Cascaded integrator comb filter with arbitrary integer decimation value and scaling for unity gain

A CIC digital filter for filtering input data samples;

• the filter having an input for data samples;
• the filter comprising:
• a shifter connected to the input;
• the shifter receiving a shift control input;
• the shift control input being pre-computed as equal to the integer portion of 2 raised to the base-2 logarithm of the gain of the CIC filter;
• a multiplier connected between the input and the shifter;
• at least four sequentially-connected integrator functions connected to the shifter;
• a decimation function receiving input from the integrator functions;
• at least four sequentially-connected differentiator functions receiving input from the decimation function;
• the decimation function having a selectable rate equal to any integer between 1 and a number equal to the predetermined maximum decimation value;
• and,
• the multiplier configured to compute the product of each input data sample by a correction factor;
• the correction factor being pre-computed as equal to the fractional portion of 2 raised to the base-2 logarithm of the gain of the CIC filter, so as to correct the gain of the CIC filter for decimation values not a power of 2.

A method of implementing a CIC filter having an input, an output, a shifter, a shift control, a multiplier, one or more integrator functions, a decimation function, and one or more differentiator functions, the method comprising:

• receiving an input stream of data samples;
• shifting each data sample by a left shift equal to the integer portion of the base-2 logarithm of the CIC filter gain;
• multiplying each so shifted data sample by the fractional portion of the base-2 logarithm of the CIC filter gain;
• integrating the shifted and multiplied data sample at least once;
• decimating the data sample by a pre-determined decimation rate;
• differentiating the decimated data sample at least once;
• and,
• rounding the differentiated data sample to a pre-determined number of bits before outputting the data sample.

An integrated circuit for performing a CIC digital filter function;

• the integrated circuit comprising:
• at least one integrator;
• a decimator;
• the decimator receiving input from the integrator;
• at least one differentiator receiving input from the decimator;
• a multiplier connected to the input;
• a shifter connected between the output of the multiplier and to the integrator;
• and,
• the multiplier configured to compute the product of each input data sample by a correction factor, so as to correct the gain of the CIC filter for decimation values not a power of 2.

An integrated circuit for performing a CIC digital filter function for filtering input data samples, comprising:

• a shifter;
• the shifter connected to the input;
• the shifter receiving a shift control input;
• the shift control input being pre-computed as equal to the integer portion of 2 raised to the base-2 logarithm of the gain of the CIC filter;
• a multiplier connected to the shifter;
• at least four sequentially-connected integrators connected to the multiplier;
• a decimator receiving input from the integrators;
• at least four sequentially-connected differentiators receiving input from the decimator;
• the decimator having a selectable rate equal to any integer between 1 and a number equal to a predetermined maximum decimation value;
• and,
• the multiplier configured to compute the product of each input data sample by a correction factor;
• the correction factor being pre-computed as equal to the fractional portion of 2 raised to the base-2 logarithm of the gain of the CIC filter, so as to correct the gain of the CIC filter for decimation values not a power of 2.

A multi-channel CIC digital filter for filtering input data samples from a multiplexed data stream, comprising:

• a multiplier connected to the input;
• a shifter connected to the multiplier;
• a demultiplexer connected to the shifter for demultiplexing the multiplexed data stream into two or more data streams;
• where each demultiplexed data stream is input to:
• at least one integrator function;
• a decimation function receiving input from the integrator function;
• at least one differentiator function receiving input from the decimation function;
• the multiplier and shifter configured to compute the product of the input data samples by a correction factor, so as to correct the gain of the multi-channel CIC filter for decimation values not a power of two.

A method of implementing a multi-channel CIC filter for filtering input data samples from a multiple data stream;

• the multi-channel CIC filter having an input, an output, a shifter, a shift control, a multiplier, a demultiplexer, and a plurality of channels;
• each channel having one or more integrator functions, a decimation function, and one or more differentiator functions, the method comprising:
• receiving an input stream of multiplexed data samples;
• shifting each data sample by a left shift equal to the integer portion of the base-2 logarithm of the CIC filter gain;
• multiplying each so shifted data sample by the fractional portion of the base-2 logarithm of the CIC filter gain;
• demultiplexing the multiplexed data stream into two or more data streams;
• for each data stream:
• integrating the shifted and multiplied data sample at least once;
• decimating the data sample by a pre-determined decimation rate;
• differentiating the decimated data sample at least once;
• and,
• rounding the differentiated data sample to a pre-determined number of bits before outputting the data sample.

An integrated circuit for performing a multi-channel CIC digital filter function on a multiplexed stream of data samples;

• the integrated circuit comprising:
• a multiplier connected to the input;
• a shifter connected to the multiplier;
• a demultiplexer connected to the shifter for demultiplexing the multiplexed data stream into two or more channels, each channel comprising a demultiplexed data stream;
• where each channel further comprises:
• at least one integrator;
• a decimator;
• the decimator receiving input from the integrator;
• at least one differentiator receiving input from the decimator;
• and, the multiplier and shifter configured to compute the product of each input data sample by a correction factor, so as to correct the gain of the CIC filter for decimation values not a power of 2.

An integrated circuit for performing a multi-channel CIC digital filter function on a multiplexed stream of data samples, comprising:

• a multiplier connected to the input;
• the multiplier configured to compute the product of each input data sample by a correction factor;
• the correction factor being pre-computed as equal to the fractional portion of 2 raised to the base-2 logarithm of the gain of the CIC filter, so as to correct the gain of the CIC filter for decimation values not a power of 2;
• and,
• a shifter connected to the multiplier;
• the shifter receiving a shift control input;
• the shift control input being pre-computed as equal to the integer portion of 2 raised to the base-2 logarithm of the gain of the CIC filter;
• a demultiplexer connected to the shifter for demultiplexing the multiplexed data stream into two or more channels;
• where each channel comprises a demultiplexed data stream;
• where each channel further comprises:
• at least four sequentially-connected integrators;
• a decimator receiving input from the integrators;
• at least four sequentially-connected differentiators receiving input from the decimator;
• the decimator having a selectable rate equal to any integer between 1 and a number equal to a predetermined maximum decimation value.