心电采集系统中模拟电路的设计英文文献.doc

ECG acquisition system in analog circuit design

ECG as the heart electrical activity in the performance of body surface, the signal is weak, the spectral range of 0.05 ~ 200Hz, voltage amplitude is 0 ~ 5mV [1], the signal source impedance of thousands to hundreds of thousands of Europe Europeand there were a lot of noise, so the ECG acquisition system can be the rational design of the right of the key components of ECG.ECG measurement conditions is quite complex, in addition to subject, including electromyography, respiratory wave signal, EEG signal interference and other outside interference in vivo has also been 50HZ electricity, baseline drift, electrode contact and interference with other electromagnetic devices in vitroTherefore, the strong noise interference how to effectively suppress a variety of ECG acquisition system will be the key to the design.

1 ECG acquisition system
Complete ECG acquisition system includes analog and digital two parts [2], which simulate some of the major to complete the pick up ECG signals, amplification and filtering, the analog portion of the digital part will receive the ECG signal analysis and processing to health carepersonnel arrive at a correct diagnosis, the ECG acquisition system in the analog circuits in the ECG monitoring system plays an important role.ECG acquisition system structure shown in Figure 1.

Figure 1. The overall structure of ECG acquisition system
ECG acquisition circuit simulation
AD conversion

Levelof uplift

Enlarge
Filter circuit
Preamplifier
ECG

Carried on by the special electrodes in the human ECG signal pick up the first amplified by the initial pre-amplifier, and a variety of interference signals in a certain inhibition into the band-pass filter to filter the ECG signal interference outside the frequency range, and then filtered by the main amplifier will amplify the signal further to the right range, and then filtered by 50Hz notch filter frequency interference, are simulated ECG signal will be converted into the AD system, analog-digital conversion, converted into digital signalsAfter the central processing unit responsible for the subsequent analysis and processing.

2 preamplifier circuit
Picked up from the body surface ECG generally only a few millivolts, in order to improve its resolution display and processing in the backend, you first need to amplify the signal.ECG acquisition process, the pre-amplifier the greatest impact on the ECG, ECG to improve the performance of the amplification of preamplifier can not choose too large (generally less than 20), otherwise they will be duelarger interference signal (refer to the electrode polarization voltage), causing the amplifier obstruction.
For the ECG, the acquisition of the signals are differential mode signals, so the amplifier is a differential amplifier circuit structure, the most common is the use of low-noise, high input impedance, high CMRR, high gain and interferencestrong ability in parallel with the differential amplifier circuit, known as the three op amp instrumentation amplifier, this system uses a common integrated operational amplifier LM324 amplifier to constitute such.LM324 is an integrated operational amplifier 4, as cheap and easy to use, it is widely used in control and signal amplification processed.We are carefully designed and repeated in the laboratory experiments, using LM324 circuit composed of successful amplification of the ECG signal processing, its main technical indicators can meet the requirements.LM324 can either use a single power supply, you can also use the dual power supply, power supply voltage has been used from the +5 V ± 15 V, and drive with low power consumption, each group of differential gain amplifier can reach 100dB.Through the rational design of peripheral circuits, making the LM324 amplifier as the main device can meet the high magnification, high stability amplification of ECG processing requirements.
It is worth mentioning that, LM324's low current noise, especially for the application of ECG, it has a very good DC Characteristics: input offset voltage is less than 5mV, the input offset voltage drift is less than 7uV / ℃, the input bias current less than40nA, common mode rejection ratio of 100dB, and its low power, low power (as low as 3V), and other features to streamline package is battery-powered portable ECG monitoring system an excellent choice.Amplification circuit shown in Figure 2.
Preamplifier circuit
Figure 2

