Datasheet VCA2612Y-2K, VCA2612Y Datasheet (Burr Brown)

1

®

VCA2612

FEATURES

● LOW NOISE PREAMP:

• Low Input Noise: 1.25nV/√Hz

• Active Termination Noise Reduction

• Switchable Termination Value

• 80MHz Bandwidth

• 5dB to 25dB Gain

• 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

APPLICATIONS

● ULTRASOUND SYSTEMS

● WIRELESS RECEIVERS

● TEST EQUIPMENT

VCA2612

®

International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: [phone]

Twx: 910-952-1111 • Internet: http://www.burr-brown.com/ • Cable: BBRCORP • Telex: 066-6491 • FAX: [phone] • Immediate Product Info: [phone]

© 2000 Burr-Brown Corporation PDS-1541B Printed in U.S.A. March, 2000

TM

Dual, VARIABLE GAIN AMPLIFIER

with Low Noise Preamp

VCA2612

For most current data sheet and other product

information, visit www.burr-brown.com

Low Noise

Preamp

5dB to 25dB

Programmable

Gain Amplifier

24 to 45dB

Voltage
Controlled
Attenuator

Analog

Control

Maximum Gain

Select

RF

2

RF

1

FB

FBSW

LNP

IN

P

LNP

IN

N

LNP

GS1

LNP

GS2

LNP

GS3

LNP

Gain Set

Input

LNP

OUT

P

SEL

VCAINP

LNP

OUT

N VCAINN VCA

CNTL

FB

CNTL

VCA

OUT

P

VCA

OUT

N

MGS

1

MGS2MGS

3

Maximum Gain Select

VCA2612

(1 of 2 Channels)

DESCRIPTION

The VCA2612 is a highly integrated, dual receive chan­nel, signal processing subsystem. Each channel of the
product consists of a low noise pre-amplifier (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 Programmable Gain Amplifier 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 single-ended or differential mode
to drive high performance analog-to-digital 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.

2

®

VCA2612

The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.

SPECIFICATIONS

At TA = +25°C, V

DD

= 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 CONDITIONS MIN TYP MAX UNITS
PREAMPLIFIER

Input Resistance 600 kΩ
Input Capacitance 15 pF
Input Bias Current 1nA
CMRR f = 1MHz, VCA

CNTL

= 0.2V 50 dB

Maximum Input Voltage Preamp Gain = +5dB 1 Vp-p

Preamp Gain = +25dB 112 mVp-p

Input Voltage Noise

(1)

Preamp Gain = +5dB 3.5 nV/√Hz

Preamp Gain = +25dB 1.25 nV/√Hz
Input Current Noise Independent of Gain 350 fA/√Hz
Noise Figure, R

S

= 75Ω, RIN = 75Ω

(1)

RF = 550Ω, PreAmp Gain = 22dB, 6.2 dB

PGA Gain = 39dB

Bandwidth Gain = 22dB 80 MHz

PROGRAMMABLE VARIABLE GAIN AMPLIFIER

Peak Input Voltage Differential 2 Vp-p
–3dB Bandwidth 40 MHz
Slew Rate 300 V/µs
Output Signal Range R

L

≥ 500Ω Each Side to Ground 2.5 ±1V
Output Impedance f = 5MHz 1 Ω
Output Short-Circuit Current ±40 mA
Third Harmonic Distortion f = 5MHz, V

OUT

= 1Vp-p, VCA

CNTL

= 3.0V –45 –71 dBc

Second Harmonic Distortion f = 5MHz, V

OUT

= 1Vp-p, VCA

CNTL

= 3.0V –45 –63 dBc

IMD, Two-Tone V

OUT

= 2Vp-p, f = 1MHz –80 dBc

V

OUT

= 2Vp-p, f = 10MHz –80 dBc
1dB Compression Point f = 5MHz, Output Referred, Differential 6 Vp-p
Crosstalk

V

OUT

= 1Vp-p, f = 1MHz, Max Gain Both Channels

68 dB

Group Delay Variation 1MHz < f < 10MHz, Full Gain Range ±2ns

ACCURACY

Gain Slope 10.9 dB/V
Gain Error ±1

(2)

dB

Output Offset Voltage ±50 mV

GAIN CONTROL INTERFACE

Input Voltage (VCA

CNTL

) Range 0 to 3.0 V
Input Resistance 1MΩ
Response Time 45dB Gain Change, MGS = 111 0.2 µs

POWER SUPPLY

Specified Operating Range 4.75 5.0 5.25 V
Power Dissipation Operating, Both Channels 410 475 mW

NOTE: (1) For preamp driving VGA. (2) Referenced to best fit dB-linear curve.

