Biopotential Amplifiers. p. 2

Transcription

1 Biopotential Ampliiers Basic unction to increase the amplitude o a weak electric signal o biological origin (next slide) typically process oltages but in some cases also process currents Typical bio-amp requirements high input impedance -greater than 10 Mohms saety: protect the organism being studied careul design to preent macro and microshocks isolation and protection circuitry to limit the current through the electrode to sae leel output impedance o the ampliier should be low to drie any external load with minimal distortion gain greater than 1000 biopotentials are typically less than a milliolt most biopotential ampliiers are dierential signals are recorded using a bipolar electrodes which are symmetrically located high common mode rejection ratio biopotentials ride on a large oset signals rapid calibration o the ampliier in laboratory conditions adjustable gains oten the change in scale is automatic thereore calibration o the equipment is ery important Biopotential Ampliiers. p. 1 Voltage and Frequency Range or Biopotentials Biopotential Ampliiers. p. 2

2 Electrocardiograph ampliiers Beating heart generates electric signal monitored to understand heart unctions Measurements are unctions o location at which the signal is detected time-dependence o the signal amplitude Dierent pairs o electrodes at dierent locations yield dierent measurements hence placement is standardized Electrical model o heart electric dipole located in a partially conducting medium (thorax) dipole represented as a cardiac ector M Mi is the dipole moment during the cardiac cycle magnitude and direction o the dipole ector will ary electric ect c potentials t appears throughout out the body and on its surace a1 M a 1, a1 M cos Biopotential Ampliiers. p. 3 Electrocardiograph Leads In clinical electrocardiography more than one lead must be recorded to describe the heart's electric actiity ully seeral leads are taken in the rontal plane and the transerse plane rontal plane: parallel to the back when lying transerse plane: parallel to the ground when standing Frontal plane lead placement called Eindhoen s triangle Additional leads unipolar measurements potential measured at electrodes wrt a reerence; aerage o the 2 electrodes Wilson central terminal three limb electrodes connected through equal-alued resistors to a common node augmented leads some nodes disconnected increase the amplitude o measurement using Biopotential Ampliiers. p. 4

3 Functional blocks o electrocardiograph Biopotential Ampliiers. p. 5 Problems in ECG Measurement Frequency distortion i ilter speciication does not match the requency content o biopotential then the result is high and low requency distortion Saturation or cuto distortion high electrode oset oltage or improperly calibrated ampliiers can drie the ampliier into saturation then the peaks o QRS waeorms are cut o Ground loops i two monitoring instruments are placed at disjoint ground points then small current could low through the patient s body Electric/magnetic ield coupling open lead wires (loating connections) pick up EMI long leads produce loop that picks up EMI (induces loop current) Intererence rom power lines (common mode intererence) can couple onto ECG signal 60Hz supply noise Coupled to ECG Biopotential Ampliiers. p. 6

4 Intererence Reduction Techniques Common-mode oltages can be responsible or much o the intererence in biopotential ampliiers. Solution 1: ampliier with a ery high common-mode rejection Solution 2: eliminate the source o intererence Ways to eliminate intererence Use shielding techniques electrostatic shielding: Place a grounded conducting plane between the source o the electric ield and the measurement system ery important or EEG measurement Magnetic shield use high permeability materials (sheet steel) Use twisted t cables to reduce magnetic lux, reduce lead loop area Biopotential Ampliiers. p. 7 One-amp dierential ampliier Dierential Ampliier gain determination Rule 1: irtual short at op-amp inputs Rule 2: no current into op-amp inr4 in 5 5 o 5 i R 3 R 4 R 3 R 4 ( ) R in in 4 o R3 Gain o dierential ampliier o R4 = Gd (not gain o op-amp) R in Vin+ characteristics no common mode gain, Gc = 1 input resistance o the di. amp is lower than ideal op-amp 3 OK or low resistance sources (like Wheatstone bridge), but not good or many biomedical applications common mode rejection ratio: G CMRR d G Vin- - Vin + i c Biopotential Ampliiers. p. 8

