This information applies to all CMG-6TD and CMG-EDU systems manufactured after 2005. These digitizers are single-channel units based on the CD24 compact digitizer module. It also applies to stand-alone CMG-CD24 digitisers.
Older EDU systems, which have a standard serial port on the processor box, use a different filter system.
Analogue signals are sampled at an initial rate of 256 kHz by a TI ADS1256 chip set, and decimated to 4 kHz using its on-board FIR filter.
After this, the digitizer uses further FIR filters to reduce the sample rate. Up to 8 filter stages may be applied in total.
For the CMG-EDU-V, the first and second stages are pre-set to decimation factors of 5 and 4 respectively. As shipped, the third stage is set to a decimation factor of a further 5, yielding 40 samples/s output (the maximum available output rate of the EDU-V digitizer.)
The CD24 digitizer is capable of single-stage decimation by factors of 2, 4 and 5. Decimation factors of 8, 10, 16, 20 and 40 are achieved by cascading two or three filters in descending order of the decimation factor.
Other information, such as bit weights (in μV/count), normalization factor, and poles and zeros, can be found on the digitizer's calibration sheet. Representative values are:
Recent CD24 firmware sets the digitiser type field and tap table lookup value in the GCF header. This allows the sequence of filters to be determined from the GCF data itself.