CONTINUE ON LEADS PLACEMENT

Transcription

1 Hert Letrure ECG leds, ECE4610, Z. Moussv CONTINUE ON LEADS PLACEMENT A I L LA I II III 0 II F III Three ddtonl leds re unpolr eletrodes euse they onsst of the potentl pperng on one eletrode tken th respet to n equvlent referene eletrode, hh s the verge of the sgnls seen t to or more eletrodes. One of suh referene eletrodes s the Wlson Centrl Termnl A LA etter use > 5 MΩ LL Wlson Centrl Termnl The voltge eteen A nd entre LA nd entre L LL nd enter F Pge 1 of 11

2 Sne shunts the rut eteen the entrl termnl nd the lm eletrode, ths tends to redue the mpltude. Modfy these leds to ugmented leds. v / A LA A LA / v L L LL L LL A LA L / LL F

3 v (1) / 1 v A v ± ± LA ± LL Compre the rut (1) th the one elo: () v ± ± ± / Thevenn s Equvlent of the ove rut ± ± v v (1) Also, the proof of one of the equtons of the prevous leture:

4 4 ) ( ) ( II I II I No, from the rut (): / No () Compre Equtons (1) nd (): 50% nrese n mpltude. Se. 6. of Wester Textook Prolems Frequently Enountered n ECG eordng 1. Frequeny Dstorton Hgh frequeny dstorton f nd pss flter hs 0 5 Hz response Lo frequeny dstorton f nd pss flter hs Hz response In lo frequeny dstorton, the selne s no longer horzontl, espelly mmedtely follong ny event n the trng. Monophs ves pper to e phs.. Sturton or Cutoff Dstorton The peks of QS n e utoff euse the output of the mplfer nnot exeed the sturton voltge or even the loer porton of the S ve ll e utoff.. Ground Loop

5 ECG G G Another mhne X ECG Mhne X No Ground Loop! In ddton, ths urrent s presentng sfety prolem, t n elevte the ptent s ody potentl to some voltge ove the loest Ground (ths se G 1 ) nd ths produes ommonmode voltge on the ECG leds tht f s not very hgh, t uses prolems. 4. Open Led Wre Alys mke sure tht the eletrodes re n good ontt. Otherse, usully hgh potentls n often e ndued n the open re s result of eletr feld. Comng from poer lne, t uses deonstntmpltude defleton on the ECG tre. 5. Artfts From Lrge Eletr Trnsents In some stutons, n hh ptent s hvng n ECG tken, rd defrllton my e requred. In suh se, hghvoltge, hghurrent eletr pulse s ppled to the ptent s hest. These potentls n e severl orders of mgntude hgher thn norml ECG potentls. When ths stuton ours, t n use n rupt defleton the ECG, hh s due to sturton of mplfers n the ECG. The pulse s suffently lrge enough to use the uld up hrge on the ouplng ptnes n the mplfer, resultng n sturton nd t 5

6 remns sturted for fnte perod of tme nd sloly drftng k to ts orgnl selne th tme (τ) determned y the lo utoff frequeny of the mplfer. Soluton Sne e nnot dsonnet the ECG mhne durng defrllton, e hve to some ho del th trnsent drftng. Also, sthng from one led to nother n use ths too lke n old ECG mhnes euse there re dfferent offset potentls under eh eletrode. In ne mhnes, there re ptors tht re dshrged durng sthng proess. They sth utomtlly. A protetng rutry n e desgned to lmt the nput voltge to ECG mplfer so to mnmze the sturton nd uld up effets due to hgh voltge. Artfts used y stt hrges on personnel n e lessened notely y redung the uld up t stt hrge through the use of ondutve lothng, shoes nd floorng, s ell s hvng personnel touh the ed efore touhng the ptent to empty of stt hrge. There re moton rtfts too espelly durng EMG reordng. Usng perfet nonpolrzle eletrodes help. 0.01µF 4.7K 150K.M Instnt mp gn of 5dB 01 1µF 0 In d oupled storge. MΩ Hgh pss flter gn 6

