_a_href_tiki_editpage_php_page_Cut_Off_title_Create_page_Cut_Off_class_wiki_wikinew_a_Frequency">Antialiasing Filter Cut-Off href="tiki-editpage.php?page=Cut-Off" title="Create page: Cut-Off" class="wiki wikinew">? Frequency

Antialiasing Filter Cut Off Frequency

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Notch Filter Q Factor

http://www.open-ecg-project.org/tiki-index.php?page=3-Lead+Wireless+ECG .diffchar { color:red; } - This 3-Lead Wireless ECG was developed by Anwar Vahed, Electronic Engineer based in South Africa. His final report presented here is also available for download as a pdf file (b57a95062f30fccbc7047546cc5d0176" rel="">b57a95062f30fccbc7047546cc5d176).
+ This 3-Lead Wireless ECG was developed by Anwar Vahed, Electronic Engineer based in South Africa. His final report presented here is also available for download as a pdf file - tiki-download_file.php?fileId=5" rel="">3Lead_Wireless_ECG_Anwar_Vahed.pdf (1.94 MB).
- Figure ab8e9f176b1cb27bf8b12918090035f0: Three Operational Amplifier Differential Amplifier
+ Figure 10: Three Operational Amplifier Differential Amplifier
- The potential created by the heart wall contraction spreads electrical currents from the heart throughout the body. The spreading electrical currents create different potentials at different points on the body. Leads are placed on the body in several pre-determined locations to provide information about heart conditions. The cardiac signal, typically 5 mV peak to peak, is an AC signal with a bandwidth of 0.05 Hz to ab8e9f176b1cb27bf8b12918090035f00 Hz (Section 4). The ECG signal is characterized by six peaks and valleys labeled with successive letters of the alphabet P, Q, R, S, T, and U (Figure 1).
+ The potential created by the heart wall contraction spreads electrical currents from the heart throughout the body. The spreading electrical currents create different potentials at different points on the body. Leads are placed on the body in several pre-determined locations to provide information about heart conditions. The cardiac signal, typically 5 mV peak to peak, is an AC signal with a bandwidth of 0.05 Hz to 100 Hz (Section 4). The ECG signal is characterized by six peaks and valleys labeled with successive letters of the alphabet P, Q, R, S, T, and U (Figure 1).
- However the paper specified that the data was only valid for healthy males in the specified age group and that the frequency components in children especially, would differ considerably. A second paper labeled 'Minimum Bandwidth Requirements for Recording of Paediatric Electrocardiograms' 9 monitored 200 infants in their study of determining the maximum frequency components of the ECG signal. The study was carried out by passing the raw ECG signal through filters of varying bandwidths while employing a sampling frequency of 1500Hz. The signals were than processed using Matlab. The results of study showed that in 95% of test subjects a 150Hz filter bandwidth was sufficient to adequately represent the ECG. However the optimal filter Bandwidth was selected to be 250Hz (where the filter bandwidth refers to the Low pass filter cut-off frequency). The majority of studies on ECG spectral analysis concentrate on the higher end of the spectral analysis. Reference ab8e9f176b1cb27bf8b12918090035f0" rel="">ab8e9f176b1cb27bf8b12918090035f0 and 8 however highlight the lower frequency component to be 0.05Hz. No indication however was given of how this value was obtained. Since Athletes typically have lower heart rates than the average person, a simple calculation was done to see if the 0.05Hz cut-off frequency was adequate in measuring the Athletes ECG signal. Assuming an athlete to have a heart rate of 32beats/min (Lance Armstrong, Tour De France Champion had a resting heart rate of 32beats/min at the peak of his powers 11) this corresponds to a frequency of:
+ However the paper specified that the data was only valid for healthy males in the specified age group and that the frequency components in children especially, would differ considerably. A second paper labeled 'Minimum Bandwidth Requirements for Recording of Paediatric Electrocardiograms' 9 monitored 200 infants in their study of determining the maximum frequency components of the ECG signal. The study was carried out by passing the raw ECG signal through filters of varying bandwidths while employing a sampling frequency of 1500Hz. The signals were than processed using Matlab. The results of study showed that in 95% of test subjects a 150Hz filter bandwidth was sufficient to adequately represent the ECG. However the optimal filter Bandwidth was selected to be 250Hz (where the filter bandwidth refers to the Low pass filter cut-off frequency). The majority of studies on ECG spectral analysis concentrate on the higher end of the spectral analysis. Reference 10 and 8 however highlight the lower frequency component to be 0.05Hz. No indication however was given of how this value was obtained. Since Athletes typically have lower heart rates than the average person, a simple calculation was done to see if the 0.05Hz cut-off frequency was adequate in measuring the Athletes ECG signal. Assuming an athlete to have a heart rate of 32beats/min (Lance Armstrong, Tour De France Champion had a resting heart rate of 32beats/min at the peak of his powers 11) this corresponds to a frequency of:
- Public health care cost consumes annually, 62 billion rand (Roughly ab8e9f176b1cb27bf8b12918090035f0%) of the government’s total budget. Public Health care however is currently desperately under resourced with people in the rural areas finding it very difficult to travel to hospitals and clinics which are largely based in the larger metropolis and there surrounding areas.
