bioamplifiers-2005
TRANSCRIPT
Raimo Sepponen 2000 Bioamplifiers 1
Biopotential amplifiers
Medical Instrumentation
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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.
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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
=
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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
+=
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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.
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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.
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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:
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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 −=
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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).
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Figure 2.34 Heart beat detector uses a comparator to determine when the R wave exceeds a threshold.
P
Q
R
S
T
Threshold
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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
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(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
+
−
+
−
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ECG - Dipole field of heart at peak of R wave
P
Q
R
S
T
Dipole moment vector M
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Relationships between two lead vectors a1, a2 and M
θcos111 MvaMv aa =⇔⋅=
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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
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Wilson´s central terminal
R > 5 MΩ
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Augmented leads aVL, aVR, aVF
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Precordial leads V1 - V6
Directions of precordiallead vectors in transverse plane
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A
Common modenoise
Differential modenoise
Signal
Vcm
Vs
Vdm
Modes of EMI Coupling
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A
Common modenoise
Differential modenoise
Signal
Vcm
Vs
Vdm
Modes of EMI Coupling
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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
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Coupling modes -COMMON MODE
Common mode noise is coupled via at least one path which is not a signal path.
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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
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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
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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
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Performance requirements for ECG units (selected from voluntary standards, USA)
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 %
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Performance requirements for ECG units (selected from voluntary standards, USA)
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
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Effect of voltage transient on ECG
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Interference Power line interference
Electromyographic interference
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Electric field pickup of ECG leads from power line
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Electric field coupling of common mode noise from power line
( ) ( ) mVkAv
Ziv
cm
Gdbcm
10502.0: valuesTypical
=Ω⋅=
=
µ
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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⋅≈−
−≈−
>>
+−
+=−
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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
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A
Yhteismuotoinenhäiriö
Signaali
Vc m
Vs
R+ R∆
R
+
-
VN 0matalataajuinen
Hajakapasitanssi
Isolaatiovahvistin
Low frequency noiseCommon mode noise
Isolation amplifier
Signal
Stray capacitance
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Vc m
id
R L
V3
V4
Ra
Ra
Rf
Ro
Vc m
Vc m
230V 50 Hz
A
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vc m
id
Ro
Rf
vovc m
id
+
R / 2a
v / Ro f2v / Rcm a
A
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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
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Magnetic field pickup by ECG leads
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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
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Overvoltage protection
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Voltage limiting devices
Low voltage breakdown
Medium voltage breakdown
High voltage breakdown
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Voltage and frequency ranges of some biopotential signals
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Instrumentation amplifier
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Electromyogram integrator
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Signal averaging for improving SNR
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EKG of mother
Thresholddetector
Signal collection blockedduring QRS-peak of mother´s EKG
EKS:s of mother and fetusFetal EKG
Analogswitch
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Typical fetal ECG obtained from maternal abdomen
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Increased electrode impedance detection
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Single-channel radiotelemetry system