ETC VCA2612Y/250

VCA2612
VCA
261
2
SBOS117B – OCTOBER 2001
Dual, VARIABLE GAIN AMPLIFIER
with Low Noise Preamp
FEATURES
DESCRIPTION
● LOW NOISE PREAMP:
• Low Input Noise: 1.25nV/√Hz
• Active Termination Noise Reduction
• Switchable Termination Value
• 80MHz Bandwidth
• 5dB to 25dB Gain Range
• Differential Input /Output
● LOW NOISE VARIABLE GAIN AMPLIFIER:
• Low Noise VCA: 3.3nV/√Hz, Differential
Programming Optimizes Noise Figure
• 24dB to 45dB Gain
• 40MHz Bandwidth
• Differential Input /Output
● LOW CROSSTALK: 52dB at Max Gain, 5MHz
● HIGH-SPEED VARIABLE GAIN ADJUST
● SWITCHABLE EXTERNAL PROCESSING
The VCA2612 is a highly integrated, dual receive channel,
signal processing subsystem. Each channel of the product
consists of a low noise preamplifier (LNP) and a Variable Gain
Amplifier (VGA). The LNP circuit provides the necessary
connections to implement Active Termination (AT), a method
of cable termination which results in up to 4.6dB noise figure
improvement. Different cable termination characteristics can
be accommodated by utilizing the VCA2612’s switchable
LNA feedback pins. The LNP has the ability to accept both
differential and single-ended inputs, and generates a differential output signal. The LNP provides strappable gains of 5dB,
17dB, 22dB, and 25dB.
The output of the LNP can be accessed externally for further
signal processing, or fed directly into the VGA. The VCA2612’s
VGA section consists of two parts: the Voltage 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 programmable
elements results in a variable gain ranging from 0dB up to a
maximum gain as defined by the user through external connections. The output of the VGA can be used in either a singleended or differential mode to drive high-performance
Analog-to-Digital (A/D) converters.
The VCA2612 also features low crosstalk and outstanding
distortion performance. The combination of low noise, and gain
range programmability make the VCA2612 a versatile building
block in a number of applications where noise performance is
critical. The VCA2612 is available in a TQFP-48 package.
APPLICATIONS
● ULTRASOUND SYSTEMS
● WIRELESS RECEIVERS
● TEST EQUIPMENT
Maximum Gain Select
FBSWCNTL
LNPOUTN
VCAINN
VCACNTL
MGS1 MGS2 MGS3
RF2
SWFB
RF1
FB
VCA2612
(1 of 2 Channels)
Analog
Control
Maximum Gain
Select
CF
Input
CC
LNPINP
LNP
Gain Set
VCAOUTN
LNPGS1
LNPGS2
Voltage
Controlled
Attenuator
Low Noise
Preamp
5dB to 25dB
Programmable
Gain Amplifier
24 to 45dB
LNPGS3
VCAOUTP
LNPINN
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.
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 © 2000, Texas Instruments Incorporated
www.ti.com
ELECTROSTATIC
DISCHARGE SENSITIVITY
ABSOLUTE MAXIMUM RATINGS
Power Supply (+VS) ............................................................................. +6V
Analog Input ............................................................. –0.3V to (+VS + 0.3V)
Logic Input ............................................................... –0.3V to (+VS + 0.3V)
Case Temperature ......................................................................... +100°C
Junction Temperature .................................................................... +150°C
Storage Temperature ...................................................... –40°C to +150°C
This integrated circuit can be damaged by ESD. Texas Instruments recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling
and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to
complete device failure. Precision integrated circuits may be more
susceptible to damage because very small parametric changes
could cause the device not to meet its published specifications.
PACKAGE/ORDERING INFORMATION
PRODUCT
PACKAGE-LEAD
PACKAGE
DESIGNATOR
PACKAGE
MARKING
ORDERING(1)
NUMBER
TRANSPORT
MEDIA, QUANTITY
VCA2612Y
TQFP-48
PFB
VCA2612Y
"
"
"
"
VCA2612Y/250
VCA2612Y/2K
Tape and Reel, 250
Tape and Reel, 2000
NOTE: (1) Models with a (/) are available only in Tape and Reel in the quantities indicated (e.g., /2K indicates 2000 devices per reel). Ordering 2000 pieces of
“VCA2612Y/2K” will get a single 2000-piece Tape and Reel.
ELECTRICAL CHARACTERISTICS
At TA = +25°C, VDDA = VDDB = VDDR = +5V, load resistance = 500Ω on 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.
