analog layout - resistors - ulisboa · pdf fileanalog layout - resistors • dummy resistor...
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Analog Layout - Resistors
• dummy resistor should be added in order to minimize the faster etchingin large areas
•Contact resistance must be taken into account for small resistance values
•In order to minimize the noise, the resistor can be designed•with a guard ring •inside a well to reduce the coupling to the substrate
•matching between resistors requires that the resistors are designed in the layout :
• with the same orientation • distributed in a interdigitized or common centroid style
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Analog Layout - Resistors
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• dummy capacitors should be added in order to minimize the faster etching in large areas
• In order to minimize the noise, the capacitor can be designed•with a guard ring •inside a well to reduce the coupling to the substrate
•matching between capacitors requires that the capacitors are designed in the layout using a common centroid style
Analog Layout - Capacitors
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• The gate resistance is reduced by dividing the gate in several sections (each section with a width < 40um). N transistors can be instantiated in parallel if the instance name is INST_NAME<1:N>
• The gate resistance is reduced also by adding contacts in both sides of the poly stripes that implement the gate
• dummy gates can be added in order to minimize the faster etching in large areas
•Guard rings are usefull to obtain noise imunity and good substrate biasing, preventing latch-up
•matching between transistors requires that the layout is designed:• using large areas for the gates• without metal overlapping the gates • distributed in a interdigitized or common centroid style
Analog Layout - Transistors
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Analog Layout - Transistors
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Analog Layout - Transistors
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Common-Centroid Layout:Matching obtained by dividing the gates in two
Topology: DASBDBSAD
BA
Analog Layout - Matching
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Analog Layout - Matching
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Examples of interdigitized MOS topologies:
1. (DASBDBSA)D A:B = 1:1
2. (SADA)(SBDBSBDB)(SADAS)
3. (SADASBDB)S(BDBSADAS)
4. (SADASBDBSADA)S A:B = 2:1
5. (SADASBDBSCDC)S(CDCSBDBSADAS) A:B:C = 1:1:1
Analog Layout - Matching
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Common-Centroid layout design guidelines:
1. Placement: The geometric center of the devices to match must be very near
2. Symmetry: The layout of the devices must be evenly distributed in bothdirections: x and y
3. Regularity: Partial devices must be distributes uniformly
4. Dispersion: The layout must be as compact and square as possible
5. Orientation: The number of partial devices oriented in each direction mustbe the same for each device to be match.
Analog Layout - Matching
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DASBD
DBSAD
B A
• A B / B A compliant with the orientation guideline
Dividing each transistorin two transistors
Common-Centroid
Analog Layout - Matching
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DASBDBSAD
DBSADASBD
Dividing each transistorin 4 transistors
Common-Centroid
Analog Layout - Matching
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DASBDBSAD
DBSADASBD
DASBDBSAD
DBSADASBD
Common-Centroid
DASBDBSADASBDBSAD
DBSADASBDBSADASBD
DASBDBSADASBDBSAD
DBSADASBDBSADASBD
DASBDBSADASBDBSAD
DBSADASBDBSADASBD
Analog Layout - Matching
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Common Source Stage : Voltage Gain
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Common Drain Stage: Output Resistance
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Common Gate Stage : Input Resistance
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Single stage basic topologies summary
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Single stage bandwidth comparison
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2 stage ampopStabilized bias circuit
Analog Layout – 2 stage AMPOP
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Analog design
Initial design criteria (after reading process parameter data):
• current budget limited• overdrive voltage: VGS-VT > 200mV• Lmin = 1µm (avoid short channel effects
and limit sub-threshold current)• W.Lmin: Offset limited• W/L : gm limited
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Overdrive in the differential the pair
> overdrive voltage ⇒ > linearity, < gm
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Vos in the differential the pair
diffdiff
VTdiffTdiffOS LW
AVV 3_max__ =∆=
99,7%
24_ // oodiffmd rrgA =
diffToodiffmdiffO VrrgV _24__ // ∆×=
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Vos in the differential the pair (c. mirror)
diffm
mirrm
mirrmirr
VTmirrOS g
gLW
AV_
_max__
3=
( ) ( )2_
2_ mirrOSdiffOSOS VVV +=
mirrToomirrmmirrO VrrgV _24__ // ∆×=
24_ // oodiffmd rrgA =
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Gain between 5,000 and 10,000
• Advantage: higher gain• Inconvenient: highly restricted common mode
•Ro = Ro2C // Ro4C= (gm2C ro2C ro2)//(gm4C ro4C ro3)
•A1 = - gm1 Ro
Analog Layout – Cascode dif. pair
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Folded-CascodeCascode versus
Analog Layout – Cascode dif. pair
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Ro = Ro2C//Ro4C= [gm2C ro2C (ro7//ro2)]//(gm4C ro4C ro3)
A = gm1 Ro
Advantage: extended common mode
Analog Layout – Cascode dif. pair
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Analog Layout – Folded cascode AMPOP
Enhanced current mirror
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VBE + Self-biasing Circuit
Analog Layout – Bandgap example