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AD8436ACPZ-RL Fiches technique(PDF) 11 Page - Analog Devices |
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AD8436ACPZ-RL Fiches technique(HTML) 11 Page - Analog Devices |
11 / 24 page Data Sheet AD8436 Rev. B | Page 11 of 24 The 16 kΩ resistor in the output converts the output current to a dc voltage that can be connected to the output buffer or to the circuit that follows. The output appears as a voltage source in series with 16 kΩ. If a current output is desired, the resistor connection to ground is left open and the output current is applied to a subsequent circuit, such as the summing node of a current summing amplifier. Thus, the core has both current and voltage outputs, depending on the configuration. For a voltage output with 0 Ω source impedance, use the output buffer. The offset voltage of the buffer is 25 μV or 50 μV, depending on the grade. FET Input Buffer Because the V-to-I input resistor value of the AD8436 rms core is 8 kΩ, a high input impedance buffer is often used between rms-dc converters and finite impedance sources. The optional JFET input op amp minimizes attenuation and uncouples common input amenities, such as resistive voltage dividers or resistors used to terminate current transformers. The wide bandwidth of the FET buffer is well matched to the rms core bandwidth so that no information is lost due to serial band- width effects. Although the input buffer consumes little current, the buffer supply is independently accessible and can be disconnected to reduce power. Optional matched 10 kΩ input and feedback resistors are provided on chip. Consult the Applications Information section to learn how these resistors can be used. The 3 dB bandwidth of the input buffer is 2.7 MHz at 10 mV rms input and approximately 1.5 MHz at 1 V rms. The amplifier gain and bandwidth are sufficient for applications requiring modest gain or response enhancement to a few hundred kilohertz (kHz), if desired. Configurations of the input buffer are discussed in the Applications Information section. Precision Output Buffer The precision output buffer is a bipolar input amplifier, laser trimmed to cancel input offset voltage errors. As with the input buffer, the supply current is very low (<50 μA, typically), and the power can be disconnected for power savings if the buffer is not needed. Be sure that the noninverting input is also disconnected from the core output (OUT) if the buffer supply pin is discon- nected. Although the input current of the buffer is very low, a laser-trimmed 16 kΩ resistor, connected in series with the inverting input, offsets any self-bias offset voltage. The output buffer can be configured as a single or two-pole low- pass filter using circuits shown in the Applications Information section. Residual output ripple is reduced, without affecting the converted dc output. As the response approaches the low frequency end of the bandwidth, the ripple rises, dependent on the value of the averaging capacitor. Figure 27 shows the effects of four combinations of averaging and filter capacitors. Although the filter capacitor reduces the ripple for any given frequency, the dc error is unaffected. Of course, a larger value averaging capacitor can be selected, at a larger cost. The advantage of using a low-pass filter is that a small value of filter capacitor, in conjunction with the 16 kΩ output resistor, reduces ripple and permits a smaller averaging capacitor, effecting a cost savings. The recommended capacitor values for operation to 40 Hz are 10 µF for averaging and 3.3 µF for filter. Dynamic Range The AD8436 is a translinear rms-to-dc converter with exceptional dynamic range. Although accuracy varies slightly more at the extreme input values, the device still converts with no spurious noise or dropout. Figure 25 is a plot of the rms/dc transfer function near zero voltage. Unlike processor or other solutions, residual errors at very low input levels can be disregarded for most applications. INPUT VOLTAGE (mV DC) 30 20 10 0 0 30 20 10 –10 –20 –30 ΔΣ OR OTHER DIGITAL SOLUTIONS CANNOT WORK AT ZERO VOLTS AD8436 SOLUTION Figure 25. DC Transfer Function near Zero |
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