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Analog IC Design- Filters -

Yves Leduc,

Advanced System Technology,Wireless Terminals Business Unit

Texas Instruments Inc.

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Introduction

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Analog? Digital?

A communication channel can support a digital or an analog signal.

By the way,

what is an Analog Signal?

what is a Digital Signal?

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There is obviously a difference:

A Hint.

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Analog and Digital Signals

Too easy

But analog and digital signals are sharing the same physical units.

Their values are represented by real numbers.

The representation of a digital signal is a sequence of integer numbers.

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Confused?

The confusion exists.

The IC design addresses two fields:

• Physics: the electrical realization.

• Mathematics: the representation of a signal.

We are interested by the electrical realization affecting the representation of the signal.

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Analog Signal

“The physical values of an analog signal belong to a unique and continuous domain of values.”

It does not mean that an analog signal is continuous.

It does not mean that an analog circuit must be linear.

Signal and noise coexist.

0.00

1.00

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Digital Signal

“The physical values of a digital signal (when settled) belong to at least two distinctive domains. In each domain, the representation of the signal is unique.”

It does not mean that a digital signal has only 2 representations (‘ 0 and 1 ’).

The gaps between domains build noise margins.

0

1

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An Analog Signal

• can be translated. offset + in[t]

• can be amplified. gain * in[t]

• can be filtered. H[s] * in[s]

• can be shaped. Min[ in[t] , 3.0 ]

• can be sampled. in[ti] ti <= t < ti+1

• can be quantized. qn qn <= in[t] < qn+1

• can be converted n qn <= in[t] < qn+1

• can be damaged… in[t]

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A Digital Signal

To process correctly a digital signal, a digital circuit must be regenerative, i.e. non linear and active.

A digital signal, noisy, distorted or attenuated, must be reshaped.

• Non linearity provides the necessary discrimination and allows the reshaping of the signal.

• Amplification is needed to recover from the natural attenuation.

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Digital Signal Processing

The Digital Signal Processing is based on the mathematical representation of the signal, on an abstraction.

A sequence of numbers can be translated, amplified, filtered, shaped, sampled, quantized, converted, and … damaged as an analog signal.

The sampled analog signal processing and the digital signal processing are based on the same mathematics.

0.0100.0050.0150.4950.7250.7800.8050.8300.8550.900...

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Analog Signal Processing

In a Continuous Time Domain.

In a Sampled Domain.

0.00

1.00

0.00

1.00

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“Mixed Signal Design”

0.00

1.00

0.00

1.00

0.0100.0050.0150.4950.7250.7800.8050.8300.8550.900...

Converters,Filters, ..

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Filtering.

At some point in the process of a signal, it is necessary to suppress spurious components.

The characteristics of a filter are very depending on the frequency.

Filters are implemented in the continuous time or the sampled analog domain, or in the digital domain.

Sampled Analog Domain

DigitalDomain

Continuous TimeAnalog Domain

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Be consistent. And careful (1).

What physical characteristics are coding the representation of the signal?

Voltage, Current, Frequency, Phase, Transition, ..?

Continuous time domain, Sampled domain?

Filtering should not damage the representation of the signal.

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Be consistent. And careful (2).

0.00

1.00

Is it a sampled signal?

Is it a (poor) reconstruction of the sampled signal in the continuous time domain?

Passing back and forth from the sampled to the continuous time domain requires specific techniques!

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Basic Elements.

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Cost. Cost. Cost. Cost. Cost. Cost.

Performances are granted.

There is a price for the Differentiation.

The Cost of Ownership is a major driving factor:• BOM (Bill Of Materials)• Cost Overhead

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Just Cost.

Integration is often (*) a solution to meet the cost:

• it may provide a differentiation

• it may reduce the BOM

• it may reduce the cost of the implementation

• it increase your market shares

• ...

(*) ‘often’ does not mean ‘always’

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IC elements.Transistors: MOSFET ..

Bipolar, JFET

Capacitors: Vertical, Lateral

MOSFET

Resistors: Polysilicon, Bulk Silicon, Metal, ..

MOSFET

Inductors: Metal, Bonding Wire ..

Wires: Several levels of Metal

Pads: Metal

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MOS Transistors.Performances: Speed (digital, analog)

Gain, Noise (analog)

Factor Of Merit: Gm / Cox (digital, analog)

Gm / Gds (analog)

Matching (analog)

Major Parameters Effective Length

Photolithography

Process Features

Layout Care B

DS

G

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MOS Transistors.Factor of Merit: NMOS / PMOS 3

Minimum Channel Size: 1 m2

(Idem, low noise input) 100 .. 1000 m2

Gate Oxide: 5 .. 10 nm

Best Practical Matching: 0.1%

Absolute Value Spread: (*)

G

D

S

B

S

G

D

B(*) sensitive information

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Capacitors.Performances: Linearity

MatchingMemory Effect

Factor of Merit: Ceff / Cpar

Density (F / m2)Absolute Value Spread

Major Parameters Dielectric and Dielectric InterfaceSymmetryLayout Care

P2

BP1

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Vertical CapacitorsElectrodes: Polysilicon, Metal

Dielectric: SiO2, Si3N4, ..

Factor of Merit: Ceff / Cpar 10

Density: 1 fF / m2

Best Practical Matching: 0.1%

Absolute Value Spread: 5%

Bulk

Electrode 2

Electrode 1

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Lateral CapacitorsFactor of Merit: Ceff / Cpar (*) ..

Density: (*) / m2

Best Practical Matching: (*) %

Absolute Value Spread: (*) %

Conventional shape or fractal.

(*) no reliable data yet available in the public domain.

Bulk

Electrode 1 Electrode 2

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Resistors.Performances: Voltage Coefficient

Temperature CoefficientMatching

Factor of Merit: Density ( / m2) Resistance / Parasitic CapacitanceAbsolute Value Spread

Major Parameters PhotolithographyLayout Care

B

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Polysilicon ResistorsResistivity: 20 .. 500 /

Best Practical Matching: 0.1 %

Absolute Value Spread: 5 %

Temperature Coefficient: process dependent

Voltage Coefficient: good

Bulk

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Bulk ResistorsResistivity: 1 .. 5000 /

Best Practical Matching: 0.1 %

Absolute Value Spread: 5 %

Voltage Coefficient: fair or poor, process dependent.

Temperature Coefficient: process dependent

BulkDoped Area

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A Few SizesMOS Transistor 10 m2

LNA MOS Transistor 500 m2

Precision Resistor 1 k 500 m2

Capacitor 1 pF 1 000 m2

Bonding Pad 10 000 m2

Inductor 5 nH 250 000 m2

Small IC 10 000 000 m2

Big IC 100 000 000 m2

[including connections]

RC FILTERS

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Conclusions

Inductors are left for RF designs.

External components should be avoided!

Integrated R*C is the preferred time constant to build the poles and zeroes of IC filters.

MOS transistors are appealing but are intrinsically non linear.

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R*C (1)The absolute resistance of an integrated resistor depends on:

• Material resistivity (doping, granularity, ..)

• Material thickness.

• (Photolithography)

The absolute capacitance of an integrated capacitor depends on:

• Thickness of the dielectric material.

• Dielectric constant

• (Photolithography)

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R*C (2)

The time constant depends on 2 uncorrelated values: R and C

R with a sigma of 5%

C with a sigma of 5%

It is impossible to rely on R*C to build accurate filters

at a correct cost.

So what?

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Exercise 1

Optimize = R * C