Comparator What is a Comparator 2 nd most

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Comparator • What is a Comparator? – 2 nd most widely used building block

Comparator • What is a Comparator? – 2 nd most widely used building block after Op Amp – The comparator is essentially a 1 -bit analog to digital converter. • Input is analog • Output is digital – Types of comparators: 1. Open-loop (op amps without compensation) 2. Regenerative (use of positive feedback - latches) 3. Combination of open-loop and regenerative comparators 1

Circuit symbol: Ideal I-O transfer curve Actual I-O transfer curve 2

Circuit symbol: Ideal I-O transfer curve Actual I-O transfer curve 2

Static Specs • Static Characteristics – Gain • Slope of the linear region •

Static Specs • Static Characteristics – Gain • Slope of the linear region • =(Voh-Vol)/(Vih-Vil) – Output high and low states • Voh, and Vol • Needs to be clearly defined as digital 1, 0 – Input resolution • Vih – vil • Vi_range / (Vih-Vil), can be expressed as bits – Offset • (Vih+Vil)/2 – Noise 3

Comparator Noise of a comparator is modeled as if the comparator were biased in

Comparator Noise of a comparator is modeled as if the comparator were biased in the transition region. Noise leads to an uncertainty in the transition point which causes jitter or phase noise. 4

Offset cancellation 1_advanced Switches off slightly ahead of Φ 1 to minimize charge injection.

Offset cancellation 1_advanced Switches off slightly ahead of Φ 1 to minimize charge injection. 5

Offset cancellation 6

Offset cancellation 6

Must have stability during phi_1 Book’s solution: compensate during phi_1 Chapter 10 Figure 05

Must have stability during phi_1 Book’s solution: compensate during phi_1 Chapter 10 Figure 05 7

Alternate solution: Offset cancellation by stage VDD vs Vin+ vin 2 8

Alternate solution: Offset cancellation by stage VDD vs Vin+ vin 2 8

Propagation Time Delay Rising propagation delay time: Propagation delay =(Rising delay + Falling delay)/2

Propagation Time Delay Rising propagation delay time: Propagation delay =(Rising delay + Falling delay)/2 or = Rising delay + Falling delay 9

Single pole response of amplifier 10

Single pole response of amplifier 10

Time delay due to dynamic response 11

Time delay due to dynamic response 11

Slew Rate of a Comparator If the rate of rise or fall of a

Slew Rate of a Comparator If the rate of rise or fall of a comparator becomes large, the dynamics may be limited by the slew rate. Slew rate comes from the relationship, i = Cdv/dt where i is the current through a capacitor and v is the voltage across it. If the current becomes limited, then the voltage rate becomes limited. Therefore for a comparator that is slew rate limited we have, tp = ∆T =∆V/SR =(VOH- VOL)/2·SR where SR = slew rate of the comparator. 12

Find the propagation delay time of an open loop comparator that has a dominant

Find the propagation delay time of an open loop comparator that has a dominant pole at 103 radians/sec, a dc gain of 104, a slew rate of 1 V/µs, and a binary output voltage swing of 1 V. Assume the applied input voltage is 10 m. V. Solution The input resolution for this comparator is 1 V/104 or 0. 1 m. V. Therefore, the 10 m. V input is 100 times larger than vin(min) giving a k of 100. Therefore, we get tp =1/103 ln(2· 100/(2· 100 -1)) = 10 -3 ln(200/199) = 5. 01µs For slew rate considerations, we get tp =1/(2· 1 x 106) = 0. 5µs Therefore, the propagation delay time for this case is the larger or 5. 01µs. But for Vin=Vin(min), tp =1/103 ln(2/(2 -1)) = 693µs. 13

Two-Stage Comparator An important category of comparators are those which use a high-gain stage

Two-Stage Comparator An important category of comparators are those which use a high-gain stage to drive their outputs between VOH and VOL for very small input voltage changes. The two-stage op amp without compensation is an excellent implementation of a high-gain, open-loop comparator. 14

Delay time 1/(√Av(0)) 15

Delay time 1/(√Av(0)) 15

Push-Pull Comparators Push-pull output higher slew rate 16

Push-Pull Comparators Push-pull output higher slew rate 16

Driving Large Capacitive Load 17

Driving Large Capacitive Load 17

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