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.

Figure 48. Block Diagram of a Second-Order Modulator

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.

Figure 49. Quantization Noise Shaping