VCA2612Y
PARAMETER
PREAMPLIFIER
Input Resistance
Input Capacitance
Input Bias Current
CMRR
Maximum Input Voltage
Input Voltage Noise(1)
Input Current Noise
Noise Figure, RS = 75Ω, RIN = 75Ω(1)
Bandwidth
CONDITIONS
MIN
f = 1MHz, VCACNTL = 0.2V
Preamp Gain = +5dB
Preamp Gain = +25dB
Preamp Gain = +5dB
Preamp Gain = +25dB
Independent of Gain
RF = 550Ω, Preamp Gain = 22dB,
PGA Gain = 39dB
Gain = 22dB
PROGRAMMABLE VARIABLE GAIN AMPLIFIER
Peak Input Voltage
Differential
–3dB Bandwidth
Slew Rate
Output Signal Range
RL ≥ 500Ω Each Side to Ground
Output Impedance
f = 5MHz
Output Short-Circuit Current
Third Harmonic Distortion
f = 5MHz, VOUT = 1Vp-p, VCACNTL = 3.0V
Second Harmonic Distortion
f = 5MHz, VOUT = 1Vp-p, VCACNTL = 3.0V
IMD, Two-Tone
VOUT = 2Vp-p, f = 1MHz
VOUT = 2Vp-p, f = 10MHz
1dB Compression Point
f = 5MHz, Output Referred, Differential
Crosstalk
VOUT = 1Vp-p, f = 1MHz, Max Gain Both Channels
Group Delay Variation
1MHz < f < 10MHz, Full Gain Range
DC Output Level, VIN = 0
–45
–45
ACCURACY
Gain Slope
Gain Error
Output Offset Voltage
GAIN CONTROL INTERFACE
Input Voltage (VCACNTL) Range
Input Resistance
Response Time
POWER SUPPLY
Operating Temperature Range
Specified Operating Range
Power Dissipation
Thermal Resistance, θJA
TYP
MAX
600
15
1
50
1
112
3.5
1.25
0.35
6.2
kΩ
pF
nA
dB
Vp-p
mVp-p
nV/√Hz
nV/√Hz
pA/√Hz
dB
80
MHz
2
40
300
2
1
±40
–71
–63
–80
–80
6
68
±2
2.5
Vp-p
MHz
V/µs
Vp-p
Ω
mA
dBc
dBc
dBc
dBc
Vp-p
dB
ns
V
10.9
±50
–40
4.75
Operating, Both Channels
TQFP-48
±1(2)
0.2 to 3.0
1
0.2
45dB Gain Change, MGS = 111
UNITS
5.0
410
56.5
dB/V
dB
mV
V
MΩ
µs
+85
5.25
495
°C
V
mW
°C/W
NOTE: (1) For preamp driving VGA. (2) Referenced to best fit dB-linear curve.
2
VCA2612
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SBOS117B
MGS1
MGS2
MGS3
VCAOUTPB
VCAOUTNB
GNDB
46
VCACNTL
VCAOUTPA
47
FBSWCNTL
VCAOUTNA
48
VCAINSEL
GNDA
PIN CONFIGURATION
45
44
43
42
41
40
39
38
37
VDDA
1
36 VDDB
NC
2
35 NC
NC
3
34 NC
VCAINNA
4
33 VCAINNB
32 VCAINPB
VCAINPA
5
LNPOUTNA
6
LNPOUTPA
7
30 LNPOUTPB
SWFBA
8
29 SWFBB
FBA
9
28 FBB
31 LNPOUTNB
VCA2612
18
19
VBIAS
VCM
20
21
22
23
24
LNPGS3B
17
LNPGS2B
16
LNPGS1B
15
GNDR
14
LNPINPB
13
VDDR
25 LNPINNB
LNPINPA
LNPINNA 12
LNPGS1A
26 COMP2B
LNPGS3A
27 COMP1B
COMP2A 11
LNPGS2A
COMP1A 10
PIN DESCRIPTIONS
PIN
DESIGNATOR
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
VDDA
NC
NC
VCAINNA
VCAINPA
LNPOUTNA
LNPOUTPA
SWFBA
FBA
COMP1A
COMP2A
LNPINNA
LNPGS3A
LNPGS2A
LNPGS1A
LNPINPA
VDDR
VBIAS
VCM
GNDR
LNPINPB
LNPGS1B
LNPGS2B
LNPGS3B
DESCRIPTION
PIN
DESIGNATOR
Channel A +Supply (+5V)
Do Not Connect
Do Not Connect
Channel A VCA Negative Input
Channel A VCA Positive Input
Channel A LNP Negative Output
Channel A LNP Positive Output
Channel A Switched Feedback Output
Channel A Feedback Output
Channel A Frequency Compensation 1
Channel A Frequency Compensation 2
Channel A LNP Inverting Input
Channel A LNP Gain Strap 3
Channel A LNP Gain Strap 2
Channel A LNP Gain Strap 1
Channel A LNP Noninverting Input
+Supply for Internal Reference (+5V)
0.01µF Bypass to Ground
0.01µF Bypass to Ground
Ground for Internal Reference
Channel B LNP Noninverting Input
Channel B LNP Gain Strap 1
Channel B LNP Gain Strap 2
Channel B LNP Gain Strap 3
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
LNPINNB
COMP2B
COMP1B
FBB
SWFBB
LNPOUTPB
LNPOUTNB
VCAINPB
VCAINNB
NC
NC
VDDB
GNDB
VCAOUTNB
VCAOUTPB
MGS3
MGS2
MGS1
VCACNTL
VCAINSEL
FBSWCNTL
VCAOUTPA
VCAOUTNA
GNDA
VCA2612
SBOS117B
www.ti.com
DESCRIPTION
Channel B LNP Inverting Input
Channel B Frequency Compensation 2
Channel B Frequency Compensation 1
Channel B Feedback Output
Channel B Switched Feedback Output
Channel B LNP Positive Output
Channel B LNP Negative Output
Channel B VCA Positive Input
Channel B VCA Negative Input
Do Not Connect
Do Not Connect
Channel B +Analog Supply (+5V)
Channel B Analog Ground
Channel B VCA Negative Output
Channel B VCA Positive Output
Maximum Gain Select 3 (LSB)
Maximum Gain Select 2
Maximum Gain Select 1 (MSB)
VCA Control Voltage
VCA Input Select, HI = External
Feedback Switch Control: HI = ON
Channel A VCA Positive Output
Channel A VCA Negative Output
Channel A Analog Ground
3
TYPICAL CHARACTERISTICS
At TA = +25°C, VDDA = VDDB = VDDR = +5V, load resistance = 500Ω on 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 ERROR vs TEMPERATURE
GAIN vs VCACNTL
65
2.0
MGS = 111
60
55
1.0
45
MGS = 100
40
35
MGS = 011
30
Gain Error (dB)
MGS = 101
50
Gain (dB)
1.5
MGS = 110
MGS = 001
20
0
–0.5
+85°C
–1.5
MGS = 000
15
+25°C
0.5
–1.0
MGS = 010
25
–2.0
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
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)
VCACNTL (V)
GAIN ERROR vs VCACNTL
GAIN ERROR vs VCACNTL
2.0
2.0
1.5
1.5
1MHz
1.0
1.0
10MHz
0.5
Gain Error (dB)
Gain Error (dB)
–40°C
0
–0.5
5MHz
MGS = 000
MGS = 011
0.5
0
–0.5
–1.0
–1.0
–1.5
–1.5
MGS = 111
–2.0
–2.0
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
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)
VCACNTL (V)
GAIN MATCH: CHA to CHB, VCACNTL = 0.2V
100
90
90
80
80
70
70
60
60
Units
Units
100
50
40
30
30
20
20
10
10
0
–0.5 –0.4 –0.3 –0.2 –0.1 0.0
0.1 0.2
0.3
0.4
0.5
–0.5 –0.4 –0.3 –0.2 –0.1 0.0
Delta Gain (dB)
4
50
40
0
GAIN MATCH: CHA to CHB, VCACNTL = 3.0V
0.1 0.2
0.3
0.4
0.5
Delta Gain (dB)
VCA2612
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SBOS117B
TYPICAL CHARACTERISTICS (Cont.)
