Large CMOS ASICs and system-on-a-chip designs often contain both analog and digital sections. Combining the two into a mixed-signal IC frequently leads to noise problems. This article, the second in a two-part series, deals with noise-reduction matters affecting the whole IC.
As discussed in the first article, engineers should think about addressing noise issues as part of the design process to avoid such difficulties during chip debug ["Noise Reduction Is Crucial To Mixed-Signal ASIC Design Success (Part I)," Electronic Design, Oct. 30, p. 123]. Dealing with the trouble after the fact can be costly. Complicating the situation is the fact that Spice simulations often don't show many noise problems. Impedance of interconnects, adjacent device coupling, and substrate noise are usually not modeled accurately.
Transition switching noise is an RF issue, with a very broad spectrum. At these frequencies, connection inductance and parasitic capacitance become significant factors.
Noise coupling is often distributed, with multiple talkers and listeners. Most effective methods of noise reduction include suppression of talkers at the source and use of noise-immune listeners throughout the IC. For our purposes, "noise reduction" refers to both reducing noise sources and using circuits and layouts that make the system less sensitive to noise. Note that including noise immunity in a design doesn't mean larger chips. Done properly, the die area usually doesn't change.
Noise-Reduction Methods Noise-reduction methods can be categorized into four areas: providing low impedance and quiet connections for power, ground, and substrate; designing analog circuits that are less sensitive to noise; reducing or silencing any noise generators; and separating the talker-listeners using proximity separation, separation in frequency, or separation in time.
The following is a summary of the previous article, which dealt with noise immunity of internal circuits.
Differential circuits and signals provide common-mode noise rejection and less sensitivity to power and ground noise.
Limiting circuit bandwidth helps to avoid noise amplification—using just enough bandwidth to get the job done. Wide-bandwidth circuits would amplify undesired system noise as well. Plus, RF filters on analog signals can cut down on parasitic coupling of noise.
Reducing the number of external analog signals minimizes opportunities for noise coupling. Using large-amplitude signals directly improves the signal-to-noise ratio.
Internally distributed power filtering placed in unused areas, under metal traces, and in similar locations can provide better power stability, especially at high frequencies.
Extensive use of grounded substrate contacts, n-well tie-ups, and guard rings (well beyond what design rules require for latch-up protection) will reduce substrate noise.
Signal separation and shielding helps to avoid noise coupling through parasitic capacitance. In addition, by using separated analog-digital routing and keeping talker-listener signals apart, noise concerns will improve.
The most significant source of noise in most mixed-signal ASICs comes from digital transition switching. Strategies to decrease digital noise generation include minimum drive-strength devices, low-noise logic types, differential output drivers, and limited slew-rate devices.
Some analog circuits produce noise due to transition switching. On/off current situations, current and voltage pulses, and any switched step voltages are the items to look for here. Removing these elements, or reducing their effects as a noise generator, will give better noise performance.
Following these eight noise-immunity tips for an IC's internal circuits will go a long way toward creating a noise-robust system. Attention then turns to integrating these circuits onto an IC. This includes circuit placement, selection of I/O drivers, pin placements, interconnect issues, and power, ground, and substrate concerns.
At high frequencies, the impedance associated with interconnect metal, bond wire, and the package's lead frame can become a significant factor in the stability of internal power/ground. For the digital section, maximizing the number of parallel power/ground pins to reduce impedance should be considered as a starting point.
Separating the power/ground connections used for the analog sections from those used for the digital areas will improve the isolation of the power supplies. Analog cells that use large current transients (large di/dt) should be considered for independent power/ground interconnects. If possible, consider redesign of these circuits to a current-steering method to avoid current transients on the power supply.
Figure 1 illustrates the possible interconnect situations. The configuration in Figure 1a is usually the most problematic method for power stability. Any transition currents convert directly to noise on the analog power. This is due to interconnect impedance and the impedance of the package and bond wire. A somewhat better layout is represented by Figure 1b. This arrangement eliminates the internal interconnect impedance as part of the analog circuit's power-supply connection.
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