Layout of Analog Circuits - MiXeDsIgNaL · PDF fileLayout of Analog Circuits Jyotirmoy Ghosh...
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Layout of Analog Circuits
Jyotirmoy Ghosh
Asudeb Dutta
Advanced VLSI Design Lab
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MOSFET (NMOS) StructureMOSFET (NMOS) Structure
p-substrate
n+ n+
OxidePolyS
GD
Leff
Ldrawn
LD
W
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MOS Device Layout
NMOS Layout
PMOS Layout
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Stick Diagram (Symbolic Layout)In stick diagram the lines represents the corresponding layers in layout .i.e. rather than drawing a rectangle to draw poly you are just drawing a line. this simplify designer's work in drawing layout "on paper" 1 Dimensionless layout entities with legend for each layer 2 Only topology is important
Stick Diagram of Inverter Actual Layout of Inverter
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Layout Steps
•Floor planningDivision of the entire die area among subcomponents to facilitate interconnection and effectively utilize the area.
•PlacementPlacing the modules in the layout.
•RoutingConnecting the modules with different metal layers.
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Issues of Analog Layout
• Use of more number vias
• Fingering and proper orientation
• Device matching
• Symmetrical and common centriod layout design
• Use of Guard ring and substrate trapping
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Resistors:1) RPD (P+ Diffusion) -> R (sheet)= 83 ohm/�
Metal 1P - diffusion
salex
Passive devicesResistance (cont’d)
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P+ Diffusion (RPD)
Equivalent Model
Passive devicesResistance (cont’d)
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Resistors:1) RND (N+ Diffusion) -> R (sheet) = 32 ohm/�
N - diffusionMetal 1
salex
Passive devicesResistance (cont’d)
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N+ Diffusion (RND)
Equivalent Model
Passive devicesResistance (cont’d)
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Resistors:1) RPP (P+ Poly) -> R (sheet) = 175 ohm/�
Metal 1Poly
salex
P+
Passive devicesResistance (cont’d)
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Equivalent Model
Passive devicesResistance (cont’d)
P+
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Resistors:1) RNP (N+ Poly) -> R (sheet) = 125 ohm/�
Metal 1Poly
salex
Passive devicesResistance (cont’d)
N+
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Equivalent Model
Passive devicesResistance (cont’d)
N+
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Choice of Resistances:Parasitic effect
Process variation,
Temperature variation,
Operating frequency
Area of resistance
There are many others resistors : RWA, PHVPP,RHVNP etc
Passive devicesResistance (cont’d)
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Passive devicesCapacitance
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Capacitor:CPP (over the substrate) : 0.86*10-3 F/m2
poly
substrate
oxide Field Oxide (FOX)Lower plateUpper plate
Passive devicesCapacitance (cont’d)
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Ca =0.8629e-3 Cf = 0.8629e-3 (F/m^2)
Passive devicesCapacitance (cont’d)
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Capacitor:CPP (over the Nwell) : 0.86*10-3 F/m2
poly
substrate
oxide Field Oxide (FOX)Lower plateUpper plate
nwell
Passive devicesCapacitance (cont’d)
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Ca =0.8629e-3 Cf = 0.8629e-3 (F/m^2)
Passive devicesCapacitance (cont’d)
Nwell
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Capacitor:Accumulation capacitor : 6.166*10-3 F/m2
substrate
oxide
nwell
N+ implant
poly
Upper plateLower plate
Passive devicesCapacitance (cont’d)
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There are many others capacitors :COMB cap, Interdigtized Cap, MOS Varactor cap
Passive devicesCapacitance (cont’d)
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Capacitor:
•Good matching accuracy
•Low voltage coefficient
•Less parasitic capacitance
•High capacitance per area
•Low temp. coefficient
Passive devicesCapacitance (cont’d)
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Interconnection
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One Via resistance = 4- 5 ohm
Interconnection (cont’d)
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p-substrate
n+ n+
GS D
p+
B
NMOS & PMOS (CMOS) on same substrate
p+ n+ n+
GDSB
p-substrate
n-well
BDG
S
p+ p+ n+
B G DS
n-substrate
p+ p+n+
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Latch up problem
p+ n+ n+
GDSB
p-substrate
n-well
BDG
S
p+ p+ n+
Vdd
Vss
Vss
Vdd
Permanent current flow between Vdd and Vss
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Substrate Coupling
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Substrate Coupling
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Metal 1
N+
POLY
Composite
Guard ring
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Guard ring and Substrate Contact
Many MOSs may in a single ring.The purpose of the ring is to bias the bulk also.
It removes the latch up problem also.It is used around the passive devices also. It reduces the interference from the adjacent blocks.Width of the ring should not be bigger than a limit to ensure proper biasing.