At TA = +25°C, VDDA = VDDB = VDDR = +5V, load resistance = 500Ω on 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
(VCA and PGA, VCACNTL = 0.2V)
GAIN vs FREQUENCY
(Pre-Amp)
30
5.0
LNP = 25dB
25
3.0
2.0
Gain (dB)
20
Gain (dB)
MGS = 111
MGS = 100
MGS = 011
MGS = 000
4.0
LNP = 22dB
15
LNP = 17dB
10
1.0
0.0
–1.0
–2.0
–3.0
5
–4.0
LNP = 5dB
0
0.1
–5.0
1
10
0.1
100
1
10
Frequency (MHz)
Frequency (MHz)
GAIN vs FREQUENCY
(VCA and PGA, VCACNTL = 3.0V)
GAIN vs FREQUENCY
(VCACNTL = 3.0V)
45
60
LNP = 25dB
MGS = 111
40
LNP = 22dB
50
35
MGS = 100
30
40
Gain (dB)
Gain (dB)
100
25
MGS = 011
20
15
LNP = 17dB
30
LNP = 5dB
20
MGS = 000
10
10
5
0
0
0.1
1
10
100
0.1
1
Frequency (MHz)
GAIN vs FREQUENCY
(LNP = 22dB)
100
OUTPUT REFERRED NOISE vs VCACNTL
1800
60
VCACNTL = 3.0V
1600
RS= 50Ω
MGS = 111
50
1400
Noise (nV/√Hz)
VCACNTL = 1.6V
40
Gain (dB)
10
Frequency (MHz)
30
20
1200
1000
800
600
MGS = 011
400
10
200
VCACNTL = 0.2V
0
0
0.1
1
10
0 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
100
VCACNTL (V)
Frequency (MHz)
VCA2612
SBOS117B
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5
TYPICAL CHARACTERISTICS (Cont.)
At TA = +25°C, VDDA = VDDB = VDDR = +5V, load resistance = 500Ω on 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.
INPUT REFERRED NOISE vs VCACNTL
24
22
RS= 50Ω
20
18
16
MGS = 111
Noise (nV√Hz
Noise (nV/√Hz)
INPUT REFERRED NOISE vs RS
10.0
14
12
10
8
6
4
2
1.0
MGS = 011
0
0.1
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
1
10
VCACNTL (V)
100
1000
RS (Ω)
NOISE FIGURE vs RS
(VCACNTL = 3.0V)
11
NOISE FIGURE vs VCACNTL
30
10
25
8
Noise Figure (dB)
Noise Figure (dB)
9
7
6
5
4
3
2
20
15
10
5
1
0
0
100
1000
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
RS (Ω)
VCACNTL (V)
LNP vs FREQUENCY
(Differential, 2Vp-p)
LNP vs FREQUENCY
(Single-Ended, 1Vp-p)
–45
–45
–50
–50
Harmonic Distortion (dBc)
Harmonic Distortion (dBc)
10
–55
3rd Harmonic
–60
–65
–70
–75
–55
2nd Harmonic
–60
–65
–70
–75
3rd Harmonic
2nd Harmonic
–80
–80
0.1
1
10
100
0.1
Frequency (MHz)
6
1
10
100
Frequency (MHz)
VCA2612
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SBOS117B
TYPICAL CHARACTERISTICS (Cont.)
At TA = +25°C, VDDA = VDDB = VDDR = +5V, load resistance = 500Ω on 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.
HARMONIC DISTORTION vs FREQUENCY
(Differential, 2Vp-p, MGS = 000)
HARMONIC DISTORTION vs FREQUENCY
(Differential, 2Vp-p, MGS = 011)
–40
–40
VCACNTL = 0.2V, H2
VCACNTL = 0.2V, H3
VCACNTL = 3.0V, H2
VCACNTL = 3.0V, H3
–50
–55
–60
–65
–70
–75
–80
–85
–55
–60
–65
–70
–75
–80
–90
0.1
–30
1
10
–45
1
Frequency (Hz)
HARMONIC DISTORTION vs FREQUENCY
(Differential, 2Vp-p, MGS = 111)
HARMONIC DISTORTION vs FREQUENCY
(Single-Ended, 1Vp-p, MGS = 000)
10
–40
VCACNTL = 0.2V, H2
VCACNTL = 0.2V, H3
VCACNTL = 3.0V, H2
VCACNTL = 3.0V, H3
–45
Harmonic Distortion (dBc)
–40
0.1
Frequency (MHz)
VCACNTL = 0.2V, H2
VCACNTL = 0.2V, H3
VCACNTL = 3.0V, H2
VCACNTL = 3.0V, H3
–35
Harmonic Distortion (dBc)
–50
–85
–90
–50
–55
–60
–65
–70
–75
–50
–55
–60
–65
–70
–75
–80
–85
–80
–90
0.1
1
10
0.1
Frequency (MHz)
HARMONIC DISTORTION vs FREQUENCY
(Single-Ended, 1Vp-p, MGS = 011)
HARMONIC DISTORTION vs FREQUENCY
(Single-Ended, 1Vp-p, MGS = 111)
–55
VCACNTL = 0.2V, H2
VCACNTL = 0.2V, H3
VCACNTL = 3.0V, H2
VCACNTL = 3.0V, H3
–35
Harmonic Distortion (dBc)
–50
10
–30
VCACNTL = 0.2V, H2
VCACNTL = 0.2V, H3
VCACNTL = 3.0V, H2
VCACNTL = 3.0V, H3
–45
1
Frequency (MHz)
–40
Harmonic Distortion (dBc)
VCACNTL = 0.2V, H2
VCACNTL = 0.2V, H3
VCACNTL = 3.0V, H2
VCACNTL = 3.0V, H3
–45
Harmonic Distortion (dBc)
Harmonic Distortion (dBc)
–45
–60
–65
–70
–75
–80
–85
–40
–45
–50
–55
–60
–65
–70
–75
–80
–90
–85
0.1
1
10
0.1
Frequency (MHz)
VCA2612
SBOS117B
1
10
Frequency (MHz)
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7
TYPICAL CHARACTERISTICS (Cont.)
