CHEAP SEIS AD: A 16-bit analog-to-digital converter for recording earthquakes
CHEAP SEIS AD is an analog-to-digital (AD) converter and multiplexer that records single-channel data from dc to 10 Hz or three-channel data from dc to 2 Hz. CHEAP SEIS is ideal for recording ground motions with periods from 20s to 0.5s from distant earthquakes.
CHEAP SEIS records data through the printer port of an IBM compatible computer. The supporting software displays the data in real time and records the data to the hard disk for later review and analysis. The standard recording options are single channel operation at 20sps, or one-, two-, or three-channel operation at 4sps. Each channel has a high-gain and a low-gain input, selectable with an internal jumper. The high-gain operation allows direct recording of low-level output sensors. Time is referenced to the internal clock of the computer. The software can synchronize the computer's clock with National Bureau of Standards by using an optional external WWVB receiver connected to the serial port.
The AD converter and multiplexer obtains its electrical power through the printer port of the computer and the unit requires no other connections except the input from a sensor. The current software records the 16 most significant bits from an 18-bit resolution sigma-delta type converter. A single channel data file recorded at 4sps consumes 145 kilobytes per hour of hard disk space. A 30-Megabyte disk will hold one week of single channel data. Appropriate anti-aliasing filters are included with each analog input channel.
Calibration pulse generation is available from the data-logging program. When triggered with a keyboard command, a fixed-current, square wave signal is applied to the calibration coil of a seismometer and the seismometer signal is recorded to a separate file. The seismometer's frequency response is then computed using that saved file in a separate utility program. Relative sensitivity is measured from the height of the response curve. Absolute sensitivity of the seismometer may be obtained by using the calibration factor provided by the seismometer manufacture or from the results of a classical weight drop test.
Recording computer: A PC (desktop or portable) having a clock rate of 50 MHz or greater, a 486 (or better) processor, and DB-25 parallel printer port. A serial port is required if a WWVB receiver is used for synchronization of the internal computer clock. Only the calibration program requires a math coprocessor.
Software requirements: DOS 3.3 or higher, Windows 3.0+/95/98/NT/.
Input: This AD configuration is a 15-bit + sign multi-slope device that gives a resolution of +30 microvolts. The normal full-scale input range to the AD is +1 volt. Signal levels above this range are clipped. Higher voltage ranges can be accommodated using resistive dividers at the analog inputs. The internal preamplifiers (60dB gain) that are selectable using the jumper connections on the AD printed circuit board will accommodate low-voltage input signals.
Figure 1. Low-cost, 16-bit analog-to-digital converter for recording long-period seismometer data
Housing: The AD unit is enclosed in a box (7.5 in. X 5 in. X 1.5 in.) with a DB-25 connector on one end for connection through a parallel connector cable to the computer, and a screw terminal connector on the other end for attaching the seismometer or other sensor signal.
Anti-aliasing filter: 3-pole low-pass filters are provided for each input channel: In standard configuration, the three 4sps channels provide -24 dB at 2 Hz. The 20sps channel provides -24 dB at 10 Hz.
Time Reference: Time reference for data collection is linked to the internal computer clock. This clock can be automatically synchronized to the National Bureau of Standards with a WWVB receiver via the serial port (receiver also derives power from computer).
The top trace was recorded with the system shown in figure 1, a 1.0 Hz geophone, CHEAP SEIS AD using the high-gain input, and a portable computer. The geophone was placed on the ATL short-period pier. The lower trace is the ATL SPZ record of the same event, that was digitized at 100 sps at Georgia Tech. The trace illustrates surface waves from a typical quarry blast.
The seismogram on the right is the Mexico earthquake of September 30, 1999, recorded with the CHEAP SEIS AD using our prototype long-period vertical sensor. The figure on the left demonstrates the frequency response measurement option included with CHEAP SEIS AD. The top trace is the response of the prototype seismometer to a variable-period calibration square wave. The lower trace in this graph is the velocity response of the seismometer under test. This curve shows that the prototype seismometer has a flat response from the folding frequency (6 Hertz in this example) to well below 0.05 Hertz (i.e., greater than a 20 second period).