+ Public health care cost consumes annually, 62 billion rand (Roughly 10%) of the government’s total budget. Public Health care however is currently desperately under resourced with people in the rural areas finding it very difficult to travel to hospitals and clinics which are largely based in the larger metropolis and there surrounding areas.
- The 7805, 7905 regulators were used in the implementation. They are both able to supply 1A of current. The power supply is simulated using electronics work bench (Appendix D). Both regulators are loaded until breakdown in the simulation. The 7805 is successfully able to deliver 9ab8e9f176b1cb27bf8b12918090035f0mA before breakdown. The 7905 however only supplied 632mA. However this is more than sufficient for the purpose of the design.
+ The 7805, 7905 regulators were used in the implementation. They are both able to supply 1A of current. The power supply is simulated using electronics work bench (Appendix D). Both regulators are loaded until breakdown in the simulation. The 7805 is successfully able to deliver 910mA before breakdown. The 7905 however only supplied 632mA. However this is more than sufficient for the purpose of the design.
- ab8e9f176b1cb27bf8b12918090035f0.png" border="0" />

Figure ab8e9f176b1cb27bf8b12918090035f0: Three Operational Amplifier Differential Amplifier
+

Figure 10: Three Operational Amplifier Differential Amplifier
- The ECG leads interface directly to the inputs of the differential amplifier. Since the ECG signal is in the millivolt range, a highly sensitive differential amplifier is required for ECG measurement. The differential amplifier is required to perform the necessary subtraction of various potentials on the surface of the body. This subtraction action is required to generate the three lead vectors. Any of the three lead vectors may be generated in hardware by simply connecting the appropriate part of the body to the inputs of the differential amplifier. Further noise reduction techniques however will have to be employed at the output of the differential amplifier to obtain noise free ECG trace. The amplifier is also required to provide amplification to these millivolt range signals while strongly attenuating signals common to both inputs. CMRR describes the ability of a differential amplifier to reject interfering voltages (VCM), common to both inputs, and to amplify only the difference between the inputs. Various types of circuits have been used in the implementation of differential amplifiers, but the most popular design is the configuration shown in figure ab8e9f176b1cb27bf8b12918090035f0 4 above. Operational amplifiers OA1 and OA2 provide infinite input impedance while simultaneously passing the common mode voltage through and amplifying the differential voltage. Its differential gain Ad, and common mode gain Ac, are given by the following equations:
+ The ECG leads interface directly to the inputs of the differential amplifier. Since the ECG signal is in the millivolt range, a highly sensitive differential amplifier is required for ECG measurement. The differential amplifier is required to perform the necessary subtraction of various potentials on the surface of the body. This subtraction action is required to generate the three lead vectors. Any of the three lead vectors may be generated in hardware by simply connecting the appropriate part of the body to the inputs of the differential amplifier. Further noise reduction techniques however will have to be employed at the output of the differential amplifier to obtain noise free ECG trace. The amplifier is also required to provide amplification to these millivolt range signals while strongly attenuating signals common to both inputs. CMRR describes the ability of a differential amplifier to reject interfering voltages (VCM), common to both inputs, and to amplify only the difference between the inputs. Various types of circuits have been used in the implementation of differential amplifiers, but the most popular design is the configuration shown in figure 10 4 above. Operational amplifiers OA1 and OA2 provide infinite input impedance while simultaneously passing the common mode voltage through and amplifying the differential voltage. Its differential gain Ad, and common mode gain Ac, are given by the following equations:
- The characteristics of the discrete differential amplifier (Figure ab8e9f176b1cb27bf8b12918090035f0) may be improved by combining the three individual amplifiers in to a single IC, commonly known as an Instrumentation Amplifier. Instrumentation Amplifiers such as the AD620 and INA114 with input impedances in the range of ab8e9f176b1cb27bf8b12918090035f0GΩ/pF, vastly outperform the discrete component implementation discussed in the previous section. The AD620 draws a maximum supply current of 1.3mA 3 as compared to a general purpose JFET operational amplifier such as the OP07 which draws 20mA 2 per chip, resulting in a total supply current of 60mA for the configuration shown in Figure 9. Table 3 illustrates some key characteristics of three different instrumentation Amplifiers. Large scale VLSI integration ensures that the resistors within these Instrumentation Amplifier are matched to within 0.1% thus reducing common-mode gain (Equation 9).
