Noise Immunity – A Mandatory Design Aspect for Integrated Circuits and Systems
By Wolfgang Damm, Director Product Marketing – Wireless Telecom Group
Ultra-fast switching speed of modern electronic components and devices requires new Vcc and GND architectures – on board and on chip.
Operating a digital device within the specified min-max-power range does not guarantee that Vcc noise will not affect electronic circuits’ switching behavior and signal response. This article describes how unwanted signals at Vcc and GND can cause phase noise and signal jitter in electronic components and systems, and what designers and test engineers can do to ensure noise immunity of their designs.
GND is regarded as a stable reference level and Vcc is thought to be a static level supply, but this is typically not the case with modern high speed, high frequency electronics. With increasing integration and growing requirements for speed and accuracy, noise will permeate supply the power and affect the functionality of integrated circuits and systems.
What Circuits are Vulnerable to Noise on VCC?
The short answer to the question is: all of them. Some applications are less critical, but many show serious distortion if unwanted noise is present at GND and Vcc.
Serial Data Circuits
High-speed data signals rely upon serial transmission: examples are USB-3, SATA-3, and PCIe systems. At some point, these signals do no longer resemble pure rectangular forms. The eye diagram reveals that the transmission path causes distortion and the signal takes on sinusoidal character. Serial data receivers recover their embedded clock from the data stream. Obviously, any variation of data timing will decrease reliability of the receiver.
Precision Analog/ Digital and Digital/Analog converters are naturally sensitive to noise at Vcc. With 16-bit, 18-bit, 20-bit of even higher resolution, these converters come with ultra stable reference sources. Still, noise on Vcc can reduce the overall accuracy easily by 1 to 3 bits. To regain the original accuracy, the converters have to be immunized against noise on Vcc.
Although linear amplifier use internal current sources to supply their circuitry, they are not immune against fast Vcc variations. While the current source compensates for any change of Vcc, some noise elements pass through, reaching the sensitive input stages. Amplification multiplies this distortion.
Phase-Locked Loop (PLL) circuits are widely used for frequency generation, clock synchronization and in demodulators or FSK decoders. Their functionality is based on phase sensitive detection of differences between input and output signals of the control oscillator.
PLLs are everywhere in the electronic world; examples are CPUs which utilize many PLLs to generate internal and external clocks for their operation and communication with other, on board devices. This applies also to other data processing devices. Low frequency clocks are fed to the chip, but are internally multiplied by PLLs to the desired frequencies. By nature, PLLs are susceptible to any unwanted signal or noise at Vcc. Special attention is necessary to achieve sufficient noise immunity.
Effects of Noise on the Vcc Power Bus?
As described above, amplifiers react to noise at their inputs, which distorts the output signal and results in lower transmission signal quality. Switching circuits, which incorporate basically every digital circuit, show internal jitter and also signal edge jitter if noise is present at Vcc. Analyzing serial transmission devices shows, that the opening in the eye-diagram becomes significantly smaller, and results in an increase in system BER.
What are the Sources for Noise on VCC?
The main culprit is the switch-mode power supply. They operate with pulse-width-modulation, switching high-amplitude, high-frequency energy. A low-pass filter should block ripple voltages at the switching frequency and the harmonic frequencies to avoid electromagnetic interference (EMI), but unfortunately, the “fingerprint” of a switched-mode power supply is all over the printed circuit board and even within the chip itself.
GND and Vcc traces are equivalent to a circuit of a series of inductors. The power supply cable, connectors and the VCC trace all behave like small inductors. The values are very small indeed, but they add up, and fast switching high speed circuits demand very fast Vcc supply responses.
Surprisingly, the other key source of Vcc noise are integrated circuits themselves. This self-induced noise comes from logical switches that cause charge carriers to change their status. Each driver of an integrated circuit output pin connected to a board trace has to deal with a residual trace capacitance. Output level changes from logical “1” to “0” require the capacitor to be discharged, and conversely, output level changes from “0” to “1” need to charge the capacitance. Currents that are required for this are significantly higher than the currents that drive the inputs. This results in Ground bounce or Vcc jitter; and is particularly critical when parallel output ports switch their status at the same time to the same level.
Tools to Improve Noise Immunity
A first tier defense against unwanted noise on Vcc are support capacitors, positioned in the direct proximity to the Vcc pin of an IC. While these capacitors reduce noise significantly, they will not eliminate it. Developers must ensure that their designs are operating in the desired way, even with residual noise at Vcc; they have to immunize their electronics against noise.
Some design tools simulate noise energy at Vcc. Results may provide some indication on the chip’s behavior. The best way to improve immunization is to inject specific amounts of noise and CW signals into the Vcc line and analyze it directly. Instruments like Noisecom’s JV9000 Vcc Noise Generator provide these signals. Defined levels of noise or CW signals are coupled onto the Vcc bus, allowing study of the circuit’s response to noise and noise immunity.
Phase noise and jitter have always been a concern of circuit designers. With increasing switching speeds, GND and Vcc sources and references should be considered frequency dependent components. Special attention should be paid to the circuits’ Vcc design to ensure the functionality of the circuit will not be compromised by noise elements on GND or Vcc busses. Special generators, like Noisecom’s JV9000 allow injecting specific amounts of noise and CW signal energy. The circuits’ response to noise on Vcc can be analyzed. Noise immunity is paramount to ensure flawless functionality of electronic circuits even under challenging supply power conditions.
Wireless Telecom Group
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