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5962-8964601PA 参数 Datasheet PDF下载

5962-8964601PA图片预览
型号: 5962-8964601PA
PDF下载: 下载PDF文件 查看货源
内容描述: 450 V / us的精密电流反馈运算放大器 [450 V/us, Precision, Current-Feedback Op Amp]
分类和应用: 运算放大器放大器电路
文件页数/大小: 12 页 / 513 K
品牌: ADI [ ADI ]
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AD846  
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 37. Overload Recovery Test Circuit  
Figure 35. Recommended Power Supply Bypassing  
THEORY OF OPERATION  
Figure 38. Overload Recovery Time Photo  
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.  
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. 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 kfeedback resistor, this error is one part in 200,000.  
For a transimpedance amplifier with 1 Mtransimpedance, 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.  
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  
C
COMP is buffered by the output stage, consisting of transistors  
Q9–Q12.  
–8–  
REV. C