Biomedical Instrumentation

Transcription

1 Bomedcal Instrumentaton Amplers & Wnter 1393 Bonab Unersty

2 Applcatons o Operatonal Ampler (In Bologcal s and Systems) The three major operatons done on bologcal sgnals usng OpAmp: 1) Amplcatons and Attenuatons 2) DC osettng: add or subtract a DC 3) Flterng: Shape sgnal s requency content 2

3 Ideal OpAmp Most boelectrc sgnals are small and requre amplcatons 3 Fgure 3.1 Opamp equalent crcut. The two nputs are 1 and 2. A derental oltage between them causes current low through the derental resstance d. The derental oltage s multpled by A, the gan o the op amp, to generate the outputoltage source. Any current lowng to the output termnal o must pass through the output resstance o.

4 Insde the OpAmp (ICchp) 20 transstors 11 resstors 1 capactor 4

5 Ideal Characterstcs 1 A = (gan s nnty) 2 V o = 0, when 1 = 2 (no oset oltage) 3 d = (nput mpedance s nnty) 4 o = 0 (output mpedance s zero) 5 Bandwdth = (no requency response lmtatons) and no phase sht 5

6 Two Basc ules ule 1 When the opamp output s n ts lnear range, the two nput termnals are at the same oltage. ule 2 No current lows nto or out o ether nput termnal o the op amp. 6

7 Inertng Ampler 10 V 10 V 10 V (a) Slope = / o o G (b) 10 V Fgure 3.3 (a) An nertng ampled. Current lowng through the nput resstor also lows through the eedback resstor. (b) The nputoutput plot shows a slope o / n the central porton, but the output saturates at about ±13 V. 7

8 Summng Ampler o

9 Voltage, V Example 3.1 The output o a bopotental preampler that measures the electrooculogram s an undesred dc oltage o ±5 V due to electrode halcell potentals, wth a desred sgnal o ±1 V supermposed. Desgn a crcut that wll balance the dc oltage to zero and prode a gan o 10 or the desred sgnal wthout saturatng the op amp. 10 kw 100 kw 10 15V 5 kw b b 20 kw 0 b /2 Tme 15 V 10 9 (a) (b)

10 Follower ( buer) Used as a buer, to preent a hgh source resstance rom beng loaded down by a lowresstance load. In another word t preents drawng current rom the source. o G 1 10

11 Nonnertng Ampler 10 V Slope = ( )/ 10 V 10 V 10 V o G 1 11

12 Derental Amplers Derental Gan G d G d o Common Mode Gan G c For deal op amp the nputs are equal then the output = 0, and the G c =0. No derental ampler perectly rejects the commonmode oltage. Commonmode rejecton rato CMM 12 G CM G d c 4 o ( 4 3) Typcal alues range rom 100 to 10,000 Dsadantage o oneopamp derental ampler s ts low nput resstance 3

13 Instrumentaton Amplers 13 Adantages: Hgh nput mpedance, Hgh CM, Varable gan (adjust 1 ) Derental Mode Gan o ) ( G d

14 Comparator No Hysteress > 2, o = 13 V 1 < 2, o = 13 V re V 10 V re 10 V 14 I ( re ) > 0 then o = 13 V else o = 13 V 1 wll preent oerdrng the opamp

15 Comparator Wth Hysteress educes multple transtons due to mv nose leels by mong the threshold alue ater each transton V Wth hysteress re V 10 V re 3 10 V 15 Wdth o the Hysteress = 4V 3

16 ecter x (1x) D 1 D 2 10 V 10 V 10 V 10 V D 3 D 4 = x x (b) (1x) (a) D 2 o 16 Fullwae precson recter: a) For > 0, D 2 and D 3 conduct, whereas D 1 and D 4 are reersebased. Nonnertng ampler at the top s acte (a)

17 ecter x x (1x) D 1 D 2 o D 4 (b) D 3 D 4 = x 10 V (a) 10 V 10 V 17 Fullwae precson recter: b) For < 0, D 1 and D 4 conduct, whereas D 2 and D 3 are reersebased. Inertng ampler at the bottom s acte (b) 10 V

18 OneOpAmp Full Wae ecter = 2 kw = 1 kw o D L = 3 kw (c) 18 For < 0, the crcut behaes lke the nertng ampler recter wth a gan o 0.5. For > 0, the op amp dsconnects and the passe resstor chan yelds a gan o 0.5.

