CAPACITIVELY COUPLED INSTRUMENTATION AMPLIFIER FOR POWER AND SIGNAL ISOLATION
Presented by
Archana Nair I
S7 AEI
College Of Engineering, Trivandrum
2007-11 batch
Presented by
Archana Nair I
S7 AEI
College Of Engineering, Trivandrum
2007-11 batch
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Overview
Biopotential and biopotential amplifier.
Bidirectional data transfer and power exchange.
Development of the instrumentation.
Evaluation of system’s suitability for biopotential measurements.
Need for isolation
Any potential or current at amplifier’s input terminals can affect biopotential.
Electric currents can result in microshock and macroshock.
The bioamplifier must have isolation and protection circuitry so that the current through the electrodes can be kept at safe levels and any artifact generated by such current can be minimized.
What is Biopotential?
Electric potential that is measured between points in living cells, tissues, and organisms, and which accompanies all biochemical processes.
Also describes the transfer of information between and within cells.
The Biopotential Amplifier
Small amplitudes, low frequencies, environmental and biological sources of interference etc.
Essential requirements for measurement equipment:
High CMRR
High input impedance
Low Noise
Stability against temperature and voltage fluctuations
Electrical safety, isolation and defibrillation protection.
Electrical isolation
Transformer isolation
high frequency AC devices
Modulation and demodulation needed
Optical isolation
Optical signal is modulated in proportion to the electric signal
Measurement Setup
Ciso-isolation capacitance
Finite amount of capacitance between input and output
Determines the alternating current characteristics of the isolation barrier
Architecture
Two main parts:
Remote biomedical transducer unit with isolation.
Process signal
Control circuit
Central unit
Data received using a transciever.
Power for transducer unit.
Block diagram of the new instrumentation system
Data transfer
Bidirectional
Bluetooth is used as the communication technology.
Power exchange
Microwaves are used.
VCO generates a continuous wave PCS (personal communication services band)(1.98GHz) carrier.
Custom amplifier gives a medium power carrier(33dbm).
Carrier is then fed to the transducer unit through the same coaxial cable.
PCS band- good availability and low cost of the commercially available components.
Barrier Arrangement
Ciso= sum of parallely placed coupling capacitors.
Impedance matching.
Rf chip inductors in series with the capacitors.
Series resonant circuit with frequency 1.98GHz.
Barrier Arrangement contd…
Diplexers
Data-bluetooth-2.45GHz
Power-microwave-i.98GHz
Separated by difference in frequency
RF/dc converter
Converts microwave to dc
Main parts
Input matching circuit
Rectifier
Bypass filter
Performance characteristics
Return loss and the output dc power of the RF/dc converter as functions of dc load.
Transducer RF circuitry
Circuit diagram of the transducer RF circuitry including the isolation barrier, the diplexer and the RF/dc converter
System Performance
Isolated dc power
Plotted return loss vs frequency
Measured available isolated dc power
Plotted output dc power vs incident RF power
Conversion efficiency=Pdc/(Pincident-Preflected)
71%
System performance
Matching of the transducer unit as a function of frequency around the PCS band.
Converted dc power in the transducer unit as a function of the central unit RF generator output power.
System performance contd…
Induced common mode voltage
Measured by connecting the cm generator to the ECG cables
Vcm=Vb(Zeo/Ziso)
=VbReojwCiso
Vcm measured as a function of frequency
Common mode rejection improved by 40dB
Common mode voltage Vs. capacitance
Effect of isolation capacitance on induced Vcm.
As capacitance decreases induced common mode voltage decreases
System Specification Summary
System capacitance=1.6pF
Barrier dielectric withstanding voltage=2500V
Barrier voltage rating=1000V
Barrier insulation resistance=1012ohm
Continuous isolated dc power=600mW
Conclusion
New concept of isolated data and power transfer using capacitive coupling explained using functional prototype.
Advantages of this system.
Improved patient safety.
Improved common mode rejection.
Isolated dc power(600mw).
Future Enhancements
Increase barrier voltage rating for medical grade isolation.
Optimisation of the microwave power supply.
Reducing the size of isolated transducer unit.
Electrical Interference Reduction
Power line interference (50 or 60 Hz) is always around us
Connects capacitively and causes common mode interference
The common mode interference would be completely rejected by the instrumentation amplifier if the matching would be ideal
Often a clever “driven right leg circuit” is used to further enhance CMRR
Average of the VCM is inverted and driven back to the body via reference electrode
Fig (a) represents a typical bed side patient monitor with passive lead trunk.
Fig (b) new solution using the transducer unit with isolation.
References
[1] J. C. Huhta and J. G. Webster, “60-Hz Interference in electrocardiography,”IEEE Trans. Biomed. Eng., vol. BME-20, no. 2, pp. 91–101,Mar. 2007.
[2] B. B. Winter and J. G. Webster, “Reduction of interference due
to common mode voltage in biopotential amplifiers,” IEEE Trans.Biomed. Eng., vol. BME-30, no. 1, pp. 58–62, Jan. 2008.
[3] A. C. Metting van Rijn, A. A. Peper, and C. A. Grimbergen, “High quality recording of bioelectric events. Part I interference reduction theory and practice,” Med. Biol. Eng. Comput., vol. 28, pp. 389–397,2007.
[4] B. B.Winter and J. G.Webster, “Driven-right-leg circuit design,” IEEE Trans. Biomed. Eng., vol. BME-30, no. 1, pp. 62–66, Jan. 2008.
