SBAS951B August 2019 – April 2020 AMC1336
The modulator implemented in the AMC1336, as conceptualized in Figure 42, is a second-order, switched-capacitor, feed-forward ΔΣ modulator. The analog input voltage VIN and the output V5 of the 1-bit digital-to-analog converter (DAC) are subtracted, 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 VIN 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.
As depicted in Figure 39, the modulator shifts the quantization noise to high frequencies. 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 from the 1-bit data stream at a high sampling rate into a higher-bit data word at a lower rate (decimation). TI's microcontroller families TMS320F2807x and TMS320F2837x offer a suitable programmable, hardwired filter structure termed a sigma-delta filter module (SDFM) optimized for usage with the AMC1336. Furthermore, the SD24_B converters on the MSP430F677x microcontrollers offer a path to directly access the integrated sinc filters for a simple system-level solution for multichannel, isolated current sensing. An additional option is to use a suitable application-specific device, such as the AMC1210 (a four-channel digital sinc-filter). Alternatively, a field-programmable gate array (FPGA) can be used to implement the filter.