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Número de pieza VCA2616
Descripción (VCA2611 / VCA2616) Variable-Gain Amplifier
Fabricantes Burr-Brown 
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VCA2616VCA2616
VCA2611
SBOS234E MARCH 2002 REVISED NOVEMBER 2004
Dual, Variable-Gain Amplifier
with Low-Noise Preamp
FEATURES
q LOW-NOISE PREAMP:
– Low Input Noise: 0.95nV/Hz
Active Termination Noise Reduction
Switchable Termination Value
80MHz Bandwidth
5dB to 25dB Gain
Differential In and Out
q LOW-NOISE VARIABLE GAIN AMPLIFIER:
Low-Noise VCA
Up to 40dB Gain Range
40MHz Bandwidth
Differential In and Out
q LOW CROSSTALK: 66dB at Max Gain, 5MHz
q HIGH-SPEED VARIABLE GAIN ADJUST
q SWITCHABLE EXTERNAL PROCESSING
APPLICATIONS
q ULTRASOUND SYSTEMS
q WIRELESS RECEIVERS
q TEST EQUIPMENT
FBCNTL
LNPOUTN VCAINN VCACNTL
RF2 FBSW
RF1 FB
VCA2616
(1 of 2 Channels)
Analog
Control
Maximum Gain Select
MGS1 MGS2 MGS3
Maximum Gain
Select
DESCRIPTION
The VCA2616 and VCA2611 are dual, Low-Noise Preamplifiers
(LNP), plus low-noise Variable Gain Amplifiers (VGA). The
VCA2611 is an upgraded version of the VCA2616. The only
difference between the VCA2616 and the VCA2611 is the input
structure to the LNP. The VCA2616 is limited to –0.3V negative-
going input spikes; the VCA2611 is limited to –2.0V negative-
going input spikes. This change allows the user to use slower
and less expensive input clamping diodes prior to the LNP input.
In some designs, input clamping may not be required.
The combination of Active Termination (AT) and Maximum
Gain Select (MGS) allow for the best noise performance. The
VCA2616 and VCA2611 also feature low crosstalk and out-
standing distortion performance.
The LNP has differential input and output capability and is
strappable for gains of 5dB, 17dB, 22dB, or 25dB. Low input
impedance is achieved by AT, resulting in as much as a 4.6dB
improvement in noise figure over conventional shunt termina-
tion. The termination value can also be switched to accommo-
date different sources. The output of the LNP is available for
external signal processing.
The variable gain is controlled by an analog voltage whose
gain varies from 0dB to the gain set by the MGS. The ability
to program the variable gain also allows the user to optimize
dynamic range. The VCA input can be switched from the LNP
to external circuits for different applications. The output can be
used in either a single-ended or differential mode to drive high-
performance Analog-to-Digital (A/D) converters, and is cleanly
limited for optimum overdrive recovery.
The combination of low noise, gain, and gain range program-
mability makes the VCA2616 and VCA2611 versatile building
blocks in a number of applications where noise performance
is critical. The VCA2616 and VCA2611 are available in a
TQFP-48 package.
Input
LNPINP
LNP
Gain
Set
LNPGS1
LNPGS2
LNPGS3
LNPINN
Low Noise
Preamp
5dB to 25dB
Voltage
Controlled
Attenuator
Programmable
Gain Amplifier
24 to 45dB
VCAOUTN
VCAOUTP
LNPOUTP VCAINP SEL
Please be aware that an important notice concerning availability, standard warranty, and use in critical applications of
Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.
All trademarks are the property of their respective owners.
PRODUCTION DATA information is current as of publication date.
Products conform to specifications per the terms of Texas Instruments
standard warranty. Production processing does not necessarily include
testing of all parameters.
www.ti.com
Copyright © 2002-2004, Texas Instruments Incorporated

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VCA2616 pdf
TYPICAL CHARACTERISTICS (Cont.)
www.datashAeteTt4Au=.c+o2m5°C, VDDA = VDDB = VDDR = +5V, load resistance = 500on each output to ground, MGS = 011, LNP = 22dB and fIN = 5MHz, unless otherwise noted.
The input to the preamp (LNP) is single-ended, and the output from the VCA is single-ended, unless otherwise noted. This results in a 6dB reduction in signal
amplitude compared to differential operation.
GAIN vs FREQUENCY
(Pre-Amp)
30
LNA = 25dB
LNA = 22dB
25
GAIN vs FREQUENCY
VCA
(VCACNTL = 0.2V)
5.0
4.0
3.0
20
15
10
5
0
100k
LNA = 17dB
LNA = 5dB
1M 10M
Frequency (Hz)
100M
2.0
1.0
0.0
1.0
2.0
MGS = 011
3.0
4.0
5.0
100k
MGS = 111
MGS = 100
MGS = 000
1M 10M
Frequency (Hz)
100M
45
40
35
30
25
20
15
10
5
0
100k
GAIN vs FREQUENCY
VCA
(VCACNTL = 3.0V)
MGS = 111
MGS = 100
MGS = 011
MGS = 000
1M 10M
Frequency (Hz)
100M
60
50
40
30
20
10
0
100k
GAIN vs FREQUENCY
LNA and VCA
(VCACNTL = 3.0V)
LNP = 25dB
LNP = 22dB
LNP = 17dB
LNP = 5dB
1M 10M
Frequency (Hz)
100M
60
50
40
30
20
10
0
100k
GAIN vs FREQUENCY
LNA and VCA
(LNP = 22dB)
VCNTL = 3.0V
VCNTL = 1.6V
VCNTL = 0.2V
1M 10M
Frequency (Hz)
100M
2000
1800
OUTPUT-REFERRED NOISE vs VCACNTL
(LNP = 25dB)
RS= 50
1600
1400
1200
1000
MGS = 111
800
600
400
200 MGS = 011
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
VCACNTL (V)
VCA2616, VCA2611
SBOS234E
www.ti.com
5

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VCA2616 arduino
where RL is the load resistor in the drains of Q3 and Q8, and
www.datashReeSt4isu.tchoemresistor connected between the sources of the input
transistors Q4 and Q7. The connections for various RS com-
binations are brought out to device pins LNPGS1, LNPGS2,
and LNPGS3 (pins 13-15 for channel A, 22-24 for channel B).