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Proper ground connection
•All the modules of the chip should be properly grounded.
•Use star ground.
•Ground metal should be wider.
•Vdd metal should also be wider.
•Try to avoid same Vdd line for a noisy and sensitive blocks.
•Use different pins for the noisy and sensitive blocks.
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Star Ground
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Things to remember
•Keep sufficient spacing between power blocks and sensitive blocks.
•Two high frequency carrying pins should not be side by side.
•Use ground pin to avoid magnetic coupling between two pins.
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Matching of the devices
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Why Special attention on Matching ?
A large variety of analog circuits rely on matching of transistors. Circuits like differential pair rely on gate to source voltage matching while current mirrors rely on current matching.
Most integrated resistors and capacitors have a tolerance of about 20% to 30%. But ratio of two similar components can be controlled to a tolerance of 15 or even 0.1% by proper matching of the components.
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Reasons of Mismatch
Systematic mismatches which are caused by :
Process biases
Mechanical stress
Temperature gradients
Polysilicon etch rates etc.
Mismatch in integrated circuits are generally of two types :
Random mismatches due to microscopic fluctuations in dimensions, doping, oxide thickness and other parameters that influence component values.
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How does mismatch affect the performance of the circuit ?
I1 I2I1 = ½ µnCoxWL
(VGS-Vt1)2
1
I2 = ½ µnCoxWL
(VGS-Vt2)2
2
Defining average and mismatch quantities, we have
I = (I1+I2)/2 , ∆I = I1-I2 , W/L = [(W/L)1 + (W/L)2]/2
Vt = (Vt1+Vt2)/2 , ∆Vt = Vt1-Vt2
Current Mirror
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Substituting these expressions and neglecting higher order terms we obtain :
∆ΙI =
∆(W/L)
W/L
∆Vt
( VGS – Vt)/2-
Thus from the above equation we can see that the mismatch in the current depend upon
1) Mismatch in the (W/L) values of the transistors.
2) Mismatch in the threshold values of the transistors which increases as the overdrive voltage ( VGS-Vt) is reduced.
How does mismatch affect the performance of the circuit ? (cont’d)
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Input Offset voltage of a differential pair
VOS = ∆Vt + ( VGS – Vt)
2- ∆ RL
RL- ∆( W/L)
W/L
Thus we see that the offset voltage depends upon two parameters :
The first component is the threshold voltage mismatch of the transistors . This depends upon the layout and it can be reduced by careful layout.
The second component of the offset scales with the overdrive voltage and is related to mismatch in the load elements and mismatch in the W/L values.
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Rules for MOS transistor matching
Place transistors in close proximity.
Orient transistors in the same direction.
Keep the layout of the transistors as compact as possible
Whenever possible use Common centroid layouts.
Place transistors segments in the areas of low stress gradients.
Place transistors well away from the power devices.
For current matching keep overdrive voltage large.
For voltage matching keep overdrive voltage smaller.
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Rules for resistor and capacitor matching
Construct matched resistors of same type.
Make matched resistors of the same width.
Orient matched resistors in the same direction.
Place matched resistors in close proximity.
Place the matched resistors in such a way that their centroids coincide i.e. interdigitate arrayed resistors.
Place dummies on either end of the resistor array.
Connect matched resistors to cancel thermoelectric effects.
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Common Centroid Layout
Gradient-induced mismatches can be minimized by reducing the distance between the centroids of the matched devices. The layouts which actually reduce the distance between centroids of the matched pair to zero are called common centroid layouts.
A B B AD SD SD SD SD
Common Centroid Layout of two MOS
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Interdigitation can also be done in 2 dimensions
DASDBSDBSDAS
DBSDASDASDBS
DASDBS
DBSDAS
Common Centroid Layout for Resistors
R1 R2 R2 R1
Common Centroid Layout (cont’d)
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Layout of Matched Resistors
r2
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Fingering of MOS and Common-centroid Layout example
Fingering of MOS and Common-centroid Layout example
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Layout of Multi-finger Transistors
G
Fingering
Drawback of Fingering
Reduces gate resistance. Improves noise and delay
Increases drain and source side-wall capacitance.
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Example of MOS Layout with fingers
•Power MOSFET layout with large W/L ratio (in the order of 105-106)
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Layout of Standard blocksCascode Transistors
Differential Pair
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Layout of Standard blocks (cont’d)
2-D Common-centriod
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PAD, PIN & PACKAGE
Pad cap ~80f-2pF
Bondwire Inductor=1nH/mm
Pin Inductor=1-2nH
Pin Cap=300fF
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Thank You