At TA = +25°C, VDDA = VDDB = VDDR = +5V, load resistance = 500Ω on 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.
HARMONIC DISTORTION vs VCACNTL
(Differential, 2Vp-p)
HARMONIC DISTORTION vs VCACNTL
(Single-Ended, 1Vp-p)
–45
–45
MGS = 000, H2
MGS = 011, H2
MGS = 111, H2
MGS = 000, H3
MGS = 011, H3
MGS = 111, H3
–55
–60
–65
–70
–75
–55
–60
–65
–70
–75
–80
–80
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
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)
VCACNTL (V)
INTERMODULATION DISTORTION
(Differential, 2Vp-p, f = 10MHz)
INTERMODULATION DISTORTION
(Single-Ended, 1Vp-p, f = 10MHz)
–5
–5
–15
–15
–25
–25
–35
–35
Power (dBFS)
Power (dBFS)
MGS = 000, H2
MGS = 011, H2
MGS = 111, H2
MGS = 000, H3
MGS = 011, H3
MGS = 111, H3
–50
Harmonic Distortion (dBc)
Harmonic Distortion (dBc)
–50
–45
–55
–65
–45
–55
–65
–75
–75
–85
–85
–95
–95
–105
–105
9.96
9.98
10
10.2
9.96
10.4
0
9.98
10
10.2
10.4
Frequency (MHz)
Frequency (MHz)
–1dB COMPRESSION vs VCACNTL
0
3rd-ORDER INTERCEPT vs VCACNTL
–5
–5
–10
–10
IP3 (dBm)
PIN (dBm)
–15
–15
–20
–25
–20
–25
–30
–35
–30
–40
–35
–45
–40
8
–50
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
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)
VCACNTL (V)
VCA2612
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SBOS117B
TYPICAL CHARACTERISTICS (Cont.)
At TA = +25°C, VDDA = VDDB = VDDR = +5V, load resistance = 500Ω on 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.
OVERLOAD RECOVERY
(Differential, VCACNTL = 3.0V, MGS = 111)
PULSE RESPONSE (BURSTS)
(Differential, VCACNTL = 3.0V, MGS = 111)
Output
500mV/div
Output
1V/div
Input
1mV/div
Input
1mV/div
200ns/div
200ns/div
GAIN RESPONSE
(Differential, VCACNTL Pulsed, MGS = 111)
CROSS TALK vs FREQUENCY
(Single-Ended, 1Vp-p, MGS = 011)
0
Output
500mV/div
–10
Cross Talk (dB)
–20
Input
2V/div
VCACNTRL = 1.5V
–30
–40
–50
VCACNTRL = 0V
–60
–70
VCACNTRL = 3.0V
–80
–90
100ns/div
1
10
100
Frequency (MHz)
CMRR vs FREQUENCY
(LNP only)
0
0
–10
–10
–20
–20
–30
VCACNTL = 0.2V
–40
CMRR (dB)
CMRR (dB)
CMRR vs FREQUENCY
(VCA only)
VCACNTL = 1.4V
–50
–60
–30
–40
–50
–60
–70
–70
–80
VCACNTL = 3.0V
–90
0.1
1
–80
10
100
0.1
Frequency (MHz)
10
100
Frequency (MHz)
VCA2612
SBOS117B
1
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9
TYPICAL CHARACTERISTICS (Cont.)
At TA = +25°C, VDDA = VDDB = VDDR = +5V, load resistance = 500Ω on 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.
GROUP DELAY vs FREQUENCY
ICC vs TEMPERATURE
80
79.5
Group Delay (ns)
ICC (mA)
79
78.5
78
77.5
77
76.5
76
–40
–25
–10
5
20
35
50
65
80
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
95
VCACNTL = 3.0V
VCACNTL = 0.2V
1
10
Temperature (°C)
100
Frequency (MHz)
PSRR vs FREQUENCY
–45
–40
–35
PSRR (dB)
–30
–25
–20
–15
–10
–5
0
5
10
10
100
1k
10k
100k
1M
10M
Frequency (Hz)
10
VCA2612
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SBOS117B
THEORY OF OPERATION
The VCA2612 is a dual-channel system consisting of three
primary blocks: a Low Noise Preamplifier (LNP), a Voltage
Controlled Attenuator (VCA), and a Programmable Gain
Amplifier (PGA). For greater system flexibility, an onboard
multiplexer is provided for the VCA inputs, selecting either
the LNP outputs or external signal inputs. Figure 1 shows a
simplified block diagram of the dual-channel system.
op amp. The “VCM” node shown in the drawing is the VCM
output (pin 19). Typical R and C values are shown, yielding
a high-pass time constant similar to that of the LNP. If a
different common-mode referencing method is used, it is
important that the common-mode level be within 10mV of
the VCM output for proper operation.
