12/7/2023 0 Comments Analog signal isolatorIn order not to overdrive the AD8646, the input signal must be lower than the power supply (5 V). Ensure that the PCB layout meets the emissions standards and isolation requirements between the two isolated sides (see the AN-0971 Application Note). Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see the MT-031 Tutorial, Grounding Data Converters and Solving the Mystery of "AGND" and "DGND," the MT-101 Tutorial, Decoupling Techniques, and the ADuC7060/ADuC7061 evaluation board layout). The circuit must be constructed on a multilayer printed circuit board (PCB) with a large area ground plane. The isoPower ® technology of the ADuM5000 uses high frequency switching elements to transfer power through a chip-scale transformer. It is used for the power supply to the isolated side of the circuit containing the AD7400A. The ADuM5000 is an isolated dc-to-dc converter based on Analog Devices, Inc., iCoupler ® technology. The filter is implemented using the dual AD8646, a rail-to-rail, input and output, low noise, single-supply op amp. Compared to other filter responses (Butterworth or Bessel), the response of the Chebyshev provides the steepest roll-off for a given filter order. A fourth-order Chebyshev filter is used for better noise attenuation. The signal is then filtered by an active filter whose order is higher than the order of the modulator. To reconstruct the analog input signal, follow the data stream by an ADG849 switch connected to a 3 V ADR443 reference to stabilize the peak-to-peak output of the MDAT. The quantization noise is shaped by a second-order Σ-Δ modulator, which shifts the noise to the higher frequencies (see the MT-022 Tutorial). ![]() The output of the AD7400A is an isolated 1-bit data stream. The 22 Ω resistors and 0.1 μF capacitor form a differential input noise reduction filter with a cutoff frequency of 145 kHz. ![]() The analog input is sampled at 10 MSPS by the AD7400A Σ-Δ modulator. A block diagram of the circuit is shown in Figure 1.
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