Overview of "AA" Conditioner Cards
Anti-Aliasing Filters
Anti-Aliasing Filters
Analog signals acquired in the real world of industry and research are inevitably accompanied by high-frequency noise and other unwanted dynamic content, which must be identified and filtered out before A/D conversion takes place, if the resulting measurement is to be valid. The above figure shows how "aliasing" occurs when a noisy data signal is sampled at a rate (here, 1 ms) that is too slow to allow accurate representation of the "real data." The apparent data has a significantly different waveform from the actual data. Conclusions based on such "aliased" data can be substantially wrong.
Fig. 1 Typical High-Frequency Aliasing of Measurement Data
Daytronic data acquisition instrumentation minimizes the aliasing problem in two principal ways, without having to resort to elaborate software techniques:
- by providing a high scan speed in those systems where A/D conversion takes place, so that data is sampled at a rate that is ordinarily well above twice the highest frequency component (the Nyquist Criterion); and
- by incorporating specially modified low-pass analog filters in the normal signal conditioning process.
Fig. 2 Comparison of Chebychev and Butterworth Filters with the Same 3-dB Cutoff Frequency (f0)
For continuous active low-pass filtering on a per-channel basis, Daytronic Conditioner Cards employ high-quality multipole Butterworth filters that have been modified using a proprietary design. Butterworth filters were selected because they yield a relatively sharp rolloff at the cutoff frequency (approximately 20 dB per decade per pole) with optimal flatness over the passband. As shown in Fig. 2, other filters with high-slope rolloff may exhibit a sharper transition from passband to rejectband, but this is achieved at the cost of significant error-producing passband ripple. The basic Butterworth design has been modified, however, to eliminate overshoot and ringing that occur with a step change in input. Fig. 3 compares the time response of five different 3-pole filters to the same 5-volt step. Compared to the other anti-aliasing filters, the Daytronic proprietary filter produces virtually no overshoot. This feature is particularly important in dynamic peak-capture and sample-hold operations.
Fig. 3 Comparison of Filter Step Response in the Time Domain
The analog filtering provisions offered by each individual Conditioner Card have been carefully designed to yield uniform frequency response over the entire measurement bandwith of the specific input type and ranges to which the card is dedicated, with exceptionally reliable, stable performance.
Additional anti-aliasing leverage is provided by the programmable analog filters of Daytronic’s new "AA" Card Series. With these cards, you can easily adjust a data channel’s corner frequency to correspond to the practical measurement bandwidth for that channel- or to correspond to changes in the sampling rate, thus ensuring that input-signal frequencies above the Nyquist rate (1/2 the sampling rate) do not generate false frequency components in the sampled data.
|
For complete Pricing and Ordering Information, write us or call Daytronic at 1-800-668-4745 |