5 Dierential Ampliier How do we ix low input resistance o 1-op-amp di amp? Option 1: Add oltage ollower to each input Problem:? Option 2: Add non-inerting amp at each input Proides additional gain Problem:? Biopotential Ampliiers. p. 9 Better option: Instrumentation Ampliier connect Ri s o input amps together eliminate ground connection This 3-op-amp circuit is called an instrumentation ampliier Input stage characteristics acte low common-mode gain -rejects common mode oltages (noise) high input impedance 3 4 2R 2 R 1 input stage gain adjusted d by R 1 Gd 1 2 R1 Biopotential Ampliiers. p. 10

6 Instrumentation Ampliier Input stage high input impedance buers gain stage no common mode gain can hae dierential gain input stage gain stage Gain stage d dierential gain, low input impedance total dierential gain 2R 2 R1 R4 ampliies only the dierential component G d R high common mode rejection ratio 1 R3 high input impedance suitable or biopotential electrodes with high output impedance Oerall ampliier Biopotential Ampliiers. p. 11 ECG Ampliier instrumentation ampliier HPF non-inerting amp With 776 op amps, the circuit was ound to hae a CMRR o 86 db at 100 Hz and a noise leel o 40 mv peak to peak at the output. t The requency response was to 150 Hz or ±3 db and was lat oer 4 to 40 Hz. The total gain is 25 (instrument amp) x 32 (non-inerting amp) = 800. Biopotential Ampliiers. p. 12

7 Motiation reduce intererence in ampliier improe patient saety Drien Right Leg System Approach patient right leg tied to output o an auxiliary amp rather than ground common mode oltage on body sensed by aeraging resistors, Ra s & ed back to right leg proides negatie eedback to reduce common mode oltage i high oltage appears between patient t and ground, auxiliary amp eectiely un-grounds the patient to stop current low Biopotential Ampliiers. p. 13 Drien Right Leg System: Example Problem: Determine the common-mode oltage cm on the patient in the drienright-leg circuit o Slide 13 when a displacement current d i lows to the patient rom the power lines. Choose appropriate alues or the resistances in the circuit so that the common-mode oltage is minimal and there is only a high-resistance path to ground when the auxiliary operational ampliier saturates. What is cm or this circuit when d i =0.2A? Answer: The equialent circuit is shown here. Note that because the commonmode gain o the input stage is 1, and because the input stage as shown has a ery high input impedance, cm at the input is isolated rom the output circuit. R RL represents the resistance o the right-leg electrode. Summing the currents at the negatie input o the operational ampliier, we get 2cm o 0 R this gies a R 2R 1 2 o but cm cm RRLid o Ra thus, substituting (1) into (2) yields R RLi d cm 1 2R / R a Biopotential Ampliiers. p. 14

8 Example continued The eectie resistance between the right leg and ground is the resistance o the right-leg electrode diided by 1 plus the gain o the auxiliary operational-ampliier circuit. When the ampliier saturates, as would occur during a large transient cm, its output appears as the saturation oltage s. The right leg is now connected to ground through this source and the parallel resistances R and Ro. To limit the current, R and Ro should be large. Values as high as 5 M are used. When the ampliier is not saturated, we would like cm to be as small as possible or, in other words, to be an eectie low-resistance path to ground. This can be achieed by making R large and Ra relatiely small. R can be equal to Ro, but Ra can be much smaller. A typical alue o Ra would be 25 k. A worst-case electrode resistance R RL would be 100 k. The eectie resistance between the right leg and ground would then be 100k 2 5 M 1 25 k For the 0.2 A displacement current, the common-mode oltage is cm μa 50 μv Biopotential Ampliiers. p. 15 Compensation o electrode artiacts - Microelectrodes detect potentials on the order o mV. -Small size implies high source impedance which also results in a large shunting capacitance. - Degraded d requency response. Biopotential Ampliiers. p. 16

9 Compensation o electrode artiacts - Compensate large shunt capacitance using a positie eedback -Circuit below realizes a negatie capacitance i 1 i dt A C 1 1 (1 A ) C i 1 ) i i dt - Total capacitance C C ( 1 A ) C s - Compensation criteria C ( A 1) C s Biopotential Ampliiers. p. 17