1kΩ
External
InA
Channel A
Input
LNP
VCA
PGA
To VCAIN
47nF
Input
Signal
1kΩ
Channel A
Output
VCM (+2.5V)
Channel B
Input
LNP
VCA
Maximum
Gain
Select
MGS
PGA
Channel B
Output
FIGURE 2. Recommended Circuit for Coupling an External
Signal into the VCA Inputs.
External
InB
FIGURE 1. Simplified Block Diagram of the VCA2612.
LNP—OVERVIEW
The LNP input may be connected to provide active-feedback
signal termination, achieving lower system noise performance than conventional passive shunt termination. Even
lower noise performance is obtained if signal termination is
not required. The unterminated LNP input impedance is
600kΩ. The LNP can process fully differential or singleended signals in each channel. Differential signal processing
results in significantly reduced 2nd-harmonic distortion and
improved rejection of common-mode and power supply
noise. The first gain stage of the LNP is AC coupled into its
output buffer with a 44µs time constant (3.6kHz high-pass
characteristic). The buffered LNP outputs are designed to
drive the succeeding VCA directly or, if desired, external
loads as low as 135Ω with minimal impact on signal distortion. The LNP employs very low impedance local feedback
to achieve stable gain with the lowest possible noise and
distortion. Four pin-programmable gain settings are available: 5dB, 17dB, 22dB, and 25dB. Additional intermediate
gains can be programmed by adding trim resistors between
the Gain Strap programming pins.
The common-mode DC level at the LNP output is nominally
2.5V, matching the input common-mode requirement of the
VCA for simple direct coupling. When external signals are
fed to the VCA, they should also be set up with a 2.5VDC
common-mode level. Figure 2 shows a circuit that demonstrates the recommended coupling method using an external
VCA—OVERVIEW
The magnitude of the differential VCA input signal (from
the LNP or an external source) is reduced by a programmable attenuation factor, set by the analog VCA Control
Voltage (VCACNTL) at pin 43. The maximum attenuation
factor is further programmable by using the three MGS bits
(pins 40-42). Figure 3 illustrates this dual-adjustable characteristic. Internally, the signal is attenuated by having the
analog VCACNTL vary the channel resistance of a set of
shunt-connected FET transistors. The MGS bits effectively
adjust the overall size of the shunt FET by switching parallel
components in or out under logic control. At any given
maximum gain setting, the analog variable gain characteristic is linear in dB as a function of the control voltage, and
is created as a piecewise approximation of an ideal dB-linear
transfer function. The VCA gain control circuitry is common to both channels of the VCA2612.
0
VCA Attenuation (dB)
Analog
Control
VCA
Control
–24
Maximum Attenuation
–45
0
3.0V
Control Voltage
FIGURE 3. Swept Attenuator Characteristic.
VCA2612
SBOS117B
Minimum Attenuation
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11
PGA OVERVIEW AND OVERALL DEVICE
CHARACTERISTICS
The differential output of the VCA attenuator is then amplified by the PGA circuit block. This post-amplifier is programmed by the same MGS bits that control the VCA
attenuator, yielding an overall swept-gain amplifier characteristic in which the VCA • PGA gain varies from 0dB
(unity) to a programmable peak gain of 24dB, 27dB, 30dB,
33dB, 36dB, 39dB, 42dB, or 45dB.
The “GAIN vs VCACNTL” curve on page 4 shows the
composite gain control characteristic of the entire VCA2612.
Setting VCACNTL to 3.0V causes the digital MGS gain
control to step in 3dB increments. Setting VCACNTL to 0V
causes all the MGS-controlled gain curves to converge at
one point. The gain at the convergence point is the LNP gain
less 6dB, because the measurement setup looks at only one
side of the differential PGA output, resulting in 6dB lower
signal amplitude.
The VCA2612 includes a built-in reference, common to
both channels, to supply a regulated voltage for critical areas
of the circuit. This reduces the susceptibility to power supply
variation, ripple, and noise. In addition, separate power
supply and ground connections are provided for each channel and for the reference circuitry, further reducing
interchannel cross-talk.
Further details regarding the design, operation and use of
each circuit block are provided in the following sections.
LOW NOISE PREAMPLIFIER (LNP)—DETAIL
The LNP is designed to achieve a low noise figure, especially when employing active termination. Figure 4 is a
simplified schematic of the LNP, illustrating the differential
input and output capability. The input stage employs low
resistance local feedback to achieve stable low noise, low
distortion performance with very high input impedance.
Normally, low noise circuits exhibit high power consumption due to the large bias currents required in both input and
output stages. The LNP uses a patented technique that
combines the input and output stages such that they share the
same bias current. Transistors Q4 and Q5 amplify the signal
at the gate-source input of Q4, the +IN side of the LNP. The
signal is further amplified by the Q1 and Q2 stage, and then
by the final Q3 and RL gain stage, which uses the same bias
current as the input devices Q4 and Q5. Devices Q6 through
Q10 play the same role for signals on the –IN side.
The differential gain of the LNP is given in Equation (1):
ADDITIONAL FEATURES—OVERVIEW
Overload protection stages are placed between the attenuator
and the PGA, providing a symmetrically clipped output
whenever the input becomes large enough to overload the
PGA. A comparator senses the overload signal amplitude
and substitutes a fixed DC level to prevent undesirable
overload recovery effects. As with the previous stages, the
VCA is AC coupled into the PGA. In this case, the coupling
time constant varies from 5µs at the highest gain (45dB) to
59µs at the lowest gain (25dB).
