975 Decimation Filter for Incremental and Regular Delta-Sigma Modulators :: Quicker, easier and cheaper to make your own chip!

Overview

The decimation filter efficiently reduces the sample frequency of Incremental and Regular Delta-Sigma Modulators (DSMs) by a factor of 16. This process minimizes high-frequency noise and downsamples data, supporting effective and accurate signal processing of oversampled ADC outputs.

Specifications

Inputs: 3 total
- Input 1 (1 bit): ADC data input
- Input 2 (1 bit): Decimation mode selection (0 = Incremental DSM, 1 = Regular DSM)
- Input 3 (1 bit): Global reset
Output: 16 bits total
- Most Significant 8 bits (MSBs): Routed to dedicated output pins
- Least Significant 8 bits (LSBs): Routed to general-purpose IO pins
Clock Frequency: 50 MHz (standard operation)

Mode Selection

The decimation mode can be configured based on the DSM type:

Incremental DSM: Set Input 2 to low.
Regular DSM: Set Input 2 to high.

How It Works

Noise Reduction and Downsampling: The decimation filter reduces high-frequency quantization noise from DSM oversampling, delivering a downsampled output with preserved signal quality.
Adaptive Output Rate:
- Incremental DSM (Input 2 Low): The output updates after accumulating 16 input samples.
- Regular DSM (Input 2 High): The output updates based on an internal timing controlled by the reset signal.
Output Simplification: The filter converts a high data rate from the oversampled ADC into a manageable downsampled rate, optimizing data processing.

Operation

The decimation filter requires an initialization pulse on the global reset input upon start-up.

Incremental DSM Mode (Input 2 Low):
- Use the ADC’s oversampling frequency as the input clock for the filter.
- Set the main reset signal to match the desired decimation rate.
- For example, with a 50 MHz ADC frequency, setting the reset signal to 25 MHz achieves a decimation factor of 2.
Regular DSM Mode (Input 2 High):
- The default decimation factor is set to 16.
- For customized decimation factors, follow the configuration steps in Incremental DSM mode.

Testing Procedure

Hardware Setup:
- Connect a 1-bit ADC output to Input 1.
- Set Input 2 to low for Incremental DSM or high for Regular DSM.
Verification:
- Incremental DSM: Set Input 2 low, connect a clock to the reset input, and observe decimated output changes.
- Regular DSM: Set Input 2 high, then observe the decimated output, which updates at a rate of 16 samples.

Output Configuration

The decimation filter’s 16-bit output is divided as follows:

Most Significant 8 Bits (MSBs): Directed to dedicated output pins.
Least Significant 8 Bits (LSBs): Directed to general-purpose IO pins.

Compatibility

This filter is compatible with 1-bit output ADCs, either Incremental or Regular Delta-Sigma Modulator (DSM) types.

#	Input	Output	Bidirectional
0	X	decimation_output[8]	decimation_output[0]
1	type_dec	decimation_output[9]	decimation_output[1]
2	global_reset	decimation_output[10]	decimation_output[2]
3		decimation_output[11]	decimation_output[3]
4		decimation_output[12]	decimation_output[4]
5		decimation_output[13]	decimation_output[5]
6		decimation_output[14]	decimation_output[6]
7		decimation_output[15]	decimation_output[7]

975 Decimation Filter for Incremental and Regular Delta-Sigma Modulators