Waveshapping Circuits and Data Converters. Lesson #17 Comparators and Schmitt Triggers Section BME 373 Electronics II J.

Transcription

1 Waeshappg Crcuts and Data Cnerters Lessn #7 Cmparatrs and Schmtt Trggers Sectn. BME 7 Electrncs II 0

2 Waeshappg Crcuts and Data Cnerters Cmparatrs and Schmtt Trggers Astable Multbratrs and Tmers ectfers, Peak Detectrs, Sample-and- Hld A/D and D/A Cnerters BME 7 Electrncs II

3 Cmparatrs Crcuts whch cmpare tw put ltages, and, and prduces a lgc utput E.g. Hgh f > Lw f < Inputs: Inerted and nn-erted = - Ideal Transfer Characterstcs Symmetrcal r Unsymmetrcal = BME 7 Electrncs II

4 eal Cmparatrs eal Cmparatrs hae a gradual transstn > 0. m t cause a change state. -m m Interface Analg t Dgtal sgnals and may hae dfferent ltage ranges 5 t 5 n the analg sde 5 t 0 n the dgtal sde BME 7 Electrncs II

5 Schmtt Trggers Nse n the put can cause undesrable transtns the utput f a cmparatr. Example: cmparg a nsy sgnal t a reference ltage. r Thrugh the use f pste feedback t the cmparatr, ths prblem can be elmated - The Schmtt Trgger BME 7 Electrncs II 4

6 Effects f Pste Feedback Frm ths crcut: = A and = f = β Because f the pste feedback s n lnger equal t zer (nt a rtual grund) S as creases the pste (negate) drectn, creases the pste (negate) drectn. Because f the pste feedback, ths wll crease the pste drectn (negate) whch wll further crease whch further crease and s n. When wll ths stp? Σ β A β ut If we had fte pwer, then neer. Hweer, we hae lmted pwer whch s gen by the amplfer s DC ltage supples: A, -A. If ges pste, then stantaneusly grws t A lts And f ges negate, then stantaneusly grws t -A lts BME 7 Electrncs II 5

7 Hysteress Assume that β= /( ) = 0. and leels are 0 (fr > 0) and 0 (fr < 0). Frst, nte that = f -. Nw, let s assume = 0 and therefre f = then as lng as s less than, then = 0 (t s hgh state) sce, the put t the cmparatr, wll be > 0. Once surpasses, < 0, and the utput wll swtch t 0. At ths pt, f = - and as lng as the > -, the utput wll stay ts lw state, -0. Nte that has the characterstc f beg a flp-flp. If ne pulses t wth hgh (>), then the utput swtches t a lw and sa ersa. f f f - 0. Fr 0; Fr 0; f 0-0 Ths s characterstc s called hysteress BME 7 Electrncs II 6

8 Inerter f f f Fr 0; A 0A 0A Fr 0; A 0A 0A A A f βa A βa Nte that A lts are the threshlds fr when the crcut swtches states. -A BME 7 Electrncs II 7

9 Other Frms f Schmtt Trggers Nn-ertg types - Specfed Threshlds SS 4 ss - t BME 7 Electrncs II 8

10 Specfc Threshlds SS t 0; A 0; A ut A t Frm nde at nnerg put: t t t SS 0 SS SS SS G G G t T T T SS T T A A 0; SS SS t T T T T A A 0; SS SS t T T T T -A t t BME 7 Electrncs II 9

11 An Example Chse the resstrs t prde threshlds f 5±0. fr utput leels f ±4.6. t SS At the nn - ertg mde, we hae : t Usg Usg fr 4. 9 fr 0 and t t SS t SS 5fr. -4.6, we hae t 4.6, we hae We need t chse ne f the resstrs. If we chse = M, then =0.55 k and = 0.8 k. If we chse resstrs t small then may draw excesse amunts f current frm ur 5 supply and create a large pwer dra n the crcut. BME 7 Electrncs II 0

12 t when 0; 4.6 therefre, t 0 ; r 5. t Example cnttued 4.6 ut t when 0; 4.6 therefre, t 0 ; r 4.9 t BME 7 Electrncs II

13 Anther Example What are the transfer characterstcs fr ths crcut f =k and = k and the threshlds leels are ±0. Fr ( 0, 0 0; Fr 0, 0 t t ) 0; ; ; 5 5 BME 7 Electrncs II

14 Hmewrk Cmparatrs and Schmtt Trgger Crcuts Prblems:.8-9 BME 7 Electrncs II