Bioamplifiers-2005

Raimo Sepponen 2000 Bioamplifiers 1

Biopotential amplifiers

Medical Instrumentation


Figure 8.6 Block diagram of an electrocardiograph. The normal locations for surface electrodes are right arm (RA), right leg (RL), left arm (LA), and left leg (LL). Physicians usually attach several electrodes on the chest of the patients as well.


Basic requirements• Increase signal amplitude and decrease

source impedance - power amplifiers• Source impedance high and unknown (e.g.

electrodes) => input impedance high > 10 MΩ

• Safety - minimal leakage currents• Limited but adequate bandwidth• High common mode voltage rejection

RUP

2

=


100 kΩ

1 MΩ

1 mVRs

Ri

Vo

Electrocardiogram

Figure 2.8 The 1 mV signal from the electrocardiogram is attenuated by theresistive divider formed by the 100 kΩ skin resistance and the 1 MΩ input resistance of the oscilloscope.

nA 91.0M1k 100

mV 1 =Ω+Ω

==RVi

( )( ) mV 91.0M 1nA 91.0io =Ω== iRVis

i

i

oRR

RVV

+=


Rd

RS

+

A(v2 − v1)

v1

v2

vo

−

(a)

Figure 2.25 (a) Equivalent circuit for op amp. (b) Symbol of op amp. Many times V+ and V– are omitted in the op amp symbol, but it is understood that they are present.


Ideal Amplifier

• Has infinite gain.

• Has zero output impedance - won’t act as voltage divider with next component.

• Has infinite input impedance - won’t act as voltage divider with previous component or signal source.

• Has infinite frequency response - won’t attenuate signal at different frequencies.

• Contributes no noise.

• Inputs tend to follow each other so treat both inputs as if they were the same.


vo

viRi

ii

Rf

A−

+

Figure 2.26 An inverting amplifier. The gain of the circuit is –Rf/Ri

ii

fo v

RRv −=⇒ 00

f

o

i

iRv

Rv −=−

i

f

i

o RR

vv −=⇒

From Ohm’s law:


R2

vo

v1

R1

R1

R2

v2

v3

v3A

−

+

Figure 2.30 A differential amplifier uses two active inputs and a common connection.

221

23 v

RRRv+

=

2

o3

1

31Rvv

Rvv

i−

=−

=

)( 121

2o vv

RRv −=


vo

vi

R1

R2

vref A−

+

(a)

vo

vi

Saturatedvoltage −vref = vi

(b)

Figure 2.33 (a) A comparator. (b) The input–output characteristic of the comparator in (a).


Figure 2.34 Heart beat detector uses a comparator to determine when the R wave exceeds a threshold.

P

Q

R

S

T

Threshold


Gai

n

Frequency (Hz)100 104102 106

1

10 Circuit gain of 10

Figure 2.35 The typical op amp open loop gain is much larger, but less constant, than the circuit gain. However the in circuit bandwidth is larger than the open loop bandwidth.

Circuit bandwidthIdealgain

Typical openloop gain


(a)

Figure 2.38 High-pass filter. (a) RC circuit. (b) RL circuit.

RC1

c =ω

ωω

c11)(ωj

T+

=

R vovi

C

+

−

+

−

(b)

LR

c =ω

ωω

c11)(ωj

T+

=

R

vovi L

+

−

+

−


ECG - Dipole field of heart at peak of R wave

P

Q

R

S

T

Dipole moment vector M


Relationships between two lead vectors a1, a2 and M

θcos111 MvaMv aa =⇔⋅=


Standard notation of lead vectors

Lead I: LA-RALead II: LL - RALead III: LL - LA

Eindhoven´s triangle

Kirchhoff´s II law: I-II+III = 0


Wilson´s central terminal

R > 5 MΩ


Augmented leads aVL, aVR, aVF


Precordial leads V1 - V6

Directions of precordiallead vectors in transverse plane


A

Common modenoise

Differential modenoise

Signal

Vcm

Vs

Vdm

Modes of EMI Coupling


Coupling modes -DIFFERENTIAL MODE

(Normal mode)

Differential mode noiseis coupled to the circuit in the same way as signal

Noise is coupled through the same cables and connectorsas the signal => Coupling is maximally effective


Coupling modes -COMMON MODE

Common mode noise is coupled via at least one path which is not a signal path.


