9.1.1 Digital Filter Usage

The modulator generates a bit stream that is processed by a digital filter to obtain a digital word similar to a conversion result of a conventional analog-to-digital converter (ADC). A very simple filter, built with minimal effort and hardware, is a sinc³-type filter, as shown in Equation 2:

Equation 2.

This filter provides the best output performance at the lowest hardware size (count of digital gates) for a second-order modulator. All the characterization in this document is also done with a sinc³ filter with an over-sampling ratio (OSR) of 256 and an output word width of 16 bits.

Equation 3.

Figure 53. Measured Effective Number of Bits versus Oversampling Ratio

An example code for an implementation of a sinc³ filter in an FPGA, see the application note Combining ADS1202 with FPGA Digital Filter for Current Measurement in Motor Control Applications (SBAA094), available for download at www.ti.com.