The second-order, switched-capacitor, feed-forward ΔΣ modulator conceptualized in Figure 7-2 is implemented in the AMC3306M25. The analog input voltage V_IN and the output V₅ of the 1-bit digital-to-analog converter (DAC) are differentiated, providing an analog voltage V₁ at the input of the first integrator stage. The output of the first integrator feeds the input of the second integrator stage, resulting in an output voltage V₃ that is differentiated with the input signal V_IN and the output of the first integrator V₂. Depending on the polarity of the resulting voltage V₄, the output of the comparator is changed. In this case, the 1-bit DAC responds on the next clock pulse by changing the associated analog output voltage V₅, causing the integrators to progress in the opposite direction, and forcing the value of the integrator output to track the average value of the input.

Figure 7-2 Block Diagram of a Second-Order Modulator

The modulator shifts the quantization noise to high frequencies, as depicted in Figure 7-1. Therefore, use a low-pass digital filter at the output of the device to increase the overall performance. This filter is also used to convert the 1-bit data stream at a high sampling rate into a higher-bit data word at a lower rate (decimation). TI's C2000™ and Sitara™ microcontroller families offer a suitable programmable, hardwired filter structure termed a sigma-delta filter module (SDFM) optimized for usage with the AMC3306M25. Alternatively, a field-programmable gate array (FPGA) or complex programmable logic device (CPLD) can be used to implement the filter.