3

®

VCA2612

ELECTROSTATIC
DISCHARGE SENSITIVITY

This integrated circuit can be damaged by ESD. Burr-Brown
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.

ABSOLUTE MAXIMUM RATINGS

Power Supply (+VS) ............................................................................. +6V

Analog Input ............................................................. –0.3V to (+V

S

+ 0.3V)

Logic Input ............................................................... –0.3V to (+V

S

+ 0.3V)

Case Temperature ......................................................................... +100°C

Junction Temperature .................................................................... +150°C

Storage Temperature...................................................... –40°C to +150°C

PACKAGE SPECIFIED
DRAWING TEMPERATURE PACKAGE ORDERING TRANSPORT

PRODUCT PACKAGE NUMBER RANGE MARKING NUMBER

(1)

MEDIA

VCA2612Y TQFP-48 Surface Mount 355 –40°C to +85°C A12 VCA2612Y/250 Tape and Reel

"""""VCA2612Y/2K "

NOTE: (1) Models with a slash (/) 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.

PACKAGE/ORDERING INFORMATION

4

®

VCA2612

PIN CONFIGURATION

1V

DD

A Channel A +Supply
2 NC Do Not Connect
3 NC Do Not Connect
4 VCA

IN

NA Channel A VCA Negative Input

5 VCA

IN

PA Channel A VCA Positive Input

6 LNP

OUT

NA Channel A LNP Negative Output

7 LNP

OUT

PA Channel A LNP Positive Output
8 SWFBA Channel A Switched Feedback Output
9 FBA Channel A Feedback Output

10 COMP1A Channel A Frequency Compensation 1
11 COMP2A Channel A Frequency Compensation 2
12 LNP

IN

NA Channel A LNP Inverting Input

13 LNP

GS3

A Channel A LNP Gain Strap 3

14 LNP

GS2

A Channel A LNP Gain Strap 2

15 LNP

GS1

A Channel A LNP Gain Strap 1

16 LNP

IN

PA Channel A LNP Noninverting Input

17 V

DD

R +Supply for Internal Reference

18 V

BIAS

0.01µF Bypass to Ground

19 V

CM

0.01µF Bypass to Ground
20 GNDR Ground for Internal Reference
21 LNP

IN

PB Channel B LNP Noninverting Input

22 LNP

GS1

B Channel B LNP Gain Strap 1

23 LNP

GS2

B Channel B LNP Gain Strap 2

24 LNP

GS3

B Channel B LNP Gain Strap 3

25 LNP

IN

NB Channel B LNP Inverting Input
26 COMP2B Channel B Frequency Compensation 2
27 COMP1B Channel B Frequency Compensation 1
28 FBB Channel B Feedback Output
29 SWFBB Channel B Switched Feedback Output
30 LNP

OUT

PB Channel B LNP Positive Output

31 LNP

OUT

NB Channel B LNP Negative Output

32 VCA

IN

PB Channel B VCA Positive Input
33 VCA

IN

NB Channel B VCA Negative Input
34 NC Do Not Connect
35 NC Do Not Connect
36 V

DD

B Channel B +Analog Supply
37 GNDB Channel B Analog Ground
38 VCA

OUT

NB Channel B VCA Negative Output

39 VCA

OUT

PB Channel B VCA Positive Output

40 MGS

3

Maximum Gain Select 3 (LSB)

41 MGS

2

Maximum Gain Select 2

42 MGS

1

Maximum Gain Select 1 (MSB)

