In addition, the cost of an ADC increases as higher sampling rates are desired. Finally, more computer processing time and storage space in memory or disk are needed to process the larger number of data points produced when the sampling rate is increased. Thus there is a tradeoff between fidelity of reproduction on the one hand, and computer storage space, computing time, and cost on the other.
Filtering Any ADC has a maximum sampling rate. In some circumstances, this maximum sampling rate is not high enough to satisfy the Nyquist conditions mentioned above. In that case, one can pass the analogue signal through a low-pass filter before sending it on to the ADC. This filter acts to remove some of the high-frequency content of the signal that would otherwise alias down in frequency, producing spurious low-frequency content along the lines illustrated above.
Note that this anti-alias filtering could remove high frequency information of physiological importance to the phenomenon under investigation. If it is important to retain these higher frequencies, one has no choice but to use a better data acquisition system that has a higher sampling rate.
A biological signal can be broken down into fundamental frequencies, with each frequency having its own intensity. Display of the intensities at all frequencies is a power spectrum.
Usually we are interested in signals of a particular frequency range or bandwidth. The bandwidth is determined by filters, which are devices that alter the frequency composition of the signal. Ideal frequency-selective filter: is a filter that exactly passes signals at one set of frequency and completely rejects the rest.
There are three types of filter:. Real filters or hardware filters alter the frequency composition of the signal. It means after filtering the signal, we cannot recover the frequencies that have been filtered. Digital filters change the frequency of the signal by performing calculations on the data. The heavily darkened curve represents the aliasing of 11 to 5 Hz from A.
Armed with the Nyquist theorem, one knows the limitations of what signal components in the source are capturable, but the question is naturally raised as to the consequence of signals that are above this Nyquist limit. If these were simply eliminated, the issue would be moot, but sadly high-frequency components above the Nyquist can actually create nefarious artifacts in the digitized output. The sampled data points can be seen to equivalently match a sinusoid at 5 Hz solid curve.
Indeed, in practice, when displayed on a screen connected by straight lines , these points would more closely match the 5-Hz sinusoid than the original Hz sinusoid.
Fortunately, methods to eliminate this artifact are quite simple. Electrical filters—specially designed circuits composed of elements such as resistors and capacitors—can be used to eliminate undesired frequencies in the analog signal prior to digitization. In particular, ADC systems incorporate an analog low-pass filter prior to digitization, with this filter designed to pass frequencies below the Nyquist frequency and eliminate those above it.
The inset just above and to right shows the power spectrum of this data, showing the white noise and the large signal at 5 Hz. The right side panels show how anti-aliasing filtering prior to digitization can prevent this problem. Top-right panel shows the tracing after a low-pass filter modeled as Butterworth third order with cutoff at 50 Hz has been applied. Bottom-right panel shows the result of digitization again at samples per second of this filtered tracing.
Frequencies above the Nyquist 64 Hz have been attenuated by the low-pass filter and are therefore not folded back, so digitization in this case yields a faithful reproduction of the signal. The anti-alias filter and ADC in Figure 7. The analog hardware anti-alias filter must obviously precede the ADC. The anti-alias filter cutoff must be appropriate for the sampling rate of the joined ADC. The filter must adequately attenuate frequencies above the Nyquist, or else high-frequency noise will be aliased into the digitized signal.
If the cutoff is too low, then signals with frequencies above this cutoff and below the Nyquist will be unnecessarily attenuated and lost, and expense will have been wasted in using an ADC with higher sampling rate than necessary.
The sampling rate does not impact whether one can distinguish a signal as being one of two closely spaced frequencies, for example, 8. Frequency resolution is determined by the duration of the acquired signal. In general, the duration required is approximately the inverse of the difference in frequency. This reflects the fact that two sinusoids with some difference in frequency will become out of phase with each other over that time frame. For instance, to reliably distinguish a 0. This applies irrespective of the absolute frequency—that is, distinguishing 8.
