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參數(shù)資料
| 型號(hào)： | LMH6504 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Wideband, Low Power, Variable Gain Amplifier |
| 中文描述： | 寬帶，低功耗，可變?cè)鲆娣糯笃?/td> |
| 文件頁數(shù)： | 15/19頁 |
| 文件大?。?/td> | 759K |
| 代理商： | LMH6504 |
Application Information
(Continued)
trim pot R
4
to null the offset voltage at the output. This will
eliminate the output stage offsets. Next set V
G
to 2V and
adjust the trim pot R
1
to null the offset voltage at the output.
This will eliminate the input stage offsets.
DIGITAL GAIN CONTROL
Digitally variable gain control can be easily realized by driv-
ing the LMH6504’s gain control input with a digital-to-analog
converter (DAC).
Figure 7
illustrates such an application.
This circuit employs National Semiconductor’s eight-bit
DAC0830, the LMC8101 MOS input op-amp (Rail-to-Rail
Input/Output), and the LMH6504 VGA. With V
REF
set to 2V,
the circuit provides up to 80 dB of gain control in 256 steps
with up to 0.05% full scale resolution. The maximum gain of
this circuit is 20 dB.
USING THE LMH6504 IN AGC APPLICATIONS
In AGC applications, the control loop forces the LMH6504 to
have a fixed output amplitude. The input amplitude will vary
over a wide range and this can be the issue that limits
dynamic range. At high input amplitudes, the distortion due
to the input buffer driving R
G
may exceed that which is
produced by the output amplifier driving the load. In the plot,
Distortion vs. Gain, total harmonic distortion (THD) is plotted
over a gain range of nearly 35 dB for a fixed output amplitude
of 0.25 V
in the specified configuration, R
= 1k, R
=
100
. When the gain is adjusted to -15 dB (i.e. 35 dB down
from A
), the input amplitude would be 1.41 V
and we
can see the distortion is at its worst at this gain. If the output
amplitude of the AGC were to be raised above 0.25 V
, the
input amplitudes for gains 40 dB down from A
would be
even higher and the distortion would degrade further. It is for
this reason that we recommend lower output amplitudes if
wide gain ranges are desired. Using a post-amp like the
LMH6714/ 6720/ 6722 family or LMH6702 would be the best
way to preserve dynamic range and yield output amplitudes
much higher than 100 mV
. Another way of addressing
distortion performance and its limitations on dynamic range,
would be to raise the value of R
. Just like any other high-
speed amplifier, by increasing the load resistance, and
therefore decreasing the demanded load current, the distor-
tion performance will be improved in most cases. With an
increased R
G
, R
F
will also have to be increased to keep the
same A
VMAX
and this will decrease the overall bandwidth. It
may be possible to insert a series RC combination across R
F
in order to counteract the negative effect on BW when a
large R
F
is used.
AUTOMATIC GAIN CONTROL (AGC) #1
Fast Response AGC Loop
The AGC circuit shown in
Figure 8
will correct a 6 dB input
amplitude step in 100 ns. The circuit includes a two op-amp
precision rectifier amplitude detector (U1 and U2), and an
integrator (U3) to provide high loop gain at low frequencies.
The output amplitude is set by R
. Some notes on building
fast AGC loops: Precision rectifiers work best with large
output signals. Accuracy is improved by blocking DC offsets,
as shown in
Figure 8
.
20084357
FIGURE 6. OFFSET ADJUST CIRCUIT
20084358
FIGURE 7. Digital Gain Control
L
www.national.com
15
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LMH6504MA | 功能描述:特殊用途放大器 RoHS:否 制造商:Texas Instruments 通道數(shù)量:Single 共模抑制比（最小值）: 輸入補(bǔ)償電壓: 工作電源電壓:3 V to 5.5 V 電源電流:5 mA 最大功率耗散: 最大工作溫度:+ 70 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-20 封裝:Reel |
| LMH6504MA/NOPB | 功能描述:特殊用途放大器 WB,Low Pwr,Variable Gain Amp RoHS:否 制造商:Texas Instruments 通道數(shù)量:Single 共模抑制比（最小值）: 輸入補(bǔ)償電壓: 工作電源電壓:3 V to 5.5 V 電源電流:5 mA 最大功率耗散: 最大工作溫度:+ 70 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-20 封裝:Reel |
| LMH6504MAX | 功能描述:特殊用途放大器 RoHS:否 制造商:Texas Instruments 通道數(shù)量:Single 共模抑制比（最小值）: 輸入補(bǔ)償電壓: 工作電源電壓:3 V to 5.5 V 電源電流:5 mA 最大功率耗散: 最大工作溫度:+ 70 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-20 封裝:Reel |
| LMH6504MAX/E7000960 | 制造商:Texas Instruments 功能描述: |
| LMH6504MAX/NOPB | 功能描述:特殊用途放大器 RoHS:否 制造商:Texas Instruments 通道數(shù)量:Single 共模抑制比（最小值）: 輸入補(bǔ)償電壓: 工作電源電壓:3 V to 5.5 V 電源電流:5 mA 最大功率耗散: 最大工作溫度:+ 70 C 最小工作溫度:- 40 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:QFN-20 封裝:Reel |
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