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AD846 데이터시트(PDF) 8 Page - Analog Devices |
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AD846 데이터시트(HTML) 8 Page - Analog Devices |
8 / 12 page REV. C AD846 –8– POWER SUPPLY CONSIDERATIONS The power supply connections to the AD846 must maintain a low impedance to ground over a bandwidth of 40 MHz or more. This is especially important when driving a significant resistive or capacitive load, since all current delivered to the load comes from the power supplies. Multiple high quality bypass capacitors are recommended for each power supply line in any critical application. A 0.1 µF ceramic and a 2.2 µF electrolytic capacitor as shown in Figure 35 placed as close as possible to the am- plifier (with short lead lengths to power supply common) will assure adequate high frequency bypassing, in most applications. A minimum bypass capacitance of 0.1 µF should be used for any application. Figure 35. Recommended Power Supply Bypassing THEORY OF OPERATION The AD846 differs from conventional operational amplifiers in that it is a transimpedance device rather than a conventional voltage amplifier. Figure 36 is a simplified schematic of the AD846. The input stage consists of a pair of transistors, Q1 and Q2, which are biased by two diode-connected transistors, Q3 and Q4. Transistors Q1 and Q2 have their emitters connected together, and this common point functions as the inverting in- put of the amplifier. Correspondingly, the common connection of the two biasing diodes acts as the noninverting input. Figure 36. AD846 Simplified Schematic When operated as a closed-loop amplifier, feedback error cur- rent, IIN: flows into the inverting input terminal and is conveyed via current mirrors (transistors Q5, Q6, Q7, and Q8) to the compensation capacitor, CCOMP. The voltage developed across CCOMP is buffered by the output stage, consisting of transistors Q9–Q12. Figure 37. Overload Recovery Test Circuit Figure 38. Overload Recovery Time Photo Because the input error signal developed is in the form of a current, not a voltage, the AD846 differs from conventional operational amplifiers. This also means that, unlike most opera- tional amplifiers which rely on negative feedback to produce a “virtual ground” at the inverting input terminal, this terminal explicitly has a low impedance. A unique circuit approach allows the AD846 to realize an open- loop transimpedance of close to 200 M Ω. This is nearly three orders of magnitude greater than that of any other operational transimpedance amplifier and results in extremely high levels of dc precision. As an example, the output voltage gain error is approximately equal to the value of the feedback resistor divided by the value of the open-loop transimpedance of the amplifier. That is, when using a 1 k Ω feedback resistor, this error is one part in 200,000. For a transimpedance amplifier with 1 M Ω transimpedance, this error is only one part in 1000; such an amplifier would barely be able to achieve 10-bit precision. Figure 39 is a simplified three-terminal model for the AD846. Figure 40 is a simplified three-terminal model for a conventional voltage op amp. The action of current feedback serves to modify the behavior of the amplifier under closed-loop conditions. The feedback resistor, RF, is somewhat analogous to the input stage transconductance of a conventional voltage amplifier; and therefore, if the value of RF is held constant, the closed-loop bandwidth also remains virtually constant, independent of closed-loop voltage gain. |
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