Some current sensing applications in the metering and power conversion space require DC current sensing alongside the AC current sensing. The DC often is an unwanted or false signal content which needs to be detected, measured, and removed. For instance, in AC power systems, DC energy dissipates in a transformer as heat because DC energy cannot be transferred across the transformer. This energy dissipation creates power losses and thermal problems. Current sensing in such applications can be realized by a shunt-based approach measuring the voltage drop across a small value sense resistor. DC signals within the AC are very small and challenge the dynamic range of a system. This application note introduces a preconditioning filter circuit to relax those requirements and improve the DC in AC measurement.
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When measuring current, often shunt-based designs are used. In grid systems, the AC content is 50/60 Hz based and contains the fundamental plus harmonic distortion. For example, in power quality analysis or DC/AC power conversion, the harmonics are analyzed individually to monitor and minimize distortion. The AC carries the main power but an unwanted DC content can be present in addition which must be eliminated by the control loop or detected as an error. Therefore, measuring the DC value at the same time is important. For example, a DC/AC system can generate a 20-A AC current sine wave. At the same time, a 20-mA DC current must be measured. This requirement increases the dynamic range of the system and challenges the analog to digital conversion. Furthermore, the voltage across the shunt can be small such that an external gain stage can be beneficial.