals, and a VOUT connection to the drain terminals. Keep in mind that the CMOS inverter does not utilize resistors in its design, which translates to higher power efficiency versus standard resistor-MOSFET inverters.

CMOS Inverter circuit diagram

Referencing the above CMOS inverter diagram, as the voltage at the input of the CMOS device varies between 5 and 0 volts, the state of the PMOS and NMOS will differ accordingly. Therefore, if we model each transistor as a simple switch that activates by VIN, then we can undoubtedly see how the CMOS inverter functions.

Noise Margins in CMOS Inverters

Now in reference to pure digital inverters, they do not immediately switch from a "1" (logic-high) to a "0" (logic-low) since there exists some level of capacitance. When an inverter is transitioning from a logic high to a logic low, there is an indistinct region in which we cannot consider the voltage either low or high. It is at this precise moment that we consider it to be our noise margin.

There are two noise margins we must consider, and they are as follows: noise margin high (NMH) and noise margin low (NML). The minimum voltage output of the driving device for a logic high (VOH min) must be larger than the minimum voltage input (VIH min) of the receiving device for a logical high. Since there is noise present on the wire, a logic high signal at the output of the driving device may arrive with a lower voltage at the input of the receiving device.

Hence, the noise margin, NMH = (VOH min – VIH min), for logical high is the range of tolerance for which you can still correctly receive a logical high signal. We can say the same for noise margin, NML = (VIL max – VOL max) for a logical low, which stipulates the range of tolerance for a logical low signal on the wire. A smaller noise margin indicates that a circuit is more sensitive to noise.

Planned circuit diagram utilizing a CMOS inverter 

Planning your layout using a CMOS inverter requires attention to electronic noise.

A noise margin is a standard of design margins to establish proper circuit functionality under specific conditions. A source of noise can include power supplies, the operation environment, electric and magnetic fields, and radiation waves. On-chip transistor switching activity can generate undesirable noise as well. Therefore, to provide proper transistor switching under specific noisy conditions, a circuit's design must include these certain noise margins.  

Using the suite of design and analysis tools available from Cadence, you and your design teams can tackle any noise issue within your designs and verify system integrity. Before jumping into analysis and verification, though, trust Allegro PCB Designer as the premier layout solution for your circuit needs. 

 If you’re looking to learn more about how Cadence has the solution for you, talk to our team of experts and us.

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