PD Dr.-Ing. Stephan Henzler
Stephan Henzler Mixed-Signal-Electronics 2011/12 1

Stephan Henzler Mixed-Signal-Electronics 2011/12 2

Stephan Henzler Mixed-Signal-Electronics 2011/12 3

Stephan Henzler Mixed-Signal-Electronics 2011/12 4

Stephan Henzler Mixed-Signal-Electronics 2011/12 5

Stephan Henzler Mixed-Signal-Electronics 2011/12 6

Comparators
Stephan Henzler Mixed-Signal-Electronics 2011/12 7

Ideal Comparator
 Compare input signal to reference and provide binary output signal
 Often same symbol as for opamp
(reasonable as open loop opamp behaves like a comparator)
 Comparator is essentially an amplifier with saturation, ideal comparator means infinite gain in VCVS  not realistic
Stephan Henzler Mixed-Signal-Electronics 2011/12 8

Static Characteristics of Comparator
 Comparator gain
 Maximum voltage for negative saturation V
DL
 Minimum voltage for positive saturation V
DH
 Comparator resolution:
(min. voltage increment, determines comparator gain)
 Offset voltage: Horizontal shift of characteristic
 Input common mode range
Stephan Henzler Mixed-Signal-Electronics 2011/12 9

Dynamic Characteristics of Comparator
Note:
Comparators work in large signal mode of operation
– basic circuit theory to reveal trade-offs and mechanisms
– simulation to determine actual performance figures
 Main dynamic performance figure: propagation delay td
Stephan Henzler Mixed-Signal-Electronics 2011/12 10

Operational Amplifier as Comparator
 Opamp in open-loop configuration is comparator
– asynchronous
– relatively slow due to high gain and stability requirement
– offset error
(may be compensated by correlated double sampling, but this also means synchronous operation)
– Consider DC operating point at input for a reference voltage ≠ 0
– Compensation cap may be disconnected during latching
Stephan Henzler Mixed-Signal-Electronics 2011/12 11

OpAmp Comparator Dynamics
 Gain-Bandwidth trade-off
– gain determined by desired resolution
– bandwidth determined by desired propagation delay
 Amplifier model
Response of Stable 1st Order Linear System
1
0.9
 Step response
0.8
0.7
0.6
0.5
0.4
0.3
 Propagation Delay
0.2
0.1
0
0 2 4 time [AU]
6 8 10
Stephan Henzler Mixed-Signal-Electronics 2011/12 12

Comparator Propagation Delay
 Linear mode of operation
Propagation delay for small input signals is determined by linear small signal dynamics of amplifier
 Slew rate limited mode of operation
Propagation delay for large input signals is dominated by slew rate of opamp output stage
 Propagation delay for slew rate limited operation
Stephan Henzler Mixed-Signal-Electronics 2011/12 13

Discrete Time Comparators
 In some applications comparator function only desired
– during certain intervals
– at certain discrete time instances
 Allows for offset compensation via auto-zeroing and other switched capacitor benefits
 Allows for amplifiers in positive feedback configuration
– full level always reached
– gain boosting (reuse one amplifier by cyclic amplification
Stephan Henzler Mixed-Signal-Electronics 2011/12 14

Track & Latch Circuit I
Stephan Henzler Mixed-Signal-Electronics 2011/12 15

Track & Latch Circuit II
Stephan Henzler Mixed-Signal-Electronics 2011/12 16

Principle of Track-and-Latch Stage
Stephan Henzler Mixed-Signal-Electronics 2011/12 17

Linear Dynamic of Latch
 Linear small signal analysis (ref. Schaltungstechnik 2)
 Node voltages
 Differential voltage
 Propagation delay
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1
0.9
0.8
0.7
Response of Instable 1st Order Linear System (Latch)
0.4
0.1
0.01
0.5
1 1.5
2 time [AU]
2.5
3 3.5
4
Stephan Henzler Mixed-Signal-Electronics 2011/12 18

Latched Comparators I
 Standard architecture for high-speed comparators
 Latch offset voltage limits resolution of latch-only comparator
 Two step approach:
– analog pre-amplifier stage(s)
– regenerative track and latch stage
Stephan Henzler Mixed-Signal-Electronics 2011/12 19

Latched Comparators II
 Pre-amplifier:
– 1-3 amplifier stages
– low gain, high-speed
– delay along amplifier chain
– separation of input from latch to reduce loading and avoid kickback effect
Stephan Henzler Mixed-Signal-Electronics 2011/12 20

Latched Comparators III
 Track & latch circuit:
– amplifies signal in track mode
– restores (regenerates) signal to full rail in regenerative latch mode (positive feedback)
Stephan Henzler Mixed-Signal-Electronics 2011/12 21

Input Referred Offset of Latch
 Input referred offset error of latch stage is reduced by gain A of pre-amplifier
Stephan Henzler Mixed-Signal-Electronics 2011/12 22

Current Mode (CML) Latch
 Combines amplifier and latch functionality
Stephan Henzler Mixed-Signal-Electronics 2011/12 23

Memory and Hysteresis in Comparators
 Hysteresis:
Switching threshold is different when switching from low to high and from high to low, respectively.
Useful to avoid bouncing outputs for small (noisy) signals near comparator threshold
 Memory effect:
Kind of hysteresis that causes the comparator decision to be dependent on previous decisions.
Has to be strongly avoided in Nyquist rate ADCs such as flash converters.
Stephan Henzler Mixed-Signal-Electronics 2011/12 24

Elimination of Memory Effect
 Precharge and equalize circuit elements eliminate all information from previous cycles and decisions
Stephan Henzler Mixed-Signal-Electronics 2011/12 25

Switched Capacitor Comparator
 Offset compensated
 Threshold determined by capacitynce ratio
Stephan Henzler Mixed-Signal-Electronics 2011/12 26