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IMPEDANCE MATCHING CIRCUIT AND FILTERS
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- 2. IMPEDANCE MATCHING CIRCUIT ECGAMPLIFIER EQUIVALENT CIRCUIT OF ECG AMPLIFIER RIGHT LEG DRIVEN SYSTEM FILTERS TYPES
- 3. ECG AMPLIFIER Theinstrumentation amplifier is used to provide very high input impedance. High common mode rejection is achieved by adjusting the potentiometer to about 47 kΩ.
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- 7. FIND IT ? 7 HPF 1µF coupling capacitor 3.3 MΩ resistor
- 8. VALUE OF RCTIME CONSTANT ? 8 R C 3.3 Sec
- 9. • 3.3 stime constant passes all frequencies above 0.05 Hz. • The output stage is a noninverting amplifier that has a gain of 32. 9
- 10. The 150kΩ and 0.01 µF low-pass filter attenuates frequencies above 100 Hz. 0.01 µF 150 kΩ 10 LPF
- 11. • Switch S1may be momentarily closed to decrease the discharge time constant when the output saturates. • This is required after lead switching to charge the l µF capacitor rapidly to the new value and return the output to the linear region. 11
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- 13. The right-legelectrode is connected to the output of an auxiliary op amp. The common-mode voltage on the body is sensed by the two averaging resistors Ra , inverted, amplified, and fed back to the right leg. The circuit can also provide some electric safety. 13
- 14. If anabnormally high voltage should appear between the patient and ground as a result of electric leakage or other cause, the auxiliary op amp saturates. This effectively ungrounds the patient, because the amplifier can no longer drive the right leg. Now the parallel resistances Rf and Ro are between the patient and ground. They can be several megohms in value large enough to limit the current. 14
- 15. EQUIVALENT CIRCUIT FORTHE INPUT OF AN ECG AMPLIFIER ▸ Vh Voltage signal generated by heart ▸ Ve Unwanted inphase signal ▸ ZI Input impedance of the preamplifier ▸ Z1 and Z2 are the skin contact impedances of the electrodes. 15
- 16. ▸ To minimizethe effects of changes in the electrode impedances it is necessary to employ preamplifier having high input impedance. ▸ A low value of input impedance gives rise to considerable distortion of the recordings. ▸ Even if CMRR of instrument is very high, still in some cases, in hospitals 50Hz artifact can be seen in ECG trace recording or monitor display. ▸ A possible cause of problem is source impedance unbalance. 16
- 17. If ZI =10MΩ , Z2– Z1 = 1kΩ, then ▸ Thus we see that the impedance unbalance due to electrodes on ECG has reduced the CMRR from 5000 to 1000 only. ▸ ZI = 100MΩ, Z2 – Z1 = 5kΩ ▸ It shows high input impedance is very necessary to obtain high CMRR. Also electrode skin resistance should be low and as nearly equal as possible. 17 If Z2 – Z1 = 5kΩ,
- 18. Method to increaseinput impedance ▸ One method to increase input impedance of op-amp in to use FET (Field Effect Transistors) in the input differential stage. ▸ A most common approach is to use an instrumentation amplifier in the preamplifier stage. 18
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- 20. Active filters Passivefilters which use only passive components, such as resistors, capacitors and inductors. Active filters use amplifiers in addition to passive components in order to obtain better performance, which is difficult with passive filters. 20
- 21. ACTIVE FILTERS High-passfilter, which only amplifies frequencies above a certain value Low-pass filter, which only amplifies frequencies below a certain value Bandpass filter, which only amplifies frequencies within a certain band 21
- 22. FREQUENCY RESPONSE 22 bandpass filterHighPass filterLoss pass filter
- 23. LOW PASS FILTER ▸Low pass filter is useful for attenuating high frequency noise. 23
- 24. 24 Advantage of LPF: Gain is high Low output impedance. Assume, R2 = Rf C = Cf
- 25. HIGH PASS FILTER High pass filter is useful for attenuating low frequency noise. 25
- 26. 26 Assume, R1 =Ri C = Ci R2 = Rf
- 27. BAND PASS FILTER •A series combination of low pass filter and high pass filter results in bandpass filter which amplifier frequencies over a desired range and attenuates higher and lower frequencies. 27
- 28. FREQUENCY RESPONSE 28 bandpass filterHighPass filterloss pass filter
- 29. 29 Band passfilter can be easily made by cascading single HPF and single LPF. The difference between the frequencies at -3dB point will determine the bandwidth of the bandpass filter.
- 30. The filterwill have peak response or resonant frequency (fr). Where , fr resonant or center frequency fL Lower -3dB cutoff frequency point fH Higher -3dB cutoff frequency point 30
- 31. Quality factor orQ factor • It is the measure of how selective or unselective the band pass filter is towards a given spread of frequencies. Where, B.w = fH – f L fr resonant frequency 31
- 32. 32 FILTER TO REMOVENOISE SIGNAL CONCEPT RECORDED VOICE SIGNAL
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