FIR filter configuration of older 6TD instruments

This document describes the sequence of FIR filters used by the 6TD instruments manufactured before the introduction of the 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 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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