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PDF VCA2618YR Data sheet ( Hoja de datos )

Número de pieza VCA2618YR
Descripción Dual/ VARIABLE GAIN AMPLIFIER with Input Buffer
Fabricantes Burr-Brown Corporation 
Logotipo Burr-Brown Corporation Logotipo



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VCA2618
VCA2618
SBOS254B – JULY 2002 – REVISED NOVEMBER 2003
Dual, VARIABLE GAIN AMPLIFIER
with Input Buffer
FEATURES
z GAIN RANGE: up to 43dB
z 30MHz BANDWIDTH
z LOW CROSSTALK: 65dB at Max Gain, 5MHz
z HIGH-SPEED VARIABLE GAIN ADJUST
z POWER SHUTDOWN MODE
z HIGH IMPEDANCE INPUT BUFFER
APPLICATIONS
z ULTRASOUND SYSTEMS
z WIRELESS RECEIVERS
z TEST EQUIPMENT
z RADAR
DESCRIPTION
The VCA2618 is a highly integrated, dual receive channel,
Variable Gain Amplifier (VGA) with analog gain control.
The VCA2618’s VGA section consists of two parts: the Volt-
age Controlled Attenuator (VCA) and the Programmable Gain
Amplifier (PGA). The gain and gain range of the PGA can be
digitally programmed. The combination of these two program-
mable elements results in a variable gain ranging from 0dB up
to a maximum gain as defined by the user through external
connections. The single-ended unity gain input buffer provides
predictable high input impedance. The output of the VGA can
be used in either a single-ended or differential mode to drive
high-performance Analog-to-Digital (A/D) converters. A sepa-
rate power-down pin reduces power consumption.
The VCA2618 also features low crosstalk and outstanding
distortion performance. The combination of low noise
and gain range programmability make the VCA2618 a versa-
tile building block in a number of applications where
noise performance is critical. The VCA2618 is available in a
TQFP-32 package.
CP2A
CP1A
VCA2618
(1 of 2 Channels)
Voltage
INA Buffer Control
Attenuator
VCACNTL
Analog
Control
Programmable
Gain Amplifier
Maximum Gain
Select
NOUTA
POUTA
MGS1
MGS2
MGS3
Maximum
Gain Select
Voltage
INB Buffer Control
Attenuator
Programmable
Gain Amplifier
NOUTB
POUTB
CP2B
CP1B
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.
Copyright © 2002–2003, Texas Instruments Incorporated
www.ti.com

1 page




VCA2618YR pdf
TYPICAL CHARACTERISTICS (Cont.)
At TA = +25°C, VDD = 5V, load resistance = 500on each output to ground, differential output (2VPP) MGS = 111, and fIN = 5MHz, unless otherwise noted.
50
45
40
35
30
25
20
15
10
5
0
100k
GAIN vs FREQUENCY
(VCACNTL = 3.0V)
MGS = 111
MGS = 011
MGS = 001
1M 10M
Frequency (Hz)
100M
50
40
30
20
10
0
–10
100k
GAIN vs FREQUENCY
VCACNTL = 3.0V
VCACNTL = 1.6V
VCACNTL = 0.2V
1M 10M
Frequency (Hz)
100M
OUTPUT REFERRED NOISE vs VCACNTL
500
450
RS= 50
400
MGS = 111
350
300
250
200
MGS = 011
150
100
50
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)
INPUT REFERRED NOISE vs VCACNTL
600
550
500
RS= 50
450
400
350
MGS = 111
300
250
200
150
100
50
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)
INPUT REFERRED NOISE vs RS
100
10
1
1 10 100 1k
RS ()
24
22
20
18
16
14
12
10
8
6
4
2
10
NOISE FIGURE vs RS
100
RS ()
1k
VCA2618
SBOS254B
www.ti.com
5

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VCA2618YR arduino
PGA POST-AMPLIFIER
Figure 5 shows a simplified circuit diagram of the PGA block.
As stated before, the input to the PGA is AC coupled by an
internal capacitor. Provisions are made so that an external
capacitor can be placed in parallel with the internal capacitor,
thus lowering the usable low-frequency bandwidth. The low-
frequency bandwidth is set by the following equation:
1
( )2 •π • 500kΩ • (220pF + CEXTERNAL )
where CEXTERNAL is the external capacitor value in farads.
Care should be taken to avoid using too large a value of
capacitor, as this can increase the power-on delay time.
As described previously, the PGA gain is programmed with
the same MGS bits that control the VCA maximum attenua-
tion factor. Specifically, the maximum PGA gain at each
MGS setting is the inverse (reciprocal) of the maximum VCA
attenuation at that setting. Therefore, the VCA + PGA overall
gain will always be 0dB (unity) when the analog VCACNTL
input is set to 0V (the maximum attenuation for VCA). For
VCACNTL = 3V (no attenuation), the VCA + PGA gain will be
controlled by the programmed PGA gain (25dB to 43dB in
3dB steps). For clarity, the gain and attenuation factors are
detailed in Table I.
The PGA architecture converts the single-ended signal from
the VCA into a differential signal. Low input noise was also
a requirement of the PGA design due to the large amount of
signal attenuation that can be asserted before the PGA. At
minimum VCA attenuation (used for small input signals), the
MGS
SETTING
000
001
010
011
100
101
110
111
ATTENUATOR GAIN
VCACNTL = 0V to 3V
Not Valid
–25dB to 0dB
–28dB to 0dB
–31dB to 0dB
–34dB to 0dB
–37dB to 0dB
–40dB to 0dB
–43dB to 0dB
TABLE I. MGS Settings.
ATTENUATOR +
DIFFERENTIAL PGA GAIN
Not Valid
0dB to 25dB
0dB to 28dB
0dB to 31dB
0dB to 34dB
0dB to 37dB
0dB to 40dB
0dB to 43dB
input buffer noise dominates; at maximum VCA attenuation
(large input signals), the PGA noise dominates. Note that if
the PGA output is used single-ended, the apparent gain will
be 6dB lower.
LAYOUT CONSIDERATIONS
The VCA2618 is an analog amplifier capable of high gain.
When working on a PCB layout for the VCA2618, it is
recommended to utilize a solid ground plane that is con-
nected to analog ground. This helps to maximize the noise
performance of the VCA2618.
Adequate power-supply decoupling must be used in order to
achieve the best possible performance. Decoupling capaci-
tors on the VCACNTL voltage should also be used to help
minimize noise. Recommended values can be obtained from
the layout diagram of Figure 6.
VCAOUTP
RL Q1
Q11
VDD
Q12
To Bias
Circuitry
Q9 RL
VCM
+In
Q3
Q4
Q2
RS1 Q13
RS2
Q14
Q5 Q6
Q8
VCM
Q7
Q10
To Bias
Circuitry
–In
VCAOUTN
FIGURE 5. Simplified Block Diagram of the PGA Section with the VCA2618.
VCA2618
SBOS254B
www.ti.com
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