Earth Data PS6-24 Digitizer Description

A high performance recorder for seismic profiling, aftershock studies, networks, noise measurements and magnetotelluric studies.

The PS6-24 is an ADC module providing three or six independent channels of extremely high dynamic range at rates of 1 to 1000 Hz. A selectable conversion word size of up to 32 bits is available on RS232 for fully representing the high dynamic range. The RS232 is also used to select the operating modes.

SIGNAL TO NOISE

A signal to noise ratio of better than 140 dB is achieved at a sample rate of 100 Hz falling to 130 dB at 1000 Hz using a 24-bit word. 150 dB is obtained below 10 Hz when a 32-bit word size is selected.

The signal bandwidth is always d.c. to 0.4 of the selected sample rate (± 0.1 dB) with over 140 dB of rejection above the half sampling frequency negating any requirement for anti-alias filtering.

INPUTS

Inputs are fully differential with a common mode rejection better than 100 dB. There are three sensitivity settings, 100 nV per bit, 1µV per bit and 2.5 µV per bit. About 4 dB of dynamic range is lost on the highest gain setting.

Input offset voltage and gain variations are held to very low levels. Optionally the offset with temperature can be mapped during manufacture and used to correct the offset during operation. Calibration occurs automatically every time the unit is switched on.

CONVERTERS

The converters use a 7th order Delta Sigma technique developed by Earth Data over a five year period. In addition to their high dynamic range they are also free from spurious tones and artefacts which affect competing designs.

GPS

The unit can be locked to GPS simply by operating the unit with an approved GPS receiver. Absolute time and global position is then included within the transmitted RS232 data and sample times can be tracked to UTC time. This is especially useful when it is necessary to correlate data from separate locations.

The unit is supplied with a 1 ppm TXVCO timing source that can maintain accurate timing without GPS.

POWER SUPPLY

Power supply voltage, current and internal temperatures are continuously monitored and included within the RS232 data. Four additional low-resolution ADC inputs are available to the user for general monitoring functions. Two digital outputs are available for driving external equipment.

CONNECTION

The PS6-24 is supplied with a Windows utility, MODMON32, for verifying correct operation of the digitizer. This provides a near real-time display of the time series for each channel, an FFT for each channel, and GPS status if the extended header is selected. MODMON32 can be used for setting the required operating parameters, and saving the settings to the non-volatile part of the digitizer memory. MODMON32 also provides the means of updating the digitizer firmware when new issues become available.

OPERATION

The PS6-24 is available in a fully sealed extruded aluminium case or as a board only OEM product. Operating temperature range is -20 °C to +60 °C. The unit is fitted with sealed rectangular 'D' type connectors as standard. MIL-C-26482 bayonet connectors are available as an option.