7 Totl gn 5 x 800 th µa776 opmp 86 db t 100 Hz Nose level 40 m PP Frequeny respone Hz nd flt over 4 40 Hz Suppose 100 m trnsent ppers ross the nput nd lsts long enough to hrge 1 µf ptor to ts pek vlue. Ho long ll t tke the selne to return to thn 1 m of ts orgnl vlue? Anser The gn of DCoupled stge s 5. Therefore, 100 m trnsent hs.5 t the nput of ouplng ptor; hene, the C ll hrge to tht voltge. It must dshrge through. MΩ resstor. So, the nput to nonnvertng opmp s: v. 5e τ C. M 1µ. S A 1 m selne offset t the nput s equvlent to 5 m offset t the. So, t τ 5m e. τ. n( 0. 01) 15. S τ 6. Interferene from Eletr Deves Poer Lne d C 1 d1 C C z z 1 A B ECG G These felds re present even f the mhne s off. z G d1 d 7

8 d1 nd d do not go through the ECG euse of hgh, ut flo through the skn eletrode mpednes Z 1, Z, Z G. Body mpedne s out 500Ω nd n e negleted ompred to z 1 nd z. A B d1 Z 1 d Z If the leds re runnng ner eh other, d1 d. For 9 m le, mpedne n deffer s muh s 0kΩ. Then, ( z z ) A B d 1 6nA 0KΩ 10µ Ths n e mnmzed y sheldng the leds. d 6nA, the eletrodes Look t ths stuton. Poer Lne d z 1 A z z n B G z n z G d The urrent n lso flo dretly from the poer lne. In ths se, e hve ommon voltge, d dsplement urrent. d Z G 8

9 Typl vlues: 0. µ A 50KΩ 10m. In poor eletrl envronment d > 1µA. For perfet mplfer th, there s no prolem, ut Z n s lso fnte. Then, A B zn zn z1 zn z z n z 1 nd z << z n A B z z1 z n 0KΩ 10m 5MΩ 40µ Ths s stll notele on the ECG nd very ojetonle on EEG. It n e mnmzed y loerng the skn eletrode mpedne nd rsng Z n. The other soure of nterferene s mgnet nduton. Current n poer lnes estlshes mgnet feld n the vnty of the lne. The nterferene voltge s proportonl to the mgnet feld strength nd the re of the effetve sngleturn ol. It n e redued y: ) redung the mgnet feld y sheldng (hrd to heve) ) keepng the ECG leds y from mgnet feld (hrd to heve) ) redung the effetve re (esy to heve) B feld ECG ECG 9

10 ght Leg System Ground loops n generte ommonmode voltges through flo of urrent n the groundng rut. Ths n e used y mgnet feld nduton or y exstene of slghtly dfferent ground potentl t dfferent deves onneted to the ptent. In mny ECG systems, the ptent s not grounded t ll. The L eletrode goes to n uxlry opmp. The dsplement urrent d flos not to ground ut rther to the opmp rut hh ths redues the pkup s fr s the ECG mplfer s onerned nd effetvely grounds the ptent. It s etter n terms of sfety too. If n normlly hgh voltge should pper eteen ptent nd ground s result of eletr lekge or other use, the uxlry opmp sturtes nd effetvely ungrounds the ptent. 10 on ptent ould rek up the trnsstors of the ECG mp nd lrge urrents ould flo to the ground ut not through the ptent. 1 4 o L 4 L 1 f d 4 o ox 10

11 Lets ssume 4 ox f ox f ut lso 1 L f L d ox f d L d f 1 s the effetve resstne eteen L nd ground. If the uxlry mp sturtes, ox s nd the ptent s grounded through ths S nd o // f. If o nd f re lrrge, out 5MΩ, the urrent s lmted. When the opmp s not sturted, should e s smll s possle. For 0.µA d hoose f 5MΩ, 5 Ω typlly nd orst se for eletrode s 100 KΩ, then 100KΩ 0.µ A 50µ..5MΩ 1 5KΩ 11