43 VCA

CNTL

VCA Control Voltage

44 VCA

IN

SEL VCA Input Select, HI = External
45 FBSW

CNTL

Feedback Switch Control: HI = ON,

46 VCA

OUT

PA Channel A VCA Positive Output

47 VCA

OUT

NA Channel A VCA Negative Output

48 GNDA Channel A Analog Ground

PIN

DESIGNATOR

DESCRIPTION PIN

DESIGNATOR

DESCRIPTION

PIN DESCRIPTIONS

36
35
34
33
32
31
30
29
28
27
26
25

VDDB
NC
NC
VCA

IN

NB

VCA

IN

PB

LNP

OUT

NB

LNP

OUT

PB
SWFBB
FBB
COMP1B
COMP2B
LNP

IN

NB

GNDA

VCA

OUT

NA

VCA

OUT

PA

FBSW

CNTL

VCA

IN

SEL

VCA

CNTL

MGS1MGS2MGS3VCA

OUT

PB

VCA

OUT

NB

GNDB

LNP

GS3

A

LNP

GS2

A

LNP

GS1

A

LNP

IN

PA

V

DD

R

V

BIAS

V

CM

GNDR

LNP

IN

PB

LNP

GS1

B

LNP

GS2

B

LNP

GS3

B

1
2
3
4
5
6
7
8

9
10
11
12

VDDA

NC
NC

VCA

IN

NA

VCA

IN

PA

LNP

OUT

NA

LNP

OUT

PA

SWFBA

FBA
COMP1A
COMP2A

LNP

IN

NA

48 47 46 45 44 43 42

41 40 39 38

13 14 15 16 17 18 19 20 21 22 233724

VCA2612

5

®

VCA2612

TYPICAL PERFORMANCE CURVES

At TA = +25°C, V

DD

= 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.

GAIN vs VCA

CNTL

VCA

CNTL

(V)

0.2 1.21.00.4 0.6 0.8 1.8 2.0 2.21.61.4 2.4 2.6 2.8 3.0

Gain (dB)

65
60
55
50
45
40
35
30
25
20
15

MGS = 111

MGS = 110

MGS = 101

MGS = 100

MGS = 011

MGS = 010

MGS = 001

MGS = 000

GAIN ERROR vs TEMPERATURE

VCA

CNTL

(V)

0.2 1.0 1.20.80.4 0.6 2.0 2.21.4 1.6 1.8 2.4 2.6 2.8 3.0

Gain Error (dB)

2.0

1.5

1.0

0.5
0

–0.5
–1.0
–1.5
–2.0

+25°C

–40°C

+85°C

GAIN ERROR vs VCA

CNTL

VCA

CNTL

(V)

0.2 1.0 1.20.80.4 0.6 2.0 2.21.4 1.6 1.8 2.4 2.6 2.8 3.0

Gain Error (dB)

2.0

1.5

1.0

0.5
0

–0.5
–1.0
–1.5
–2.0

10MHz

1MHz

5MHz

GAIN ERROR vs VCA

CNTL

VCA

CNTL

(V)

0.2 1.0 1.20.80.4 0.6 2.0 2.21.4 1.6 1.8 2.4 2.6 2.8 3.0

Gain Error (dB)

2.0

1.5

1.0

0.5
0

–0.5
–1.0
–1.5
–2.0

MGS = 011

MGS = 000

MGS = 111

GAIN MATCH: CHA to CHB = 0.2V

Delta Gain (dB)

–0.5 –0.4 –0.3 –0.2 –0.1 0.0 0.1 0.2 0.3 0.4 0.5

Units

100

90
80
70
60
50
40
30
20
10

0

GAIN MATCH: CHA to CHB = 3.0V

Delta Gain (dB)

–0.5 –0.4 –0.3 –0.2 –0.1 0.0 0.1 0.2 0.3 0.4 0.5

Units

100

90
80
70
60
50
40
30
20
10

0

6

®

VCA2612

TYPICAL PERFORMANCE CURVES (Cont.)

At TA = +25°C, V

DD

= 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.

