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5962-9755701HPA Datasheet(PDF) 14 Page - Agilent(Hewlett-Packard) |
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5962-9755701HPA Datasheet(HTML) 14 Page - Agilent(Hewlett-Packard) |
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14 / 16 page  14 inductor can be used in series with the input of the regulator to form a low-pass filter with the regulator’s input bypass capacitor. As shown in Figure 24, a 0.1 µF bypass capacitor (C2, C4) should be located as close as possible to the input and output power supply pins of the HCPL-7850. The bypass capacitors are required because of the high- speed digital nature of the signals inside the isolation amplifier. A 0.01 µF bypass capacitor (C3) is also recommended at the input pin(s) due to the switched- capacitor nature of the input circuit. The input bypass capacitor should be at least 1000 pF to maintain gain accuracy of the isolation amplifier. Inductive coupling between the input power-supply capacitor and the input circuit, including the input bypass capacitor and the input leads of the HCPL-7850, can introduce additional DC offset in the circuit. Several steps can be taken to minimize the mutual coupling between the two parts of the circuit, thereby improving the offset performance of the design. Separate the two bypass capacitors C2 and C3 as much as possible (even putting them on opposite sides of the PC board), while keeping the total lead lengths, including traces, of each bypass capacitor less than 20 mm. PC board traces should be made as short as possible and placed close together or over ground plane to minimize loop area and pickup of stray magnetic fields. Avoid using sockets, as they will typically increase both loop area and inductance. And finally, using capacitors with small body size and orienting them perpendicular to each other on the PC board can also help. For more information concerning this effect, see Application Note 1078, Designing with Agilent Technologies Isolation Amplifiers . Shunt Resistor Selections The current-sensing shunt resistor should have low resistance (to minimize power dissipation), low inductance (to minimize di/dt induced voltage spikes which could adversely affect operation), and reasonable tolerance (to maintain overall circuit accuracy). The value of the shunt should be chosen as a compromise between minimizing power dissipation by making the shunt resistance smaller and improving circuit accuracy by making it larger and utilizing the full input range of the HCPL- 7850. Agilent Technologies recommends four different shunts which can be used to sense average currents in motor drives up to 35 A and 35 hp. Table 1 shows the maximum current and horsepower range for each of the LVR-series shunts from Dale. Even higher currents can be sensed with lower value shunts available from vendors such as Dale, IRC, and Isotek (Isabellenhuette). When sensing currents large enough to cause significant heating of the shunt, the temperature coefficient of the shunt can introduce nonlinearity due to the signal dependent temperature rise of the shunt. Using a heat sink for the shunt or using a shunt with a lower tempco can help minimize this effect. The Application Note 1078, Designing with Agilent Technologies Isolation Amplifiers , contains additional information on designing with current shunts. The recommended method for connecting the isolation amplifier to the shunt resistor is shown in Figure 24. Pin 2 (VIN+) is connected to the positive terminal of the shunt resistor, while pin 3 (VIN–) is shorted to pin 4 (GND1), with the power- supply return path functioning as the sense line to the negative terminal of the current shunt. This allows a single pair of wires or PC board traces to connect the isolation amplifier circuit to the shunt resistor. In some applications, however, supply currents flowing through the power-supply return path may cause offset or noise problems. In this case, better performance may be obtained by connecting pin 3 to the negative terminal of the shunt resistor separate from the power supply return path. When connected this way, both input pins should be bypassed. Whether two or three wires are used, it is recommended that twisted-pair wire or very close PC board traces be used to connect the current shunt to the isolation amplifier circuit to minimize electromagnetic interference to the sense signal. The 68 Ω resistor in series with the input lead forms a low-pass anti-aliasing filter with the input bypass capacitor with a 200 kHz bandwidth. The resistor performs another important function as well; it dampens any ringing which might be present in the circuit formed by the shunt, the input bypass capacitor, and the wires or traces connecting the two. Undampened ringing of the input circuit near the input sampling frequency can alias into the baseband producing what might appear to be noise at the output of the device. To be |
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