Figure 2 Design of the amplifier consists of two components, U1C and U1D constitute the first stage of the differential input and output stage, U2A differential ratio for the basic circuit, the total voltage gain Au, is equal to the product of two-stage gain.The first stage uses the same phase as input, a higher input resistance, U1C and U1D op amp selection of the same characteristics, so that their common-mode voltage output voltage and drift are equal, then through the second differential fractional U2A circuit composed ofcan cancel each other out, and the second differential amplifier circuit will be point differential input single ended output changing to adapt to the needs of ground load.After the two-stage cascade circuit, they are mutually reinforcing, so that combination of this circuit has high input impedance, voltage gain adjustment is convenient, common mode rejection is high and offset drift, and a series of advantages.In order to improve CMRR and reduce temperature drift effects, and further improve the circuit performance, measuring amplifier with symmetrical structure, ie a few carefully selected external resistor R10, R11, R12, R13, R14, R15, R16 and regulation of RW2, makes the R11= R12, R13 = R14, R16 = R15 + adjusted RW2.So by adjusting the size of an external resistor RW2 can easily change the measurement of amplifier gain.
Removal of the body to carry communication mode interference is an effective method is to use the right leg drive circuit [3], in the Figure 2 system, the right leg RL side is not directly grounded, but the amplifier output.Shielded from the driver output common mode voltage detection, it is the reverse by the auxiliary amplifier U0 amplified, and then through the feedback resistor R1 to the right leg, right leg drive thus be the name of that displacement current is not flowing into the human bodyland, but the flow of R1 and auxiliary amplifier output of U0.But right leg drive circuit of the AC interference voltage feedback loop, while the AC current flows through the body may have become unsafe, the current limiting resistor is not very small, usually taking more than 1M in Europe, where R1 is a security protection, between when the patient and to high voltage, the auxiliary amplifier saturation U0, right leg drive circuit does not work, U0 equivalent to grounding, the resistor R1 since then to limit protection.Interference signal sent to the right leg drive amplifier inverting amplifier, driven right leg electrode RL reached for the depth of interference signal is a negative feedback, which effectively weakens the human body sensing the common mode interference signals.

Figure 3
The main amplification and filtering circuits

3 main amplifier and filter circuit
The main ECG amplification and filter circuit shown in Figure 3.Detection signal as the main interference in the signals between the electrode plates and the polarization voltage, 50Hz frequency interference, the instrument around the internal noise and electromagnetic interference equipment and so on.To obtain a clear ECG signal stability, filter design is also critical, especially 50Hz band stop filter is particularly important.Collected from the ECG, 200Hz signal is stronger than the interference, and interference signals below 0.05Hz relatively weak, so take the first filter circuit in the low-pass filter out signals below 200Hz, and then take the way of high-passto filter out the polarization voltage and high frequency interference.In the circuit U2B and resistors, capacitors, composed of band-pass filter, while the circuit with high input impedance [4].Filter circuit using RC coupling circuit, high input impedance low output impedance, and input and output between the good isolation.Its role is mainly to pre-amplifier across the DC voltage and DC polarization voltage, coupled ECG.In order to ensure that no distortion of ECG coupled to the next level, must be coupled RC.RC product of the larger low-frequency response of the amplifier the better, but increase the RC values can not be unlimited, because the R value is limited by the input impedance, C is too large not only large in size and increase in leakage current can also cause drift, but alsocharge and discharge time will be extended loop.ECG body low frequency filter circuit with the RC active filter can effectively avoid the general-purpose integrated circuit operational amplifier as the bandwidth is narrow and not suitable for high frequency range of defects.
4 level lift circuit
Amplified ECG signal after filtering will be sent to A / D converter analog to digital conversion, and the system chosen AD converter is a single 5V supply the MAX187, so the need to add a filter after the uplift of the circuit level, theECG signal raised to the range of 0 ~ + 5V.Circuit design is very simple, shown in Figure 4.

Figure 4
Circuit-level uplift

5 Analysis
Through this method successfully designed and implemented the analog ECG acquisition system circuit, which is simple and reliable circuit-level uplift, amplifier with high gain, high input impedance, high CMRR, low noise, low drift, rightbandwidth and dynamic range characteristics, the circuit can be in strong background noise, the sensor surface without distortion by the weak ECG signal detected and enlarged, uplifted to the extent appropriate, to obtain better shown in Figure 5 ECGFig.
Figure 5
ECG measured

The test proved that the design paper ECG acquisition circuit can be clinically valuable ECG data, ECG follow-up treatment to provide a reliable guarantee.
This view of innovation: overcoming the traditional ECG instrumentation amplifier used in collecting some of the shortcomings, the ordinary op amp realized under the weak ECG signal in strong noise amplification and filtering, through careful design, circuit implementation is simple, reliable, access to thevaluable ECG.