This message from Bob McClure to the Public Seismic Network E-mail list explains how the DATAQ AD's work and why an anti-alias filter is not required when using one.


Date Fri, 6 Sep 2002 004510 EDT

Subject Re Seismic "Hookup"



Hi Ed,

I use the DI-194RS and the DI-154RS for seismic recording. The DI-154 is preferable over the DI-194 because it has 12-bit A/D conversion, but at the same time, it costs $150 compared to the $25 paid for the RS-194. It will work with your setup, I believe, but your preamp should have a full scale range of +/- 10 volts in order to get all of the dynamic range available from the Dataq starter kit.

The Dataq unit samples at a fixed rate of 240 samples per second, but records at any desired rate that is an integer divide of 240. All of the acquired data is input signal averaged, which improves resolution, increases signal-to-noise ratio, attenuates aliasing, and totally rejects 60 Hz and 120 Hz hum. For example, I usually record at 5 samples per second. During the 0.2 second sample interval, 48 data samples taken at the 240 samples per second rate are averaged to yield the recorded sample.

The data is resolved to 14 bits. The lower 2 bits of the data word are reserved for marker bits, such as time marks. I equipped a WWVB wall clock with an optical pickoff to mark my recordings at a once-per-minute rate.

I developed a drum plot program for display and filtering of recorded data. This program can run simultaneously with the Dataq recorder and display data from the file as it is being recorded. It can output filtered versions of the original file. I find the "Broadband" filter to be most useful, since it extends the usual response of my 3.4 second seismometer out to periods of 20 seconds or more. It is available for downloading from the Dataq web site. It is also on the installation disc for the Dataq Lite Software, in the catalog section.


Bob McClure