These Gain Strap pins allow the user to establish one of four
fixed LNP gain options as shown in Table I.
LNP PIN STRAPPING
LNPGS1, LNPGS2, LNPGS3 Connected Together
LNPGS1 Connected to LNPGS3
LNPGS1 Connected to LNPGS2
All Pins Open
LNP GAIN (dB)
25
22
17
5
TABLE I. Pin Strappings of the LNP for Various Gains.
It is also possible to create other gain settings by connecting
an external resistor between LNPGS1 on one side, and
LNPGS2 and/or LNPGS3 on the other. In that case, the
internal resistor values (see Figure 4) should be combined
with the external resistor to calculate the effective value of RS
for use in Equation 1. The resulting expression for external
resistor value is given in Equation 2:
REXT
=
2RS1RL
+ 2RFIXRL Gain × RS1RFIX
Gain × RS1 2RL
(2)
where REXT is the externally selected resistor value needed
to achieve the desired gain setting, RS1 is the fixed parallel
resistor in Figure 4, and RFIX is the effective fixed value of the
remaining internal resistors: RS2, RS3, or (RS2 || RS3), de-
pending on the pin connections.
Note that the best process and temperature stability will be
achieved by using the pre-programmed fixed-gain options of
Table I, since the gain is then set entirely by internal resistor
ratios, which are typically accurate to ±0.5%, and track quite
well over process and temperature. When combining exter-
nal resistors with the internal values to create an effective RS
value, note that the internal resistors have a typical tempera-
ture coefficient of +700ppm/°C and an absolute value toler-
ance of approximately ±5%, yielding somewhat less predict-
able and stable gain settings. With or without external resis-
tors, the board layout should use short Gain Strap connec-
tions to minimize parasitic resistance and inductance effects.
The overall noise performance of the VCA2616 and VCA2611
will vary as a function of gain. Table II shows the typical input-
and-output-referred noise densities of the entire VCA2616 and
VCA2611 for maximum VCA and PGA gain; that is, VCACNTL
set to 3.0V and all MGS bits set to 1. Note that the input-
referred noise values include the contribution of a 50fixed
source impedance, and are therefore somewhat larger than
the intrinsic input noise. As the LNP gain is reduced, the noise
contribution from the VCA/PGA portion becomes more signifi-
cant, resulting in higher input-referred noise. However, the
output-referred noise, which is indicative of the overall SNR at
that gain setting, is reduced.
To preserve the low-noise performance of the LNP, the user
should take care to minimize resistance in the input lead. A
parasitic resistance of only 10will contribute 0.4nV/Hz.
LNP GAIN (dB)
25
22
17
5
NOISE (nV/Hz)
Input-Referred
Output-Referred
1.35
1.41
1.63
4.28
2260
1650
1060
597
TABLE II. Equivalent Noise Performance for MGS = 111 and
VCACNTL = 3.0V with 50source impedance.
The LNP is capable of generating a 2VPP differential signal.
The maximum signal at the LNP input is therefore 2VPP
divided by the LNP gain. An input signal greater than this
would exceed the linear range of the LNP, an especially
important consideration at low LNP gain settings.
The VCA2611 is an upgraded version of the VCA2616. The
only difference between the VCA2616 and the VCA2611 is the
input structure to the LNP. The VCA2616 is limited to 0.3V
negative-going input spikes; the VCA2611 is limited to 2.0V
negative-going input spikes. This change allows the user to
use slower and less expensive input clamping diodes prior to
the LNA input. In some designs, input clamping may not be
required.
ACTIVE FEEDBACK WITH THE LNP
One of the key features of the LNP architecture is the ability
to employ active-feedback termination to achieve superior
noise performance. Active-feedback termination achieves a
lower noise figure than conventional shunt termination, es-
sentially because no signal current is wasted in the termina-
tion resistor itself. Another way to understand this is to
consider first that the input source, at the far end of the signal
cable, has a cable-matching source resistance of RS. Using
conventional shunt termination at the LNP input, a second
terminating resistor of value RS is connected to ground.
Therefore, the signal loss is 6dB due to the voltage divider
action of the series and shunt RS resistors. The effective
source resistance has been reduced by the same factor of 2,
but the noise contribution has been reduced by only the 2,
only a 3dB reduction. Therefore, the net theoretical SNR
degradation is 3dB, assuming a noise-free amplifier input. (In
practice, the amplifier noise contribution will degrade both
the unterminated and the terminated noise figures, some-
what reducing the distinction between them.)
See Figure 5 for an amplifier using active feedback. This
diagram appears very similar to a traditional inverting ampli-
fier. However, the analysis is somewhat different because
the gain A in this case is not a very large open-loop op amp
gain; rather, it is the relatively low and controlled gain of the
LNP itself. Thus, the impedance at the inverting amplifier
terminal will be reduced by a finite amount, as given in the
familiar relationship of Equation 3:
RIN
=
RF
(1+ A)
(3)
where RF is the feedback resistor (supplied externally be-
tween the LNPINP and FB terminals for each channel), A is
VCA2616, VCA2611
SBOS234E
www.ti.com
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