Novel Analog-to-Analog Isolator Using an Isolated Sigma-Delta Modulator, Isolated DC-to-DC Converter, and Active Filter (CN0185)
The circuit shown in Figure 1 is a complete low cost implementation of an analog-to-analog isolator. The circuit provides isolation of 2500 V rms (1 minute per UL 1577).
The circuit is based on the AD7400A, a second-order, sigma-delta (Σ-Δ) modulator with a digitally isolated 1-bit data stream output. The isolated analog signal is recovered with a fourth-order active filter based on the dual, low noise, rail-to-rail AD8646 op amp. With the ADuM5000 as the power supply for the isolated side, the two sides are completely isolated and use only one power supply for the system. The circuit has 0.05% linearity and benefits from the noise shaping provided by the modulator of the AD7400A and the analog filter. The applications of the circuit include motor control and shunt current monitoring, and the circuit is also a good alternative to isolation systems based on optoisolators.
Figure 1. Analog Isolator using AD7400A (Simplified Schematic: All Connections and Decoupling Not Shown)
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