參數(shù)資料
型號(hào)： VCA2616
英文描述： Dual, Variable-Gain Amplifier with Low-Noise Preamp
中文描述： 雙路，可變?cè)鲆娣糯笃鞯牡驮肼暻爸梅糯笃?/td>
文件頁(yè)數(shù)： 14/19頁(yè)
文件大?。?/td> 389K
代理商： VCA2616
VCA2616, VCA2611
SBOS234E
14
www.ti.com
The capacitance that is determined in Equation 5 should be
added to the capacitance of Equation 4 to determine the
overall bandwidth of the LNP. The LNP
IN
NA (pin 12) and the
LNP
IN
NB (pin 25) should be bypassed to ground by the
shortest means possible to avoid any inductance in the lead.
LNP OUTPUT BUFFER
The differential LNP output is buffered by wideband class AB
voltage followers which are designed to drive low impedance
loads. This is necessary to maintain LNP gain accuracy,
since the VCA input exhibits gain-dependent input imped-
ance. The buffers are also useful when the LNP output is
brought out to drive external filters or other signal processing
circuitry. Good distortion performance is maintained with
buffer loads as low as 135
. As mentioned previously, the
buffer inputs are AC-coupled to the LNP outputs with a
3.6kHz high-pass characteristic, and the DC common-mode
level is maintained at the correct V
CM
for compatibility with
the VCA input.
VOLTAGE-CONTROLLED ATTENUATOR (VCA)
—
DETAIL
The VCA is designed to have a dB-linear attenuation charac-
teristic; that is, the gain loss in dB is constant for each equal
increment of the VCA
CNTL
control voltage. See Figure 1 for a
block diagram of the VCA. The attenuator is essentially a
variable voltage divider consisting of one series input resis-
tor, R
S
, and ten identical shunt FETs, placed in parallel and
controlled by sequentially activated clipping amplifiers. Each
clipping amplifier can be thought of as a specialized voltage
comparator with a soft transfer characteristic and well-con-
trolled output limit voltages. The reference voltages V1 through
V10 are equally spaced over the 0V to 3.0V control voltage
range. As the control voltage rises through the input range of
each clipping amplifier, the amplifier output will rise from 0V
(FET completely ON) to V
CM
–
V
T
(FET nearly OFF), where
V
CM
is the common source voltage and V
T
is the threshold
voltage of the FET. As each FET approaches its OFF state
and the control voltage continues to rise, the next clipping
amplifier/FET combination takes over for the next portion of
the piecewise-linear attenuation characteristic. Thus, low
control voltages have most of the FETs turned ON, while
high control voltages have most turned OFF. Each FET acts
to decrease the shunt resistance of the voltage divider
formed by R
S
and the parallel FET network.
The attenuator is comprised of two sections, with five parallel
clipping amplifier/FET combinations in each. Special refer-
ence circuitry is provided so that the (V
CM
–
V
T
) limit voltage
will track temperature and IC process variations, minimizing
the effects on the attenuator control characteristic.
In addition to the analog VCA
CNTL
gain setting input, the
attenuator architecture provides digitally programmable ad-
justment in eight steps, via the three MGS bits. These adjust
the maximum achievable gain (corresponding to minimum
attenuation in the VCA, with VCA
CNTL
= 3.0V) in 3dB incre-
ments. This function is accomplished by providing multiple
FET sub-elements for each of the Q
1
to Q
10
FET shunt
elements (see Figure 12). In the simplified diagram of
Figure 13, each shunt FET is shown as two sub-elements,
Q
NA
and Q
NB
. Selector switches, driven by the MGS bits,
activate either or both of the sub-element FETs to adjust the
maximum R
ON
and thus achieve the stepped attenuation
options.
The VCA can be used to process either differential or single-
ended signals. Fully differential operation will reduce 2nd-
harmonic distortion by about 10dB for full-scale signals.
Input impedance of the VCA will vary with gain setting, due
to the changing resistances of the programmable voltage
divider structure. At large attenuation factors (that is, low gain
settings), the impedance will approach the series resistor
value of approximately 135
.
As with the LNP stage, the VCA output is AC-coupled into the
PGA. This means that the attenuation-dependent DC com-
mon-mode voltage will not propagate into the PGA, and so
the PGA
’
s DC output level will remain constant.
Finally, note that the VCA
CNTL
input consists of FET gate
inputs. This provides very high impedance and ensures that
multiple VCA2616 and VCA2611 devices may be connected
in parallel with no significant loading effects. The nominal
voltage range for the VCA
CNTL
input spans from 0V to 3V.
Overdriving this input (
≤
5V) does not affect the performance.
INPUT OVERLOAD RECOVERY
One of the most important applications for the VCA2616 and
VCA2611 is processing signals in an ultrasound system. The
ultrasound signal flow begins when a large signal is applied to
a transducer, which converts electrical energy to acoustic
energy. It is not uncommon for the amplitude of the electrical
signal that is applied to the transducer to be
±
50V or greater.
FIGURE 13. Programmable Attenuator Section.
R
S
Q
1A
A1
B1
B2
VCM
INPUT
OUTPUT
Programmable Attenuator Section
Q
1B
Q
2A
A2
Q
2B
Q
3A
A3
Q
3B
Q
4A
A4
Q
4B
Q
5A
A5
Q
5B
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相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
VCA2616EVM 功能描述:放大器 IC 開(kāi)發(fā)工具 VCA2616 Eval Mod RoHS:否 制造商:International Rectifier 產(chǎn)品:Demonstration Boards 類型:Power Amplifiers 工具用于評(píng)估:IR4302 工作電源電壓:13 V to 23 V
VCA2616YR 功能描述:特殊用途放大器 Dual Variable Gain 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
VCA2616YT 功能描述:特殊用途放大器 Dual Variable Gain 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
VCA2617 制造商:BB 制造商全稱:BB 功能描述:Dual, Variable Gain Amplifier
VCA2617EVM 功能描述:放大器 IC 開(kāi)發(fā)工具 VCA2617 Eval Mod RoHS:否 制造商:International Rectifier 產(chǎn)品:Demonstration Boards 類型:Power Amplifiers 工具用于評(píng)估:IR4302 工作電源電壓:13 V to 23 V
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