Magnitude or Amplitude. Digitization occurs along the magnitude axis as well. Instead, only the value or magnitude at each successive point is stored. Digital equipment is optimized when storing each successive magnitude in a predefined length.
The number of binary digits bits set aside for each measurement is the same. This storage length can differ between equipment manufacturers. Early digitizers used 8, 10, or 12 bits to store each measurement. Digitizers now routinely use 16 to 24 bits. The number of bits determines explicitly the number of distinguishable magnitudes of the measurements.
Note carefully that for the 6-bit depth, the digital step is most apparent at the peak and trough, where for each time step the voltage change is small, but these changes are smaller than the smallest discrete step and so cannot be represented. The three parameters—maximum value or dynamic range , minimum distinguishable difference, and digitization depth— are interconnected, and not independent.
Choosing any two specifies the third. The more bits available, the more accurate each sample. For very low-amplitude signals e. Both sampling rate and digitization depth have an impact on acquired file lengths. Increasing the sampling rate to Hz would increase the file sizes by 2.
The ACNS guideline 9 suggests that bit depth including sign bit is the minimum, with 12 bits or more being preferred. The minimum amplitude resolution is recommended to be 0. Since the ADC rate and anti-aliasing filter are hardware determined, these are under the control of the manufacturer but should be investigated and documented clearly when purchasing decisions are considered.
ADC Hardware. Some of the basic principles of the hardware involved in AD conversion are worth mentioning. First, no AD converter operates instantaneously. The input voltage is typically measured over some finite time obviously less than the time between samples. The time over which the signal is actually measured is called the conversion time. Sampling skew is most problematic with high-frequency activity and may cause misalignment of high-frequency events such as spikes between the first and the last channels sampled.
A second method is to have a series of ADCs, one for each channel, all triggered by a single computer command. There are various ADC hardware structures available. These structures include direct conversion or flash, successive approximation, ramp-compare, delta-encoded, pipeline, and sigma-delta. A single resistor sets the gain from 1 to The reference pin can be used to apply a precise offset to the output voltage.
Frequency domain measurements can use the wideband linear phase filter. The wideband and sinc5 filters can be selected and run on a per channel basis. The ability to vary the decimation filtering optimizes noise performance to the required input bandwidth. Embedded analog functionality on each ADC channel makes design easier, such as a precharge buffer on each analog input that reduces analog input current and a precharge reference buffer per channel reduces input current and glitches on the reference input terminals.
The AD integrates key analog and digital signal conditioning blocks to allow users to configure an individual setup for each analog input channel in use. Each feature can be user selected on a per channel basis.
Integrated true rail-to-rail buffers on the analog inputs and external reference inputs provide easy to drive high impedance inputs. The precision 2. The digital filter allows simultaneous 50 Hz and 60 Hz rejection at a The user can switch between different filter options according to the demands of each channel in the application.
The ADC automatically switches through each selected channel. Further digital processing functions include offset and gain calibration registers, configurable on a per channel basis. The auditory tone bursts are accomplished by a specially modified tone synthesizing component that is included within the system.
It is able to generate the required range of auditory stimuli, and because it programmable, offers a much better dynamic range. The somatosensory component has been developed to drive an off-the-shelf stimulus isolation unit so that patient safety is maintained.
The digital signals are then sent over the telephone line using a baud modem that is attached to the microcomputer, where they are analyzed and saved at the data collection center. The stimulator computer switches the telephone line from analog mode to digital transmission mode for modem transmission via a computerized switch. During EEG recording, as an option, the digital data can be displayed on the monitor 38 of the stimulator computer The stimulator computer controls the data acquisition process and monitors the communications line.
In case of communication line interruption, the digital data are saved, and streamed to the data collection center once communication or telephone contact is resumed. The transmitted signals from the remote site is received at the data collection center through the main control computer 42 which monitors the incoming communications lines and routes the incoming signal either to the receiving unit 44 in the case of analog signals where an analog output is obtained and the signal is analyzed by the next available analysis station, or directly to the analysis station in the case of incoming digital signals.
