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slide 1 of 13University of Toronto D.A. Johns, K. Martin, 1997

Comparators

David Johns and Ken MartinUniversity of Toronto

(johns@eecg.toronto.edu)(martin@eecg.toronto.edu)

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slide 2 of 13University of Toronto D.A. Johns, K. Martin, 1997

Opamp does not work well!

Slow

Compensation capacitor lowers slew-rate (shoulddisconnect capacitor for higher speed)

Inaccurate

Offset of input stage might be 2mV - 10mV

Limited input range

Common-mode input range poor not good if is near

one of power supplies (flash converters)

Vin

Vr

Vout Vin Vr( )sgn=

Vr

A/D

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slide 4 of 13University of Toronto D.A. Johns, K. Martin, 1997

Comparators - Offset Cancellation

Shown phases does not require charging of C (on ,

capacitor is not charged)

is advanced version of to reduce charge-injection

effects (more below)

Opamp compensation cap should be disconnected duringevaluation phase for faster slew-rate

Advantages Very low offset possible

Input common-mode range of amplifier near zero

Also reduces 1/f noise

Disadvantages

Requires floating capacitor and switches

2

1a 1

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slide 6 of 13University of Toronto D.A. Johns, K. Martin, 1997

Making Charge Injection Signal Independent

Advance Q3 so it turns off first [Haigh, 83]

Reasoning

Q3 off causes right side of C to be open circuited

When Q2 turns off, charge injection cannot change chargeon C

When Q1 on, goes to and goes back to correct

value (opamp offset voltage)

V Vin

V

1a

1

2a

2

2a

2

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slide 7 of 13University of Toronto D.A. Johns, K. Martin, 1997

Minimizing Charge Injection Errors

Can use large C but slows down circuit

Fully differential design

Only mismatch in charge injection cause errors

Likely 10 times smaller than single-ended case

C Q3a

C Q3b

Vin Vout

1

1 1a

1a

2

2

Vr+

Vr-

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slide 8 of 13University of Toronto D.A. Johns, K. Martin, 1997

Mult-Stage Comparator

[Poujois, 78] [Vittoz, 85]

Clock feedthrough stored on coupling caps

Used together with fully-diff design

OA1OA2

Vin2 C1

1

C2C3

OA3

Vout

2

1

1 1 1

1

1

Vr

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slide 9 of 13University of Toronto D.A. Johns, K. Martin, 1997

Multi-Stage Comparator

When goes low, charge OA1 output glitches but

recovers

OA1 input side goes negative causing OA1 output to gopositive

Since OA2 still being reset, error stored on C2

1

C3

OA2

C2

OA1

Cov1Cov1

C1

Negative

VDD

VSS

Q1

1

1

charge

injection

Vr

1

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slide 10 of 13University of Toronto D.A. Johns, K. Martin, 1997

Latched Comparators

1 or 2 stages of preamp followed by track-and-latch

Vin+

Vin

Vout

Vout+

Vltch

Track-and-latch stagePreamplifier stage

Vltch

VltchVltch

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slide 11 of 13University of Toronto D.A. Johns, K. Martin, 1997

Latched Comparators

Preamp used to improve resolution and reduce kickback

Kickback result of charge transfer to inputs when goingfrom track to latch mode

Use preamp and match input impedances Preamp and track both have gain

Positive feedback regenerates signal to logic levels

Hysteresis effects reduced by setting internal nodes toknown values each switching interval

Known values likely setting latch to its trip point

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slide 12 of 13University of Toronto D.A. Johns, K. Martin, 1997

Example CMOS Comparator

[Song, 90]

Preamp Gain stage Positive feedback

Trk

Vout

Vin

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slide 13 of 13University of Toronto D.A. Johns, K. Martin, 1997

Example CMOS Comparator

[Norsworthy, 89]

Vin

Preamp

Positive feedback

Latch

Digital signal-

level restoration

Vout

Latch