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參數(shù)資料
| 型號： | OPA2658UB |
| 元件分類： | 運算放大器 |
| 英文描述： | Dual Wideband, Low Power, Current Feedback OPERATIONAL AMPLIFIER |
| 中文描述： | 雙寬帶，低功耗，電流反饋運算放大器 |
| 文件頁數(shù)： | 9/13頁 |
| 文件大小： | 230K |
| 代理商： | OPA2658UB |
9
OPA2658
The feedback resistor value acts as the frequency response
compensation element for a current feedback type amplifier.
The 402
used in setting the specification achieves a nomi-
nal maximally flat Butterworth response while assuming a
2pF output pin parasitic. Increasing the feedback resistor
will over compensate the amplifier, rolling off the frequency
response, while decreasing it will decrease phase margin,
peaking up the frequency response. Note that a non-invert-
ing, unity gain buffer application still requires a feedback
resistor for stability (560
for SO-8, 402
for PDIP and
560
for MSOP-8).
d) Connections to other wideband devices
on the board
may be made with short direct traces or through on-board
transmission lines. For short connections, consider the trace
and the input to the next device as a lumped capacitive load.
Relatively wide traces (50 to 100 mils) should be used,
preferably with ground and power planes opened up around
them. Estimate the total capacitive load and set R
ISO
from
the plot of recommended R
ISO
vs capacitive load. Low
parasitic loads may not need an R
ISO
since the OPA2658 is
nominally compensated to operate with a 2pF parasitic load.
If a long trace is required and the 6dB signal loss intrinsic to
doubly terminated transmission lines is acceptable, imple-
ment a matched impedance transmission line using microstrip
or stripline techniques (consult an ECL design handbook for
microstrip and stripline layout techniques). A 50
environ-
ment is not necessary on board, and in fact a higher imped-
ance environment will improve distortion as shown in the
distortion vs load plot. With a characteristic impedance
defined based on board material and desired trace dimen-
sions, a matching series resistor into the trace from the
output of the amplifier is used as well as a terminating shunt
resistor at the input of the destination device. Remember
also that the terminating impedance will be the parallel
combination of the shunt resistor and the input impedance of
the destination device; the total effective impedance should
match the trace impedance. Multiple destination devices are
best handled as separate transmission lines, each with their
own series and shunt terminations.
If the 6dB attenuation loss of a doubly terminated line is
unacceptable, a long trace can be series-terminated at the
source end only. This will help isolate the line capacitance
from the op amp output, but will not preserve signal integrity
as well as a doubly terminated line. If the shunt impedance
at the destination end is finite, there will be some signal
attenuation due to the voltage divider formed by the series
and shunt impedances.
e) Socketing a high speed part like the OPA2658 is not
recommended.
The additional lead length and pin-to-pin
capacitance introduced by the socket creates an extremely
troublesome parasitic network which can make it almost
impossible to achieve a smooth, stable response. Best results
are obtained by soldering the part onto the board. If socket-
ing for the DIP package is desired, high frequency flush
mount pins (e.g., McKenzie Technology #710C) can give
good results.
The effective noise at the output, generated by the op amp,
can be determined by taking the root sum of the squares of
equation (4) and applying the spectral noise values found in
the Typical Performance Curve graph section. This applies to
noise from the op amp only. Note that both the noise figure
(NF) and the equivalent input offset voltages improve as the
closed loop gain increases (by keeping R
FB
fixed and reduc-
ing R
FF
with R
N
= 0
).
INCREASING BANDWIDTH AT HIGH GAINS
The closed-loop bandwidth can be extended at high gains by
reducing the value of the feedback resistor R
FB
(see Equation
3). This bandwidth reduction is caused by the feedback
current being split between R
S
and R
FF
(refer to Figure 1).
As the gain increases (for a fixed R
FB
), more feedback
current is shunted through R
FF
, which reduces closed-loop
bandwidth.
CIRCUIT LAYOUT AND BASIC OPERATION
Achieving optimum performance with a high frequency am-
plifier like the OPA2658 requires careful attention to layout
parasitics and selection of external components. Recommen-
dations for PC board layout and component selection include:
a) Minimize parasitic capacitance
to any ac ground for all
of the signal I/O pins. Parasitic capacitance on the output
and inverting input pins can cause instability; on the non-
inverting input it can react with the source impedance to
cause unintentional bandlimiting. To reduce unwanted ca-
pacitance, a window around the signal I/O pins should be
opened in all of the ground and power planes. Otherwise,
ground and power planes should be unbroken elsewhere on
the board.
b) Minimize the distance
(< 0.25") from the two power pins
to high frequency 0.1
μ
F decoupling capacitors. At the pins,
the ground and power plane layout should not be in close
proximity to the signal I/O pins. Avoid narrow power and
ground traces to minimize inductance between the pins and
the decoupling capacitors. Larger (2.2
μ
F to 6.8
μ
F) decoupling
capacitors, effective at lower frequencies, should also be
used. These may be placed somewhat farther from the
device and may be shared among several devices in the same
area of the PC board.
c) Careful selection and placement of external compo-
nents will preserve the high frequency performance of the
OPA2658
. Resistors should be a very low reactance type.
Surface mount resistors work best and allow a tighter overall
layout. Metal film or carbon composition axially-leaded
resistors can also provide good high frequency performance.
Again, keep their leads as short as possible. Never use
wirewound type resistors in a high frequency application.
Since the output pin and the inverting input pin are most
sensitive to parasitic capacitance, always position the feed-
back and series output resistor, if any, as close as possible to
the package pins. Other network components, such as non-
inverting input termination resistors, should also be placed
close to the package.
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
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| OPA2662AU | 功能描述:高速運算放大器 Dual Wide Band RoHS:否 制造商:Texas Instruments 通道數(shù)量:1 電壓增益 dB:116 dB 輸入補償電壓:0.5 mV 轉(zhuǎn)換速度:55 V/us 工作電源電壓:36 V 電源電流:7.5 mA 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Tube |
| OPA2670IRGVR | 功能描述:高速運算放大器 Sgl PortHi Out Crnt VDSL2 Line Driver RoHS:否 制造商:Texas Instruments 通道數(shù)量:1 電壓增益 dB:116 dB 輸入補償電壓:0.5 mV 轉(zhuǎn)換速度:55 V/us 工作電源電壓:36 V 電源電流:7.5 mA 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Tube |
| OPA2670IRGVT | 功能描述:高速運算放大器 Sgl PortHi Out Crnt VDSL2 Line Driver RoHS:否 制造商:Texas Instruments 通道數(shù)量:1 電壓增益 dB:116 dB 輸入補償電壓:0.5 mV 轉(zhuǎn)換速度:55 V/us 工作電源電壓:36 V 電源電流:7.5 mA 最大工作溫度:+ 85 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:SOIC-8 封裝:Tube |
| OPA2670IRGVT | 制造商:Texas Instruments 功能描述:IC OP-AMP 120MHZ 5000V/ VQFN-16 制造商:Texas Instruments 功能描述:IC, OP-AMP, 120MHZ, 5000V/ us, VQFN-16 |
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