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HCPL7840 bảng dữ liệu(PDF) 10 Page - Agilent(Hewlett-Packard) |
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HCPL7840 bảng dữ liệu(HTML) 10 Page - Agilent(Hewlett-Packard) |
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10 / 12 page  1-257 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 Hewlett- Packard Isolation Amplifiers . Shunt Resistor Selection The current-sensing shunt resis- tor 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-7840. Hewlett-Packard 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 (Isabellen- huette). When sensing currents large enough to cause significant heating of the shunt, the tempera- ture 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, Design- ing with Hewlett-Packard 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 23. Pin 2 (VIN+) is con- nected 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 recom- mended 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. Undamped 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 effective, the damping resistor should be at least 39 Ω. PC Board Layout In addition to affecting offset, the layout of the PC board can also affect the common mode rejec- tion (CMR) performance of the isolation amplifier, due primarily to stray capacitive coupling between the input and the output circuits. To obtain optimal CMR performance, the layout of the printed circuit board (PCB) should minimize any stray coup- ling by maintaining the maximum possible distance between the input and output sides of the circuit and ensuring that any ground plane on the PCB does not pass directly below the HCPL-7840. Using surface mount components can help achieve many of the PCB objectives discussed in the preceding para- graphs. An example through-hole PCB layout illustrating some of the more important layout recommendations is shown in Figures 25 and 26. See Applica- tion Note 1078, Designing with Hewlett-Packard Isolation Amplifiers , for more information on PCB layout considerations. Post-Amplifier Circuit The recommended application circuit (Figure 23) includes a post-amplifier circuit that serves three functions: to reference the output signal to the desired level (usually ground), to amplify the signal to appropriate levels, and |
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