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Analog layout


Analog layout definitions tells that it is the layout of ciruit bears analog signals, it is mainly divided into two types, one is on PCB and the other is on silicon,
in both the cases pronciples are same, here is some analog layout issues and lecture notes
In doing layouts for digital circuits, the speed and the area are the two most important issues. In contrast, in doing layout for analog circuits, everything should be considered simultaneously. In addition to the speed and the area, other equally critical considerations should be taken into account.
For example, for amplifier design, good matching in devices is necessary to minimize the offset voltage, and good shielding is required to protect critical nodes from being disturbed. Without proper layout, the mismatches and the coupled noise would be quite large and would significantly degrade the performance of the amplifiers.

Analog layout Issues

Matching of Devices:
Matching of individual devices is of paramount concern in analog circuit design. Infact almost all of the analog layout techniques are actually methods for improving matching between different devices on a chip. Matching is important because most analog circuit designs use a ratio based design technique(e.g. current mirrors). Some common techniques that help improve device mathcing are MULTI-GATE FINGER LAYOUT and COMMON-CENTROID LAYOUT.
Noise:
Noise is important in all analog circuits because it limits dynamic range. In general there are two types of noise, random noise and environmental noise. Random noise refers to noise generated by resistors and active devices in an integrated circuit; environmental noise refers to unwanted signals that are generated by humans. Two common examples of environmental noise are switching of digital circuits and 60 Hz hum In general, random noise is dealt with at the circuit design level. However the are some layout techniques which can help to reduce random noise. MULTI-GATE FINGER LAYOUT reduces the gate resistance of the poly-silicon and the neutral body region, which are both random noise sources. Generous use of SUBSTRATE PLUGS will help to reduce the resistance of the neutral body region, and thus will minimize the noise contributed by this resistance.
Enivironmental noise is also dealt with at the circuit level. One common design technique used to minimize the effects of environmental noise is to employ a fully-differential circuit design, since environmental noise generally appears as a common-mode signal. However SUBSTRATE PLUGGING is also very useful for reducingsubstrate noise, which is a particularly troublesome form of environmental noise encountered in highly integrated mixed-signal systems and Systems-On-a-Chip (SOC). Substrate noise occurs when a large amount digital circuits are present on a chip. The switching of a large number of circuits discharges large dynamic currents to the substrate, which cause the substrate voltage to bounce. The modulation of the substrate voltage can then couple into analog circuits via the body effect or parasitic capacitances. SUBSTRATE PLUGGING minimizes substrate noise because it provides a low impedance path to ground for the noise current.
Note:
Issues that are important in digital circuits are still important in analog layout. Foremost among these is parasitic aware layout. It is important to minimize series resistance in digital circuits because it slows switching speed. Series resistance also slows analog circuits, plus it introduces unwanted noise. Parasitic capacitance is avoided in digital circuits because it slows switching speed and/or increases dynamic power dissipation. Stray capacitance has the same effect in analog circuits (bias current must be increased to maintain bandwidth and/or slew rate when extra load capacitance is present) plus it can lead to instability in high gain feedback systems.
Common error reduction technique :
Use large area to reduce random error
Common Centroid layout to reduce linear gradient errors
Use unit element arrays
Interdigitize for matching
Use of symmetry of photolithographic invariance
Controlled edge or corner effects
Dummy device for similar vicinity
Guard rings for isolation
Careful floor planning,
More details
CMOS Technology :
Flow varies with process types company
– N-Well CMOS
– Twin-Well CMOS
• Start with substrate selection
– Type: n or p
– Doping level, ?resistivity
– Orientation, 100, or 101, etc
– Other parameterscmos technology
What is BiCMOS?
BiCMOS technology combines Bipolar and CMOS transistors onto a single integrated circuit where the advantages of both can be utilized.
bicmos process
failure mechanisms Failure Modes
• Thermal Secondary breakdown – high power, small junction causes junction melting
• Metallization Melt – ESD causes the metal to melt and
bond wires to fuse, usually causes and open circuit
• Dielectric Breakdown – high potential difference across a
dielectric region cause a punch through
• Bulk Breakdown – changes in junction parameters caused
by excessive temperature at the junction
failure mechanismWhat is Charge Spreading ?
Charge spreading is the mechanism underlying the formation of channels.
It requires the presence of static electric charges at the insulating interface.
These charges consists primarily of electrons.
Hot carrier injection also contributes to charge spreading along with integrated circuits that do not
produce hot carriers.
capacitors in vlsi
• Coupling AC Signals
• Constructing timing networks
• Constructing phase shift networks
• Feedback loop compensation
cmos capacitorsMatching of Resistors and Capacitors
matching of passive devices