R 
Gain = 2 •  L 
 RS 
VDD
COMP2A
COMP1A
RL
93Ω
Q2
RL
93Ω
LNPOUTN
To Bias
Circuitry
Q9
LNPOUTP
Buffer
CCOMP
(External
Capacitor)
(1)
Buffer
Q3
Q8
RS1
105Ω
RW
RS2
34Ω
Q4
LNPINP
LNPGS1
RW
Q7
LNPINN
LNPGS2
RS3
17Ω
Q10
LNPGS3
Q1
To Bias
Circuitry
Q5
Q6
FIGURE 4. Schematic of the Low Noise Preamplifier (LNP).
12
VCA2612
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SBOS117B
where RL is the load resistor in the drains of Q3 and Q8, and
RS is the resistor connected between the sources of the input
transistors Q4 and Q7. The connections for various RS
combinations 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
LNP GAIN (dB)
LNPGS1, LNPGS2, LNPGS3 Connected Together
LNPGS1 Connected to LNPGS3
LNPGS1 Connected to LNPGS2
All Pins Open
25
22
17
5
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 shown in 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).
2 R S1R L + 2 R FIX R L – Gain • R S1R FIX
Gain • R S1 – 2 R L
NOISE (nV/√Hz)
(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)
depending 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 external resistors with the internal values to create an effective RS
value, note that the internal resistors have a typical temperature coefficient of +700ppm/°C and an absolute value tolerance of approximately ±5%, yielding somewhat less predictable and stable gain settings. With or without external
resistors, the board layout should use short Gain Strap
connections to minimize parasitic resistance and inductance
effects.
The overall noise performance of the VCA2612 will vary as
a function of gain. Table II shows the typical input-and
output-referred noise densities of the entire VCA2612 for
maximum VCA and PGA gain; i.e., VCACNTL set to 3.0V
and all MGS bits set to “1”. Note that the input-referred
noise values include the contribution of a 50Ω fixed 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
significant, resulting in higher input-referred noise. However, the output-referred noise, which is indicative of the
overall SNR at that gain setting, is reduced.
Input-Referred
Output-Referred
25
22
17
5
1.54
1.59
1.82
4.07
2260
1650
1060
597
The LNP is capable of generating a 2Vp-p differential
signal. The maximum signal at the LNP input is therefore
2Vp-p 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.
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,
essentially because no signal current is wasted in the termination resistor itself. Another way to understand this is as
follows: 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, somewhat reducing the distinction between them.)
See Figure 5 for an amplifier using active feedback. This
diagram appears very similar to a traditional inverting amplifier. 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):
R IN =
RF
(1 + A)
(3)
where RF is the feedback resistor (supplied externally between the LNPINP and FB terminals for each channel), A is
the user-selected gain of the LNP, and RIN is the resulting
amplifier input impedance with active feedback. In this case,
unlike the conventional termination above, both the signal
voltage and the RS noise are attenuated by the same factor of
VCA2612
SBOS117B
LNP GAIN (dB)
TABLE II. Noise Performance for MGS = 111 and VCACNTL = 3.0V.
TABLE I. Pin Strappings of the LNP for Various Gains.
R EXT =
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 10Ω will contribute 0.4nV/√Hz.
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13
VCA NOISE = 3.8nV√Hz, LNP GAIN = 20dB
14
RF
LNP Noise
nV/√Hz
6.0E-10
8.0E-10
1.0E-09
1.2E-09
1.4E-09
1.6E-09
1.8E-09
2.0E-09
12
RS
Noise Figure (dB)
LNPIN
A
RIN
RIN =
RF
1+A
Active Feedback
= RS
10
8
6
4
2
RS
0
0
A
100 200 300 400 500 600 700 800 900 1000
RS
Source Impedance (Ω)
FIGURE 7. Noise Figure for Conventional Termination.
Conventional Cable Termination
FIGURE 5. Configurations for Active Feedback and Conventional Cable Termination.
two (6dB) before being re-amplified by the “A” gain setting.
This avoids the extra 3dB degradation due to the square-root
effect described above, the key advantage of the active
termination technique.
As mentioned above, the previous explanation ignored the
input noise contribution of the LNP itself. Also, the noise
contribution of the feedback resistor must be included for a
completely correct analysis. The curves given in Figures 6
and 7 allow the VCA2612 user to compare the achievable
noise figure for active and conventional termination methods. The left-most set of data points in each graph give the
results for typical 50Ω cable termination, showing the worst
noise figure but also the greatest advantage of the active
feedback method.
A switch, controlled by the FBSWCNTL signal on pin 45,
enables the user to reduce the feedback resistance by adding
an additional parallel component, connected between the
LNPINP and SWFB terminals. The two different values of
feedback resistance will result in two different values of
active-feedback input resistance. Thus, the active-feedback
impedance can be optimized at two different LNP gain
settings. The switch is connected at the buffered output of
the LNP and has an “ON” resistance of approximately 1Ω.
When employing active feedback, the user should be careful
to avoid low-frequency instability or overload problems.
Figure 8 illustrates the various low-frequency time constants. Referring again to the input resistance calculation of
Equation (3), and considering that the gain term “A” falls
off below 3.6kHz, it is evident that the effective LNP input
impedance will rise below 3.6kHz, with a DC limit of
approximately RF. To avoid interaction with the feedback
pole/zero at low frequencies, and to avoid the higher signal
levels resulting from the rising impedance characteristic, it
is recommended that the external RFCC time constant be set
to about 5µs.
RF
VCA NOISE = 3.8nV√Hz, LNP GAIN = 20dB
9
7
Noise Figure (dB)
VCM
LNP Noise
nV/√Hz
6.0E-10
8.0E-10
1.0E-09
1.2E-09
1.4E-09
1.6E-09
1.8E-09
2.0E-09
8
6
5
4
3
CF
0.001µF
1MΩ
44pF
CC
Buffer
LNPOUTN
RS
44pF
2
LNPOUTP
1
0
0
100 200 300 400 500 600 700 800
FIGURE 6. Noise Figure for Active Termination.
