Input Coupling Analog input channels 1 through 8 can be independently set in

Input coupling analog input channels 1 through 8 can

This preview shows page 51 - 53 out of 122 pages.

Input Coupling Analog input channels 1 through 8 can be independently set in software to AC Coupling or to DC Coupling. When AC Coupling is selected, the input signal will go through either a 0.1 Hz or 1 Hz high pass filter, as set in software. When DC Coupling is selected the high-pass filter is bypassed. Programmable Gain Amplifier (PGA) The dynamic conditioning circuit provides programmable gains of 1, 2, 5, 10, 20, 50, 100, and 200. These correspond to bipolar input ranges of 5V, 2.5V, 1V, 500mV, 250mV, 100mV, 50mV, and 25mV. Additionally, there is a 25V DC-coupled only range that is suitable for proximity sensor measurements. Range selection is on a per channel basis. ZonicBook/618E 878595 Analog Signals 6-3
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6-4 Analog Signals 878595 ZonicBook/618E Low-Pass Anti-Aliasing Filter Each of the 8 channels has its own low-pass filter to provide alias protection and to allow for the removal of undesired frequencies from the measured response. What is Aliasing? Aliasing is a phenomenon of sampled data systems wherein a high frequency signal is misrepresented as a low frequency signal when the A/D converter sampling rate being used is too slow. This misrepresentation can result in severe data corruption and incorrect FFT results. Aliasing is a well-documented data acquisition effect, and interested users are encouraged to research detailed information that is available on-line from companies such as Analog Devices and Texas Instruments. This text aims to not supplant those resources, but to provide most users with sufficient knowledge to avoid most aliasing problems through proper filter and sampling rate configuration. For a given sampling rate, F S , input signals of frequency up to F S /2 will be processed correctly. However, input signals above F S /2 are subject to aliasing. For example, a sampling rate of 100 kHz can process signals up to 50 kHz without aliasing. An input signal of 90 kHz, however, will be aliased. Specifically, it will appear in the sampled data as a signal of frequency F S -F IN , which in this case is 100 kHz-90 kHz = 10 kHz. Aliasing, and its prevention, should be a consideration in all sampled data systems. This is especially important in mechanical vibration measurements, because most mechanical systems exhibit a resonance apart from their fundamental frequency. That is, there may be signal energy present that has the potential to be aliased that is unknown to the user. And the worst part of aliasing is that its effects are indistinguishable from real input signals. That is, in the given example, it is not apparent to the user whether the 10 kHz energy is real or an alias. Aliasing Protection F IN /F C Gain (dB) <0.1 -0.15 0.1 -0.15 0.2 -0.18 0.3 -0.22 0.4 -0.27 0.5 -0.33 0.6 -0.39 0.7 -0.47 0.8 -0.68 0.9 -1.4 1.0 -3.6 1.25 -16 1.5 -28 1.75 -39 2.0 -48 2.5 -63 2.8 -70 3.0 -75 3.3 -80 3.5 -86 ZonicBook/618E’s conditioning circuit provides alias rejection via the unit’s 8-pole low pass filter. This filter has an extremely steep roll-off characteristic, very closely achieving an ideal “brick-wall” response. It consequently passes frequencies of interest without significant attenuation, but significantly attenuates frequencies just above.
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