Sampling Rates and THE LAW

Nyquist criterion demands that we sample at GREATER THAN twice the highest frequency applied to our analog-to-digital converter (A/D). This is required to avoid aliasing errors from the A/D. Following this LAW will provide enough meaningful information from our A/D to determine the frequency content in the measured signal. However, frequency content may not be enough information! What if you want to re-draw the waveform?

For example, if you took your A/D data from from above (sampled at just a little over twice the highest frequency applied to your A/D) and tried to use it in Excel, or other graphing software, it would look terrible! The frequency would be correct yet the appearance would be terrible.

So, how much more do you need to sample in order to reconstruct a "reasonable" graph of the original signal? How much more to reconstruct an "excellent" graph? Of course, it depends on your definitions of reasonable and excellent. Moreover, it depends upon your ability to look at ONLY the frequency of interest and to block out all the other frequencies.

To observe only the frequencies of interest requires an analog filter BEFORE the A/D converts the analog signal to digital data! There are many different types of analog filters. There are benefits and drawbacks for each type. Each filter will affect your data in different and sometimes undesirable ways. Moreover, the performance of a filter is determined by its design characteristics and number of poles in the filter.

Dataforth uses up to seven (7) poles of a modified Butterworth filter in the design of its SCM5B signal conditioners and a Bessel filter in the design of its SCM7B and DIN mount DSCA conditioners.

The type of filter selected will then determine the amount of additional ( or over-sampling) required to reconstruct the original signal's amplitude versus time into a good looking graph.

Read our application note AN115, Data Acquisition and Control Sampling Law

Remember, our Application Engineers can assist you with signal conditioner selection over the phone or via fax and email. Call us at our manufacturing facility in Tucson at 520-741-1404 (fax 520-741-0762) or Email us at support@dataforth.com.