1MΩ
900 1000
Source Impedance (Ω)
14
Buffer
Gain
Stage
(VCA) LNP
VCM
FIGURE 8. Low Frequency LNP Time Constants.
VCA2612
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SBOS117B
Achieving the best active feedback architecture is difficult
with conventional op amp circuit structures. The overall
gain “A” must be negative in order to close the feedback
loop, the input impedance must be high to maintain low
current noise and good gain accuracy, but the gain ratio must
be set with very low value resistors to maintain good voltage
noise. Using a two-amplifier configuration (noninverting for
high impedance plus inverting for negative feedback reasons) results in excessive phase lag and stability problems
when the loop is closed. The VCA2612 uses a patented
architecture that achieves these requirements, with the additional benefits of low power dissipation and differential
signal handling at both input and output.
For greatest flexibility and lowest noise, the user may wish
to shape the frequency response of the LNP. The COMP1
and COMP2 pins for each channel (pins 10 and 11 for
channel A, pins 26 and 27 for channel B) correspond to the
drains of Q3 and Q8 in Figure 4. A capacitor placed between
these pins will create a single-pole low-pass response, in
which the effective “R” of the “RC” time constant is approximately 186Ω.
COMPENSATION WHEN USING ACTIVE
FEEDBACK
The typical open-loop gain versus frequency characteristic
for the LNP is shown in Figure 9. The –3dB bandwidth is
approximately 180MHz and the phase response is such that
when feedback is applied the LNP will exhibit a peaked
response or might even oscillate. One method for compensating for this undesirable behavior is to place a compensation capacitor at the input to the LNP, as shown in Figure 10.
This method is effective when the desired –3dB bandwidth
is much less than the open-loop bandwidth of the LNP. This
compensation technique also allows the total compensation
capacitor to include any stray or cable capacitance that is
–3dB Bandwidth
Gain
25dB
180MHz
FIGURE 9. Open-Loop Gain Characteristic of LNP.
RF
RI
Input
C
A
Output
associated with the input connection. Equation 4 relates the
bandwidth to the various impedances that are connected to
the LNP.
BW =
(A + 1) R I + R F
2pC(R I )(R F )
(4)
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 impedance. 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 135Ω. 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 VCM for
compatibility with the VCA input.
VOLTAGE-CONTROLLED ATTENUATOR (VCA)—DETAIL
The VCA is designed to have a “dB-linear” attenuation
characteristic, i.e. the gain loss in dB is constant for each
equal increment of the VCACNTL control voltage. See
Figure 11 for a diagram of the VCA. The attenuator is
essentially a variable voltage divider consisting of one series
input resistor, RS, 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-controlled 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 VCM –VT (FET nearly “OFF”), where VCM is the common
source voltage and VT 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 piecewiselinear 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
RS and the parallel FET network.
The attenuator is comprised of two sections, with five
parallel clipping amplifier/FET combinations in each. Special reference circuitry is provided so that the (VCM –VT)
limit voltage will track temperature and IC process variations, minimizing the effects on the attenuator control characteristic.
In addition to the analog VCACNTL gain setting input, the
attenuator architecture provides digitally programmable adjustment in eight steps, via the three Maximum Gain Setting
(MGS) bits. These adjust the maximum achievable gain
FIGURE 10. LNP with Compensation Capacitor.
VCA2612
SBOS117B
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15
Attenuator
Input
RS
A1 - A10 Attenuator Stages
Attenuator
Output
QS
Q1
VCM
A1
Q2
A2
C1
A3
C2
V1
Q3
A4
C3
V2
Q4
A5
C4
V3
V4
Control
Input
Q5
Q6
A6
C5
A7
C6
V5
Q7
V6
Q8
A8
C7
V7
Q9
A9
C8
Q10
A10
C9
V8
C10
V9
V10
C1 - C10 Clipping Amplifiers
0dB
–4.5dB
Attenuation Characteristic of Individual FETs
VCM-VT
0
V1
V2
V3
V4
V5
V6
V7
V8
V9
Characteristic of Attenuator Control Stage Output
V10
OVERALL CONTROL CHARACTERISTICS OF ATTENUATOR
0dB
–4.5dB
0.3V
Control Signal
3V
FIGURE 11. Piecewise Approximation to Logarithmic Control Characteristics.
(corresponding to minimum attenuation in the VCA, with
VCACNTL = 3.0V) in 3dB increments. This function is
accomplished by providing multiple FET sub-elements for
each of the Q1 to Q10 FET shunt elements shown in
Figure 11. In the simplified diagram of Figure 12, each shunt
FET is shown as two sub-elements, QNA and QNB. Selector
16
switches, driven by the MGS bits, activate either or both of
the sub-element FETs to adjust the maximum RON and thus
achieve the stepped attenuation options.
The VCA can be used to process either differential or singleended signals. Fully differential operation will reduce 2ndharmonic distortion by about 10dB for full-scale signals.
VCA2612
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SBOS117B
RS
OUTPUT
INPUT
Q1A
Q1B
Q2A
Q2B
Q3A
Q3B
Q4A
Q4B
Q5A
Q5B
VCM
A1
A2
A3
A4
A5
B1
B2
PROGRAMMABLE ATTENUATOR SECTION
FIGURE 12. Programmable Attenuator Section.
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 (i.e., low gain
settings), the impedance will approach the series resistor
value of approximately 135Ω.
As with the LNP stage, the VCA output is AC coupled into
the PGA. This means that the attenuation-dependent DC
common-mode voltage will not propagate into the PGA, and
so the PGA’s DC output level will remain constant.
Finally, note that the VCACNTL input consists of FET gate
inputs. This provides very high impedance and ensures that
multiple VCA2612 devices may be connected in parallel
with no significant loading effects. The nominal voltage
range for the VCACNTL input spans from 0V to 3V. Over
driving this input (≤ 5V) does not affect the performance.