Coupling modes -COMMON MODE

Z

Z+∆∆∆∆ZVs

VCM

ZCM

ZCM

Vne = VCMZCM ZCM

ZCM +Z ZCM +Z +∆∆∆∆Z

Common mode noise is transformed to differential mode in unbalanced impedances


One should always try to prevent a generation of differential mode noise

Often differential mode noise contains spectrum components outside the frequency range of the

Filtering of differential mode noise is practical when the frequency ranges of signal and noise differs from each other

Coupling modes -DIFFERENTIAL MODE


Performance requirements for ECG units (selected from voluntary standards, USA)

Overall signal accuracy up to +-5mV and 125 mV/s

Max +- 5 %Max +- 40 µV

Upper cut off 3 dB Min 150 Hz

Response to 20 ms, 1.5 mVtriangular input

Min. 13,5 mm

Response after 3 mV, 100 msimpulse

Max 0.1 mVMax 0.3 mV/s

Error in lead weighting factors 5 %

Hysteresis after 15 mmdeflection from baseline

Max 0.5 mm



Input impedance at 10 Hz Min 2.5 MΩ

DC current any input lead Max 0.1 µA

DC current any patient electrode Max 1.0 µA

Common mode rejection:Allowable noise with 20 V, 60Hz and +- 300 mV DC, 51 kΩimbalance

Max 10 mm,Max 1 mV

System noise: RTI, pp Max 30 µV

Multichannel crosstalk Max 2 %



Baseline control and stability:Return after resetReturn after lead switchBaseline drift rate RTITotal baseline drift RTI (2 min period)

Max 3 sMax 1 sMax 10 µV/sMax 500 µV

Overload protection:No damage diff. 60 Hz, 1Vpp,10 s application timeNo damage after defibrillatorsurges, 5 kV, 360 JRecovery time Max 8 s


Effect of voltage transient on ECG


Interference Power line interference

Electromyographic interference


Electric field pickup of ECG leads from power line


Electric field coupling of common mode noise from power line

( ) ( ) mVkAv

Ziv

cm

Gdbcm

10502.0: valuesTypical

=Ω⋅=

=

µ


Transformation of CM noise to DM noise by impedance imbalance

( ) VMkmVvv

ZZZvvv

ZZZZZ

ZZZ

Zvvv

BA

incmBA

in

in

in

in

incmBA

µ4052010: valuesTypical

, :Because

12

21

21

=ΩΩ⋅≈−

−≈−

>>

+−

+=−


What is the necessary common moderejection ratio (CMRR), if 0.1 mA noise currentflows through the body and the noise visible in ECG

should be below 10 µV? The impedance ofthe body is 1 kΩ.

0,1 mA

Z k≈ 1 ΩV mA k mVV V

CMRR VV

dB

CM

NMax

CM

NMax

≈ ⋅ =

=

= = =

0 1 1 10010

10 000 80

, Ω

µ


A

Yhteismuotoinenhäiriö

Signaali

Vc m

Vs

R+ R∆

R

+

-

VN 0matalataajuinen

Hajakapasitanssi

Isolaatiovahvistin

Low frequency noiseCommon mode noise

Isolation amplifier

Signal

Stray capacitance


Vc m

id

R L

V3

V4

Ra

Ra

Rf

Ro

Vc m

Vc m

230V 50 Hz

A


vc m

id

Ro

Rf

vovc m

id

+

R / 2a

v / Ro f2v / Rcm a

A


If the amplifier connected to the right legis operating in linear range

2vR

vR

0

v - 2RR

v

v R i vv = R i

1 2 R RExample:

R 5 MR = 25 k

Effective resistance connected to the right leg:5 M

1+ 2 5 M25 k

12,5 k

Amount of current coupled capacitively:i = 0,2 A

v 12,5 k 0,2 A = 2,5 mV

CM

a

0

f

0f

aCM

CM 0 d 0

CM0 d

f a

0

a

d

CM

+ =

⇒ =

= +

+

=

⋅ =

⇒ = ⋅

ΩΩ

ΩΩ

Ω

Ω

Ωµ

µ

Vc m

id

R L

V3

V4

Ra

Ra

Rf

Ro

Vc m

Vc m

230V 50 Hz

A

vc m

id

Ro

Rf

vovc m

id

+

R / 2a

v / Ro f2v / Rcm a

A


Magnetic field pickup by ECG leads


Sources Electromagnetic fieldsSource Frequency range Intensity rangeLightning 1 Hz-1 kHz 10 kV/m

SWMW diathermy

27 MHz2.45 GHz

2 kV/m

Surgigal Diathermy 0.4 – 2.4 MHz 1 kV/m

Home appliances 50 – 60 Hz 250 V/m10 µT

Microwave ovens 2.45 GHz 50 W/m2

Portable phones >500 MHz 1 W/m2


Overvoltage protection


Voltage limiting devices

Low voltage breakdown

Medium voltage breakdown

High voltage breakdown


Voltage and frequency ranges of some biopotential signals


Instrumentation amplifier


Electromyogram integrator


Signal averaging for improving SNR


EKG of mother

Thresholddetector

Signal collection blockedduring QRS-peak of mother´s EKG

EKS:s of mother and fetusFetal EKG

Analogswitch


Typical fetal ECG obtained from maternal abdomen


Increased electrode impedance detection


Single-channel radiotelemetry system

Bioamplifiers-2005

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