+ The characteristics of the discrete differential amplifier (Figure 10) may be improved by combining the three individual amplifiers in to a single IC, commonly known as an Instrumentation Amplifier. Instrumentation Amplifiers such as the AD620 and INA114 with input impedances in the range of 10GΩ/pF, vastly outperform the discrete component implementation discussed in the previous section. The AD620 draws a maximum supply current of 1.3mA 3 as compared to a general purpose JFET operational amplifier such as the OP07 which draws 20mA 2 per chip, resulting in a total supply current of 60mA for the configuration shown in Figure 9. Table 3 illustrates some key characteristics of three different instrumentation Amplifiers. Large scale VLSI integration ensures that the resistors within these Instrumentation Amplifier are matched to within 0.1% thus reducing common-mode gain (Equation 9).
- The AD620 has a maximum gain of ab8e9f176b1cb27bf8b12918090035f000 2 ideally we would like to set the gain here to some maximum value thus eliminating the requirements for an additional gain stage prior to microcontroller stage. The gain selection of the AD620 is however critical to the design of the entire system. The gain must be set such that output saturation of the ±5V power supply does not occur. As mentioned above the maximum input is ±5 mV plus a variable normal-mode dc offset of up to ±300 mV 8. By setting the gain resistor to 6.2k it ensures that the maximum output swing is 3.07V i.e. Gain = ab8e9f176b1cb27bf8b12918090035f0. The gain was verified by grounding the negative input and passing waves of varying amplitude into the amplifier while observing both the input and output waves on an oscilloscope.
+ The AD620 has a maximum gain of 1000 2 ideally we would like to set the gain here to some maximum value thus eliminating the requirements for an additional gain stage prior to microcontroller stage. The gain selection of the AD620 is however critical to the design of the entire system. The gain must be set such that output saturation of the ±5V power supply does not occur. As mentioned above the maximum input is ±5 mV plus a variable normal-mode dc offset of up to ±300 mV 8. By setting the gain resistor to 6.2k it ensures that the maximum output swing is 3.07V i.e. Gain = 10. The gain was verified by grounding the negative input and passing waves of varying amplitude into the amplifier while observing both the input and output waves on an oscilloscope.
- Initially the output of the differential amplifier was interfaced directly to the Anti-aliasing filter with out removing the D.C offset component. Observing the signal at the output of the filter while using this configuration showed that the signal ranged anywhere between 45mV – 3.7V. This was unacceptable as the 45mV signal was too small to be adequately represented digitally with a 12 bit Analogue to Digital Converter (See Resolution). Simply adding a gain stage would cause the output to saturate if it fell in the volt range. Since the magnitude of this signal could not be reliably predicted it was decided that the DC offset component be removed. This was achieved by simply placing a ab8e9f176b1cb27bf8b12918090035f00μF capacitor, at the output of the differential amplifier. Observing the output of the filter after placing the capacitor yields an output ranging from24-63mV.
+ Initially the output of the differential amplifier was interfaced directly to the Anti-aliasing filter with out removing the D.C offset component. Observing the signal at the output of the filter while using this configuration showed that the signal ranged anywhere between 45mV – 3.7V. This was unacceptable as the 45mV signal was too small to be adequately represented digitally with a 12 bit Analogue to Digital Converter (See Resolution). Simply adding a gain stage would cause the output to saturate if it fell in the volt range. Since the magnitude of this signal could not be reliably predicted it was decided that the DC offset component be removed. This was achieved by simply placing a 100μF capacitor, at the output of the differential amplifier. Observing the output of the filter after placing the capacitor yields an output ranging from24-63mV.
- The DSPic’s on board Analogue to Digital converter peripheral is uni-polar, i.e. it cannot digitize signals below ground level. For this reason the signal being input to the DSPic must be level shifted. The DSPic runs of a 3.3V supply. This requires that the input to the DSPic be within the 0-3.3V range. The ab8e9f176b1cb27bf8b12918090035f0k and 4.7k resistors form a voltage divider. The output of the divider is given by:
+ The DSPic’s on board Analogue to Digital converter peripheral is uni-polar, i.e. it cannot digitize signals below ground level. For this reason the signal being input to the DSPic must be level shifted. The DSPic runs of a 3.3V supply. This requires that the input to the DSPic be within the 0-3.3V range. The 10k and 4.7k resistors form a voltage divider. The output of the divider is given by:
- The purpose of this circuit is to provide an inverted version of the common-mode interference to the user’s right leg, with the intention of cancelling out the interference. Additionally it serves as a virtual ground, for the ECG signal. The operational amplifier utilized in this circuit is the OP07 which is a low power, high precision junction field effect transistor (JFET) operational amplifier with an extremely high CMRR of about ab8e9f176b1cb27bf8b12918090035f06 dB minimum. A common sense point is established in order to drive this amplifier, also known as a force amplifier. The output from the circuit passes current through the user until the net sum output from the differential amplifier is zero. The amplifier is given a low pass cut off frequency of ab8e9f176b1cb27bf8b12918090035f00Hz by setting resistor Rhf and capacitor CLP (RLP = 12k CLP = ab8e9f176b1cb27bf8b12918090035f00nF).