From VCA
Output
PGA
Comparators
Gain = A
Selection
Logic
E = Maximum Peak Amplitude
–
E E
A A
FIGURE 13. Overload Protection Circuitry.
VCACNTL = 0.2V, DIFFERENTIAL, MGS = 100, (0dB)
1V/div
Output
Input
200ns/div
FIGURE 14. Overload Recovery Response For Minimum Gain.
VCACNTL = 3.0V, DIFFERENTIAL, MGS = 100, (36dB)
Output
1V/div
OVERLOAD RECOVERY CIRCUITRY—DETAIL
With a maximum overall gain of 70dB, the VCA2612 is
prone to signal overloading. Such a condition may occur in
either the LNP or the PGA depending on the various gain
and attenuation settings available. The LNP is designed to
produce low-distortion outputs as large as 1Vp-p singleended (2Vp-p differential). Therefore the maximum input
signal for linear operation is 2Vp-p divided by the LNP
differential gain setting. Clamping circuits in the LNP ensure that larger input amplitudes will exhibit symmetrical
clipping and short recovery times. The VCA itself, being
basically a voltage divider, is intrinsically free of overload
conditions. However, the PGA post-amplifier is vulnerable
to sudden overload, particularly at high gain settings. Rapid
overload recovery is essential in many signal processing
applications such as ultrasound imaging. A special comparator circuit is provided at the PGA input which detects
overrange signals (detection level dependent on PGA gain
setting). When the signal exceeds the comparator input
threshold, the VCA output is blocked and an appropriate
fixed DC level is substituted, providing fast and clean
overload recovery. The basic architecture is shown in
Figure 13. Both high and low overrange conditions are
sensed and corrected by this circuit.
Figures 14 and 15 show typical overload recovery waveforms with MGS = 100, for VCA + PGA minimum gain
(0dB) and maximum gain (36dB), respectively. LNP gain is
set to 25dB in both cases.
Input
200ns/div
FIGURE 15. Overload Recovery Response For Maximum Gain.
VCA2612
SBOS117B
www.ti.com
17
INPUT OVERLOAD RECOVERY
One of the most important applications for the VCA2612 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. To prevent damage, it is necessary to place a
protection circuit between the transducer and the VCA2612,
as shown in Figure 16. Care must be taken to prevent any
signal from turning the ESD diodes on. Turning on the ESD
diodes inside the VCA2612 could cause the input coupling
capacitor (CC) to charge to the wrong value.
attenuation at that setting. Therefore, the VCA + PGA overall
gain will always be 0dB (unity) when the analog VCACNTL
input is set to 0V (= maximum attenuation). For VCACNTL = 3V
(no attenuation), the VCA + PGA gain will be controlled by the
programmed PGA gain (24dB to 45dB in 3dB steps).
For clarity, the gain and attenuation factors are detailed in
Table III.
MGS
ATTENUATOR GAIN DIFFERENTIAL
SETTING VCACNTL = 0V to 3V
PGA GAIN
000
001
010
011
100
101
110
111
VDD
CF
RF
–24dB to 0dB
–27dB to 0dB
–30dB to 0dB
–33dB to 0dB
–36dB to 0dB
–39dB to 0dB
–42dB to 0dB
–45dB to 0dB
24dB
27dB
30dB
33dB
36dB
39dB
42dB
45dB
ATTENUATOR +
DIFF. PGA GAIN
0dB to 24dB
0dB to 27dB
0dB to 30dB
0dB to 33dB
0dB to 36dB
0dB to 39dB
0dB to 42dB
0dB to 45dB
TABLE III. MGS Settings.
LNPINP
Protection
Network
LNP
The PGA architecture consists of a differential, programmable-gain voltage to current converter stage followed by
transimpedance amplifiers to create and buffer each side of
the differential output. The circuitry associated with the voltage to current converter is similar to that previously described
for the LNP, with the addition of eight selectable PGA gainsetting resistor combinations (controlled by the MGS bits) in
place of the fixed resistor network used in the LNP. Low input
noise is also a requirement of the PGA design due to the large
amount of signal attenuation which can be inserted between
the LNP and the PGA. At minimum VCA attenuation (used
for small input signals) the LNP 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.
LNPOUTN
ESD Diode
FIGURE 16. VCA2612 Diode Bridge Protection Circuit.
PGA POST-AMPLIFIER—DETAIL
Figure 17 shows a simplified circuit diagram of the PGA
block. As described previously, the PGA gain is programmed
with the same MGS bits which control the VCA maximum
attenuation factor. Specifically, the PGA gain at each MGS
setting is the inverse (reciprocal) of the maximum VCA
VDD
To Bias
Circuitry
Q1
RL
Q11
VCAOUTP
Q12
Q9
Q3
RL
VCAOUTN
Q8
VCM
RS1
VCM
Q13
RS2
Q4
+In
Q7
–In
Q14
Q2
Q10
Q5
Q6
To Bias
Circuitry
FIGURE 17. Simplified Block Diagram of the PGA Section Within the VCA2612.
18
VCA2612
www.ti.com
SBOS117B
PACKAGE DRAWING
MTQF019A – JANUARY 1995 – REVISED JANUARY 1998
PFB (S-PQFP-G48)
PLASTIC QUAD FLATPACK
0,27
0,17
0,50
36
0,08 M
25
37
24
48
13
0,13 NOM
1
12
5,50 TYP
7,20
SQ
6,80
9,20
SQ
8,80
Gage Plane
0,25
0,05 MIN
0°– 7°
1,05
0,95
Seating Plane
1,20 MAX
0,75
0,45
0,08
4073176 / B 10/96
NOTES: A. All linear dimensions are in millimeters.
B. This drawing is subject to change without notice.
C. Falls within JEDEC MS-026
VCA2612
SBOS117B
www.ti.com
19
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