The receiving unit 44 is part of the data collection center hardware and is connected directly to the main control computer via a software-controlled switch. Incoming analog signals are routed to the receiving unit The specifications of a preferred implementation of the receiving unit are as follows:.
Signal to Noise Ratio: Better than , with 50 microvolts input calibration signal. Power Requirements: A. The incoming channel analog signals are decoded by removing the carrier frequencies and outputting the signal to separate lines. Each line contains a signal of each channel from the transmitting unit The technician is able to talk to the transmitting site directly through the receiving unit.
In case the volume control on the transmitting unit is turned down, the receiving center can sound a tone to alert the user that the data collection center needs to talk to him. The brain function monitoring subsystem operated at the data collection center is preferably a microcomputer-based electrophysiological data collection and analysis system integrated with microcomputer hardware and software.
The hardware components of this subsystem are as follow:. Although the above microcomputer configuration is currently used, any suitable equipment which will run the software can be used.
The system can take advantage of subsequent improvements in hardware. A special interface, indicated by the component 44a in FIG.
An 11 Hz. Also employed is a 60 Hz. A built-in bypass switch is provided for filtered or non-filtered recording necessary for evoked potential recording. Software-controlled switching circuitry is used for software controlled switching between calibration and data acquisition modes. Software feedback circuitry checks for proper software control during data acquisition. The analog to digital converter currently used has bit resolution, and is capable of acquiring channels under software control, with software-controllable variable gain 2.
In this way all text outputs can be printed on the laser printer faster and more economically than using the color ink jet printer for all outputs. All printing is done under software control therefore universal printer drivers have been developed. With these drivers any kind of off-the-shelf printer, such as dot matrix or thermal printers or plotters, can be used with the system depending on the application. The software for the data collection center includes the components described below and as illustrated by function blocks in FIG.
Patient information acquisition software is matched to the software at the data transmission site. When the computer 34 is used, an automatic data acquisition screen appears. The user is asked to enter the initials name is not used to guarantee confidentiality of the patient , date of birth, sex, handedness, and social security number of the patient. Using the mouse, the user answers clinical questions necessary for the proper interpretation of the data to be obtained.
When all the patient data has been entered, then the remote system enters the transmit mode. The automatic communications switch is switched so that the modem obtains a telephone line, and the data collection center is dialed. Once the data collection center is on line, the entered patient information is transmitted.
Once the transmission is completed, the switch is switched so that the electrophysical data transmission subsystem is on line. The same procedures can be applied if satellite transmission is used. With the mouse the user selects the appropriate stimulation protocol and prepares the patient.
Once the stimulation is started then instructions to the patient appear on the screen and pressing the mouse initiates the testing. During the testing a trigger signal for each testing sweep and stimulation control information are sent on different channels though the communications link.
During the EP or ERP testing the system can be paused and resumed at any time by pressing the appropriate mouse buttons. At the data collection center, the software to control and acquire the electrophysiological data is installed. The software switches the communications link to the receiving modem, and the software is in the answer mode. When a remote site calls in, the telephone is automatically answered, and the patient information is collected.
Once all patient data has been received, the system switches the communications link to receive EP, ERP, or EEG data and the software performs a calibration. This is done by switching the interface 44a into a calibration mode, and acquiring the calibration signal. The results of the calibration are displayed on the monitor 45 where the recording technician can check the signal integrity and decide whether to commence or to restart the procedure.
Once the technician signals that the calibration is acceptable, then the software switches the interface into data acquisition mode, and the system is ready to acquire the incoming electrophysiological data. The technician then signals the beginning of the recording to the transmitting site, and the center executes the predetermined protocol for recording.
If the test is an EEG, the resultant signal is sent to the data recorder 46 which can be an EEG machine or amplifiers with an analog writer unit attached and an analog paper record is generated. During the recording the digital signal is also displayed on the monitor, and maps for every 5 seconds are also displayed.
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