19 Logarthmc Amplers Uses o Log Ampler 1. Multply and dde arables 2. ase arable to a power 3. Compress large dynamc range nto small ones 4. Lnearze the output o deces I c /9 V BE (a) V BE 0.06log I I C S o 0.06log Fgure 3.8 (a) A logarthmc ampler makes use o the act that a transstor's V BE s related to the logarthm o ts collector current. For range o I c equal 10 7 to 10 2 and the range o o s.36 to 0.66 V.

20 Logarthmc Amplers o V BE I c /9 10 V V BE 9V BE 10 V 10 V 1 (a) (b) 10 V 10 Fgure 3.8 (a) Wth the swtch thrown n the alternate poston, the crcut gan s ncreased by 10. (b) Inputoutput characterstcs show that the logarthmc relaton s obtaned or only one polarty; 1 and 10 gans are ndcated. 20

21 Integrators 21 C j j V j V C j j V j V o o 1 c o C 2 1 or < c o t c o Z Z j V j V dt C ) ( ) ( A large resstor s used to preent saturaton

22 A threemode ntegrator 22 Fgure 3.9 A threemode ntegrator Wth S 1 open and S 2 closed, the dc crcut behaes as an nertng ampler. Thus = c and can be set to any desred ntal conducton. Wth S 1 closed and S 2 open, the crcut ntegrates. Wth both swtches open, the crcut holds constant, makng possble a lesurely readout.

23 Example 3.2 The output o the pezoelectrc sensor may be ed drectly nto the negate nput o the ntegrator as shown below. Analyze the crcut o ths charge ampler and dscuss ts adantages. s sc = s = 0 o = c dq s / dt = s = K dx/dt C 23 1 t 1 C 0 Kdx dt dt o Kx C Pezoelectrc sensor sc s FET Long cables may be used wthout changng sensor senstty or tme constant.

24 Derentators d o C dt V j Z o( ) V ( j) Z jc Fgure 3.11 A derentator The dashed lnes ndcate that a small capactor must usually be added across the eedback resstor to preent oscllaton. 24

25 Acte Flters LowPass Flter Gan = G = G / V V o j 1 j 1 j C (a) C / c = 1/2 C req 25 Acte lters (a) A lowpass lter attenuates hgh requences

26 Acte Flters (HghPass Flter) Gan = G = G V V o j jc j 1 jc C / / (b) c = 1/2 C req 26 Acte lters (b) A hghpass lter attenuates low requences and blocks dc.

27 Acte Flters (BandPass Flter) V V o j j C j 1 j C 1 j C G C C o / (c) / cl = 1/2 C ch = 1/2 C req 27 Acte lters (c) A bandpass lter attenuates both low and hgh requences.

28 Frequency esponse o opamp and Ampler OpenLoop Gan Compensaton ClosedLoop Gan Loop Gan Gan Bandwdth Product (4MHz here) Slew ate (addtonal lmtaton or large sgnals, 15/μs) 28

29 Oset Voltage and Bas Current The 2 opamp nputs dre the bases o transstors BasetoEmtter oltage drop may be derent 1 2 = ew m o =0 So, amplyng tny oltages (stran gage, thermocouples) attenton Nullng: educe ths oltage to zero by external pot (alterng emtter currents n the 2 transstors) Drt: educng ths oltage to zero happens at a certan temperature temp. change? large drt temp. controlled chps (more expense) Nose: All semconductor junctons produce nose lmt small sgnal detecton 29

30 Oset Voltage and Bas Current Nose: All semconductor junctons produce nose lmt small sgnal detecton Nosecurrent source Noseoltage source For low source mpedances, only the nose oltage s mportant Bas Current: The 2 OpAmp nputs dre transstors Base/gate currents should be always enough (200pA) It also passes through eedback resstor errors 30

31 Input and Output esstance Example: Follower o d d A d o L CL a ( A1) d ao o o o A1 31 Typcal alue o d = 2 to 20 MW Typcal alue o o = 40 W

32 Phase Modulator or Lnear arable derental transormer LVDT 32

33 Phase Modulator or Lnear arable derental transormer LVDT 33

34 PhaseSenste Demodulator Used n many medcal nstruments or sgnal detecton, aeragng, and Nose rejecton 34

35 The ng Demodulator (a practcal phase senste demodulator) I c s poste then D 1 and D 2 are orwardbased and A = B. So o = DB I c s negate then D 3 and D 4 are orwardbased and A = c. So o = DC c 2 35