FIR filter configuration of the Affinity

This information applies to all Güralp Affinity digitisers.

Data from the modulator are decimated in stages down to 4000 Hz within the Texas Instruments ADS1282 ADC chip, which is configured to provide an initial fifth-order sincAlthough T.I. call this a sinc filter, it is more accurately described as a Cascaded Integrator-Comb or CIC filter. A classic sinc filter's shape is sinc in time and rectangular in frequency whereas a CIC filter's shape is rectangular in time and sinc in frequency. For more information, see Wikipedia's article on CIC filters and the Beginner's Guide To Cascaded Integrator-Comb (CIC) Filters on DSPrelated.com. filter and four low-pass FIRFinite Impulse Response is a class of filter whose response to an impulse output will settle to zero in finite time - see Wikipedia's article on FIR filters for further information. filters.

After this, a configurable chain of FIR filters are used to decimate the data down to the user-configured sample rate(s). Each filter stage provides ÷2, ÷4 or ÷5 decimation. The filters are arranged in increasing order of divisor so zero or one ÷2 filter is followed by zero, one or two ÷4 filters and, finally, zero, one, two or three ÷5 filters. The specific filter chains used for each sample rate are given in the decimation chain table below.

The general arrangement is given in this diagram:

The filter chains used for each desired sample rate are:

For more details, please see Chapter 6 of the Affinity manual.

Characteristics and coefficients

The z-domain characteristics of the sinc filter are

The coefficients of the five initial filters are as follows:

• Coefficients for initial fifth-order Sinc (CIC) ÷ 8
• Coefficients for FIR stage 1: 11-pt symmetric (linear phase) ÷2
• Coefficients for FIR stage 2: 15-pt symmetric (linear phase) ÷2
• Coefficients for FIR stage 3: 49-pt asymmetric ÷4
• Coefficients for FIR stage 4: 110-pt asymmetric ÷2

The coefficients of the FIR filters in the configurable chain are as follows:

• Coefficients for 501-pt FIR ÷2
• Coefficients for 501-pt FIR ÷4
• Coefficients for 501-pt FIR ÷5