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5962-9755701HXA Datasheet(PDF) 15 Page - Agilent(Hewlett-Packard) |
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5962-9755701HXA Datasheet(HTML) 15 Page - Agilent(Hewlett-Packard) |
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15 / 16 page  15 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 rejection (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 coupling 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-7850. Using surface mount components can help achieve many of the PCB objectives discussed in the preceding paragraphs. An example through- hole PCB layout illustrating some of the more important layout recommendations is shown in Figures 26 and 27. See Applications Note 1078, Designing with Agilent Technologies Isolation Amplifiers , for more information on PCB layout consideration. Post-Amplifier Circuit The recommended application circuit (Figure 24) 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 to help filter output noise. The particular op-amp used in the post-amp is not critical; however, it should have low enough offset and high enough bandwidth and slew rate so that it does not adversely affect circuit performance. The offset of the op-amp should be low relative to the output offset of the HCPL- 7850, or less than about 5 mV. To maintain overall circuit bandwidth, the post-amplifier circuit should have a bandwidth at least twice the minimum bandwidth of the isolation amplifier, or about 200 kHz. To obtain a bandwidth of 200 kHz with a gain of 5, the op-amp should have a gain-bandwidth greater than 1 mHz. The post- amplifier circuit includes a pair of capacitors (C5 and C6) that form a single-pole low-pass filter. These capacitors allow the bandwidth of the post-amp to be adjusted independently of the gain and are useful for reducing the output noise from the isolation amplifier (doubling the capacitor values halves the circuit bandwidth). The component values shown in Figure 24 form a differential amplifier with a gain of 5 and a cutoff frequency of approximately 100 kHz, and were chosen as a compromise between low noise and fast response times. The overall recommended application circuit has a bandwidth of 66 kHz, a rise time of 5.2 µs and a delay to 90% of 8.5 µs. The gain-setting resistors in the post-amp should have a tolerance of 1% or better to ensure adequate CMRR and gain tolerance for the overall circuit. Resistor networks with even better ratio tolerances can be used which offer better performance, as well as reducing the total component count and board space. The post-amplifier circuit can be easily modified to allow for single-supply operation. Figure 25 shows a schematic for a post amplifier for use in 5 V single supply applications. One additional resistor is needed and the gain is decreased to 1 to allow circuit operation over the full input voltage range. See Application Note 1078, Designing with Agilent Technologies Isolation Amplifiers , for more information on the post-amplifier circuit. Other Information As mentioned above, reducing the bandwidth of the post amplifier circuit reduces the amount of output noise. Figure 22 shows how the output noise changes as a function of the post-amplifier bandwidth. The post-amplifier circuit exhibits a first-order low- pass filter characteristic. For the same filter bandwidth, a higher- order filter can achieve even better attenuation of modulation noise due to the second-order noise shaping of the sigma-delta modulator. For more information on the noise characteristics of the HCPL-7850, see Application Note 1078, Designing with Agilent Technologies Isolation Amplifiers . The HCPL-7850 can also be used to isolate signals with amplitudes larger than its recommended input range through the use of a resistive voltage divider at its input. The only restrictions are that the impedance of the divider be relatively small (less than 1 K Ω so that the input resistance (480 K Ω) and input bias current (0.6 A) do not affect the accuracy of the measurement. An input bypass capacitor is still required, although the 68 Ω series damping resistor is not. (The resistance of the voltage divider provides the same function.) The low pass filter formed by the divider resistance and the input bypass capacitor may limit the achievable bandwidth. |
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