+ The purpose of this circuit is to provide an inverted version of the common-mode interference to the user’s right leg, with the intention of cancelling out the interference. Additionally it serves as a virtual ground, for the ECG signal. The operational amplifier utilized in this circuit is the OP07 which is a low power, high precision junction field effect transistor (JFET) operational amplifier with an extremely high CMRR of about 106 dB minimum. A common sense point is established in order to drive this amplifier, also known as a force amplifier. The output from the circuit passes current through the user until the net sum output from the differential amplifier is zero. The amplifier is given a low pass cut off frequency of 100Hz by setting resistor Rhf and capacitor CLP (RLP = 12k CLP = 100nF).
- Due to the sensitivity of the AD620 Instrumentation amplifier, the output of the amplifier is highly susceptible to any variation in contact resistance between the skin and electrode. This condition results in a deviation of the D.C content of the amplified differential signal and manifests itself as a drift in the baseline of the ECG signal. This phenomenon is often referred to as baseline wander. The problem is overcome by utilizing an analog integrator scheme highlighted in Figure 15. The integrator scheme integrates the dc content of the ab8e9f176b1cb27bf8b12918090035f0× amplified Raw ECG signal and feeds it back to the reference pin of theAD620. The feedback action allows the AD620 to maintain a constant DC level at the output, regardless of the change in skin contact resistance.
+ Due to the sensitivity of the AD620 Instrumentation amplifier, the output of the amplifier is highly susceptible to any variation in contact resistance between the skin and electrode. This condition results in a deviation of the D.C content of the amplified differential signal and manifests itself as a drift in the baseline of the ECG signal. This phenomenon is often referred to as baseline wander. The problem is overcome by utilizing an analog integrator scheme highlighted in Figure 15. The integrator scheme integrates the dc content of the 10× amplified Raw ECG signal and feeds it back to the reference pin of theAD620. The feedback action allows the AD620 to maintain a constant DC level at the output, regardless of the change in skin contact resistance.
- ab8e9f176b1cb27bf8b12918090035f0.jpg" border="0" />
+
- The first major operation is to take a sample of an analog input and hold that sample for analysis. The sample is then held in the sample and hold amplifier. Sufficient time must be given to the converter to capture an accurate representation of the signal. The A/D will then analyze the output of the sample-and-hold amplifier and convert the information into a digital number. The DSPic30F4013 datasheet 13 specifies that the conversion time be set to a minimum of ab8e9f176b1cb27bf8b12918090035f0μs.
+ The first major operation is to take a sample of an analog input and hold that sample for analysis. The sample is then held in the sample and hold amplifier. Sufficient time must be given to the converter to capture an accurate representation of the signal. The A/D will then analyze the output of the sample-and-hold amplifier and convert the information into a digital number. The DSPic30F4013 datasheet 13 specifies that the conversion time be set to a minimum of 10μs.
- For the purpose of the wearable ECG, a second second-order digital notch filter having a notch frequency at 50Hz and a 3-dB notch bandwidth of 6Hz (Due to tolerance levels of mains frequency) is required. The sampling frequency employed is ab8e9f176b1cb27bf8b12918090035f000Hz. The normalised angular notch frequency and the normalised angular 3-dB bandwidth are therefore given by
+ For the purpose of the wearable ECG, a second second-order digital notch filter having a notch frequency at 50Hz and a 3-dB notch bandwidth of 6Hz (Due to tolerance levels of mains frequency) is required. The sampling frequency employed is 1000Hz. The normalised angular notch frequency and the normalised angular 3-dB bandwidth are therefore given by
- The code snippet below uses the butter() function to generate the necessary coefficients for a second order Butterworth filter with a ab8e9f176b1cb27bf8b12918090035f00Hz cut-off frequency. The ecgsig() function is then employed to generate a an ECG signal with 200Hz interference (figure 24a). The generated ECG signal is then filtered using convolution and the resultant trace is plotted (figure 24b). Finally figure 24c tests the filter as it will be programmed on the DSPic30F4013.
+ The code snippet below uses the butter() function to generate the necessary coefficients for a second order Butterworth filter with a 100Hz cut-off frequency. The ecgsig() function is then employed to generate a an ECG signal with 200Hz interference (figure 24a). The generated ECG signal is then filtered using convolution and the resultant trace is plotted (figure 24b). Finally figure 24c tests the filter as it will be programmed on the DSPic30F4013.
]]> Ivor Sat, 04 Jul 2009 23:35:16 +0100