8.3.2 Modulator

The modulator implemented in the AMC1305 is a second-order, switched-capacitor, feed-forward ΔΣ modulator, such as the one conceptualized in Figure 48. 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 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 its analog output voltage V₅, causing the integrators to progress in the opposite direction while forcing the value of the integrator output to track the average value of the input.

The modulator shifts the quantization noise to high frequencies; see Figure 49. Therefore, use a low-pass digital filter at the output of the device to increase 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 family TMS320F2837x offers a suitable programmable, hardwired filter structure termed a sigma-delta filter module (SDFM) optimized for usage with the AMC1305 family. Also, SD24_B converters on the MSP430F677x microcontrollers offer a path to directly access the integrated sinc-filters, thus offering a 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 digital filter.