FIR filter configuration of older CMG-6TD instruments

This document describes the sequence of FIR filters used by the CMG-6TD instruments manufactured before the introduction of the CMG-CD24 digitiser.

Data from the modulator is decimated within the DSP to 1000 Hz. This is done using cascaded ÷5 and ÷4 filters with the coefficients given below.

After this, four further sets of FIR filters are used to decimate the data. ÷2, ÷4 and ÷5 filters are available; however, up to two FIR filters may be applied at each tap, allowing you to choose decimation factors up to 10.

The CMG-6TD allows the user to select a decimation factor for all four taps. However, the hardware does not support a 500 samples/s output rate, so the first tap must be set to a factor of ÷4 or ÷5.

Other information, such as bit weights (in μV/count), normalisation factor, and poles and zeros, can be found on the instrument’s calibration sheet.

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