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OPA2652UG4 参数 Datasheet PDF下载

OPA2652UG4图片预览
型号: OPA2652UG4
PDF下载: 下载PDF文件 查看货源
内容描述: 双通道, 700MHz的,电压反馈运算放大器 [Dual, 700MHz, Voltage-Feedback OPERATIONAL AMPLIFIER]
分类和应用: 运算放大器光电二极管
文件页数/大小: 20 页 / 508 K
品牌: BB [ BURR-BROWN CORPORATION ]
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OPA2652  
www.ti.com  
SBOS125AJUNE 2000REVISED MAY 2006  
OPERATING SUGGESTIONS  
Optimizing Resistor Values  
causes the phase margin to approach 90° and the  
bandwidth to more closely approach the predicted  
value of (GBP/NG). At a gain of +5, the 45MHz  
bandwidth shown in the Electrical Characteristics is  
close to that predicted using this simple formula.  
Because the OPA2652 is a unity gain stable voltage  
feedback op amp, a wide range of resistor values  
may be used for the feedback and gain setting  
resistors. The primary limits on these values are set  
by dynamic range (noise and distortion) and parasitic  
capacitance considerations. For a noninverting unity  
gain follower application, the feedback connection  
should be made with a 25resistor, not a direct  
short. This configuration isolates the inverting input  
capacitance from the output pin and improves the  
frequency response flatness. Usually, the feedback  
resistor value should be between 200and 1.5k.  
Below 200, the feedback network presents  
additional output loading that can degrade the  
harmonic distortion performance of the OPA2652.  
Above 1.5k, the typical parasitic capacitance  
(approximately 0.2pF) across the feedback resistor  
may cause unintentional bandlimiting in the amplifier  
response.  
Inverting Amplifier Operation  
Because the OPA2652 is  
a
general-purpose,  
wideband voltage feedback op amp, all of the  
familiar op amp application circuits are available to  
the designer. Inverting operation is one of the more  
common  
requirements  
and  
offers  
several  
performance benefits. Figure 29 shows a typical  
inverting configuration.  
In the inverting configuration, three key design  
consideration must be noted. First, the gain resistor  
(RG) becomes part of the signal channel input  
impedance. If input impedance matching is desired  
(which is beneficial whenever the signal is coupled  
through a cable, twisted pair, long PCB trace or other  
transmission line conductor), RG may be set equal to  
the required termination value and RF adjusted to  
give the desired gain. This approach is the simplest,  
and results in optimum bandwidth and noise  
performance. However, at low inverting gains, the  
resulting feedback resistor value can present a  
significant load to the amplifier output. For an  
inverting gain of –1, setting RG to 50for input  
matching eliminates the need for RM but requires a  
50W feedback resistor. This configuration has the  
interesting advantage that the noise gain becomes  
equal to 2 for a 50source impedance—the same  
as the noninverting circuits considered above.  
However, the amplifier output now sees the 50Ω  
feedback resistor in parallel with the external load. In  
general, the feedback resistor should be limited to  
the 200to 1.5krange. In this case, it is preferable  
to increase both the RF and RG values as shown in  
Figure 29, and then achieve the input matching  
impedance with a third resistor (RM) to ground. The  
total input impedance becomes the parallel  
combination of RG and RM.  
A good rule of thumb is to target the parallel  
combination of RF and RG (see Figure 28) to be less  
than approximately 300. The combined impedance  
RF || RG interacts with the inverting input  
capacitance, placing an additional pole in the  
feedback network, and thus a zero in the forward  
response. Assuming a 2pF total parasitic on the  
inverting node, holding RF || RG < 300keeps this  
pole above 250MHz. By itself, this constraint implies  
that the feedback resistor RF can increase to several  
kat high gains. This increase is acceptable as long  
as the pole formed by RF and any parasitic  
capacitance appearing in parallel is kept out of the  
frequency range of interest.  
Bandwidth vs Gain: Noninverting Operation  
Voltage feedback op amps exhibit decreasing  
closed-loop bandwidth as the signal gain is  
increased. In theory, this relationship is described by  
the Gain Bandwidth Product (GBP) shown in the  
specifications. Ideally, dividing GBP by the  
noninverting signal gain (also called the Noise Gain,  
or NG) predicts the closed-loop bandwidth. In  
practice, this prediction only holds true when the  
phase margin approaches 90°, as it does in high  
gain configurations. At low gains (increased  
feedback factor), most amplifiers exhibit a wider  
bandwidth and lower phase margin. The OPA2652 is  
The second major consideration, touched on in the  
previous paragraph, is that the signal source  
impedance becomes part of the noise gain equation  
and influences the bandwidth. For the example in  
Figure 29, the RM value combines in parallel with the  
external 50source impedance, yielding an effective  
driving impedance of 50|| 57.6= 26.8. This  
impedance is added in series with RG for calculating  
the noise gain (NG). The resulting NG is 1.94 for  
Figure 29 (an ideal source would cause NG = 2.00).  
compensated to give  
noninverting gain of  
a
flat response in  
a
1
(see Figure 28). This  
configuration results in a typical gain of +1 bandwidth  
of 700MHz, far exceeding that predicted by dividing  
the 200MHz GBP by NG = 1. Increasing the gain  
The third important consideration in inverting  
amplifier design is setting the bias current  
cancellation resistor on the noninverting input (RB). If  
this resistor is set equal to the total DC resistance  
looking out of the inverting node, the output DC  
13  
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