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參數資料
| 型號： | LMH6738 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Very Wideband, Low Distortion Triple Op Amp |
| 中文描述： | 非常寬帶，低失真三運算放大器 |
| 文件頁數： | 9/12頁 |
| 文件大小： | 432K |
| 代理商： | LMH6738 |
Application Section
(Continued)
/Gain. Thus for an inverting gain of 7 V/V and an optimal
value for R
F
the input impedance is equal to 50
. Using a
termination resistor this can be brought down to match a
25
source, however, a 150
source cannot be matched. To
match a 150
source would require using a 1050
feedback
resistor and would result in reduced bandwidth.
For more information see Application Note OA-13 which
describes the relationship between R
and closed-loop fre-
quency response for current feedback operational amplifiers.
The value for the inverting input impedance for the LMH6738
is approximately 30
. The LMH6738 is designed for opti-
mum performance at gains of +1 to +10 V/V and 1 to 9
V/V. Higher gain configurations are still useful, however, the
bandwidth will fall as gain is increased, much like a typical
voltage feedback amplifier.
ACTIVE FILTER
When using any current feedback Operational Amplifier as
an active filter it is necessary to be careful using reactive
components in the feedback loop. Reducing the feedback
impedance, especially at higher frequencies, will almost cer-
tainly cause stability problems. Likewise capacitance on the
inverting input should be avoided. See Application Notes
OA-7 and OA-26 for more information on Active Filter appli-
cations for Current Feedback Op Amps.
When using the LMH6738 as a low pass filter the value of R
F
can be substantially reduced from the value recommended
in the R
F
vs. Gain charts. The benefit of reducing R
F
is
increased gain at higher frequencies, which improves at-
tenuation in the stop band. Stability problems are avoided
because in the stop band additional device bandwidth is
used to cancel the input signal rather than amplify it. The
benefit of this change depends on the particulars of the
circuit design. With a high pass filter configuration reducing
R
will likely result in device instability and is not recom-
mended.
DRIVING CAPACITIVE LOADS
Capacitive output loading applications will benefit from the
use of a series output resistor R
OUT
.
Figure 5
shows the use
of a series output resistor, R
, to stabilize the amplifier
output under capacitive loading. Capacitive loads of 5 to 120
pF are the most critical, causing ringing, frequency response
peaking and possible oscillation. The charts “Suggested
R
vs. Cap Load” give a recommended value for selecting
a series output resistor for mitigating capacitive loads. The
values suggested in the charts are selected for .5 dB or less
of peaking in the frequency response. This gives a good
compromise between settling time and bandwidth. For appli-
cations where maximum frequency response is needed and
some peaking is tolerable, the value of R
OUT
can be reduced
slightly from the recommended values.
An alternative approach is to place Rout inside the feedback
loop as shown in
Figure 6
. This will preserve gain accuracy,
but will still limit maximum output voltage swing.
INVERTING INPUT PARASITIC CAPACITANCE
Parasitic capacitance is any capacitance in a circuit that was
not intentionally added. It comes about from electrical inter-
action between conductors. Parasitic capacitance can be
reduced but never entirely eliminated. Most parasitic capaci-
tances that cause problems are related to board layout or
lack of termination on transmission lines. Please see the
section on Layout Considerations for hints on reducing prob-
lems due to parasitic capacitances on board traces. Trans-
mission lines should be terminated in their characteristic
impedance at both ends.
High speed amplifiers are sensitive to capacitance between
the inverting input and ground or power supplies. This shows
up as gain peaking at high frequency. The capacitor raises
device gain at high frequencies by making R
G
appear
smaller. Capacitive output loading will exaggerate this effect.
In general, avoid introducing unnecessary parasitic capaci-
tance at both the inverting input and the output.
20097507
FIGURE 4. Typical Video Application
20097508
FIGURE 5. Decoupling Capacitive Loads
20097509
FIGURE 6. Series Output Resistor Inside
Feedback Loop
L
www.national.com
9
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相關代理商/技術參數 |
參數描述 |
|---|---|
| LMH6738MQ | 功能描述:運算放大器 - 運放 RoHS:否 制造商:STMicroelectronics 通道數量:4 共模抑制比（最小值）:63 dB 輸入補償電壓:1 mV 輸入偏流（最大值）:10 pA 工作電源電壓:2.7 V to 5.5 V 安裝風格:SMD/SMT 封裝 / 箱體:QFN-16 轉換速度:0.89 V/us 關閉:No 輸出電流:55 mA 最大工作溫度:+ 125 C 封裝:Reel |
| LMH6738MQ/NOPB | 功能描述:運算放大器 - 運放 Triple Video Op-Amp RoHS:否 制造商:STMicroelectronics 通道數量:4 共模抑制比（最小值）:63 dB 輸入補償電壓:1 mV 輸入偏流（最大值）:10 pA 工作電源電壓:2.7 V to 5.5 V 安裝風格:SMD/SMT 封裝 / 箱體:QFN-16 轉換速度:0.89 V/us 關閉:No 輸出電流:55 mA 最大工作溫度:+ 125 C 封裝:Reel |
| LMH6738MQX | 功能描述:運算放大器 - 運放 RoHS:否 制造商:STMicroelectronics 通道數量:4 共模抑制比（最小值）:63 dB 輸入補償電壓:1 mV 輸入偏流（最大值）:10 pA 工作電源電壓:2.7 V to 5.5 V 安裝風格:SMD/SMT 封裝 / 箱體:QFN-16 轉換速度:0.89 V/us 關閉:No 輸出電流:55 mA 最大工作溫度:+ 125 C 封裝:Reel |
| LMH6738MQX/NOPB | 功能描述:運算放大器 - 運放 RoHS:否 制造商:STMicroelectronics 通道數量:4 共模抑制比（最小值）:63 dB 輸入補償電壓:1 mV 輸入偏流（最大值）:10 pA 工作電源電壓:2.7 V to 5.5 V 安裝風格:SMD/SMT 封裝 / 箱體:QFN-16 轉換速度:0.89 V/us 關閉:No 輸出電流:55 mA 最大工作溫度:+ 125 C 封裝:Reel |
| LMH6739 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Very Wideband, Low Distortion Triple Video Buffer |
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