GAIN vs FREQUENCY

(Pre-Amp)

Frequency (MHz)

0.1 1 10 100

Gain (dB)

30

25

20

15

10

5

0

LNP = 25dB

LNP = 22dB

LNP = 17dB

LNP = 5dB

GAIN vs FREQUENCY

(VCA and PGA, VCA

CNTL

= 0.2V)

Frequency (MHz)

0.1 1 10 100

Gain (dB)

5.0

4.0

3.0

2.0

1.0

0.0
–1.0
–2.0
–3.0
–4.0
–5.0

MGS = 111
MGS = 100
MGS = 011
MGS = 000

GAIN vs FREQUENCY

(VCA and PGA, VCA

CNTL

= 3.0V)

Frequency (MHz)

0.1 1 10 100

Gain (dB)

45
40
35
30
25
20
15
10

5
0

MGS = 111

MGS = 100

MGS = 011

MGS = 000

GAIN vs FREQUENCY

(VCA

CNTL

= 3.0V)

Frequency (MHz)

0.1 1 10 100

Gain (dB)

60

50

40

30

20

10

0

LNP = 25dB

LNP = 22dB

LNP = 5dB

LNP = 17dB

GAIN vs FREQUENCY

(LNP = 22dB)

Frequency (MHz)

0.1 1 10 100

Gain (dB)

60

50

40

30

20

10

0

VCA

CNTL

= 3.0V

VCA

CNTL

= 1.6V

VCA

CNTL

= 0.2V

OUTPUT REFERRED NOISE vs VCA

CNTL

VCA

CNTL

(V)

0 1.0 1.20.4 0.6 0.8 1.8 2.01.4 1.6 2.2 2.4 2.6 2.8 3.0

Noise (nv/√Hz)

1800
1600
1400
1200
1000

800
600
400
200

0

RS= 50Ω

MGS = 111

MGS = 011

7

®

VCA2612

TYPICAL PERFORMANCE CURVES (Cont.)

At TA = +25°C, V

DD

= 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.

INPUT REFERRED NOISE vs R

S

RS (Ω)

1 10 100 1000

Noise (nV√Hz

10.0

1.0

0.1

NOISE FIGURE vs R

S

(VCA

CNTL

= 3.0V)

R

S

(Ω)

10 100 1000

Noise Figure (dB)

11
10

9
8
7
6
5
4
3
2
1
0

NOISE FIGURE vs VCA

CNTL

VCA

CNTL

(V)

Noise Figure (dB)

30

25

20

15

10

5

0

0.2 1.0 1.20.4 0.6 0.8 1.8 2.01.4 1.6 2.2 2.4 2.6 2.8 3.0

INPUT REFERRED NOISE vs VCA

CNTL

VCA

CNTL

(V)

0.2 1.0 1.20.4 0.6 0.8 1.8 2.01.4 1.6 2.2 2.4 2.6 2.8 3.0

Noise (nV/√Hz)

24
22
20
18
16
14
12
10

8
6
4
2
0

MGS = 011

MGS = 111

RS= 50Ω

LNP vs FREQUENCY

(Differential, 2Vp-p)

Frequency (MHz)

0.1 1 10 100

Harmonic Distortion (dBc)

–45

–50

–55

–60

–65

–70

–75

–80

3rd Harmonic

2nd Harmonic

LNP vs FREQUENCY
(Single-Ended, 1Vp-p)

Frequency (MHz)

0.1 1 10 100

Harmonic Distortion (dBc)

–45

–50

–55

–60

–65

–70

–75

–80

2nd Harmonic

3rd Harmonic

8

®

VCA2612

TYPICAL PERFORMANCE CURVES (Cont.)

At TA = +25°C, V

DD

= 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.

HARMONIC DISTORTION vs FREQUENCY

(Differential, 2Vp-p, MGS = 000)

Frequency (MHz)

0.1 1 10

Harmonic Distortion (dBc)

–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90

VCA

CNTL

= 0.2V, H2

VCA

CNTL

= 0.2V, H3

VCA

CNTL

= 3.0V, H2

VCA

CNTL

= 3.0V, H3

HARMONIC DISTORTION vs FREQUENCY

(Differential, 2Vp-p, MGS = 011)

Frequency (Hz)

0.1 1 10

Harmonic Distortion (dBc)

–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90

VCA

CNTL

= 0.2V, H2

VCA

CNTL

= 0.2V, H3

VCA

CNTL

= 3.0V, H2

VCA

CNTL

= 3.0V, H3

HARMONIC DISTORTION vs FREQUENCY

(Differential, 2Vp-p, MGS = 111)

Frequency (MHz)

0.1 1 10

Harmonic Distortion (dBc)

–30
–35
–40
–45
–50
–55
–60
–65
–70
–75
–80

VCA

CNTL

= 0.2V, H2

VCA

CNTL

= 0.2V, H3

VCA

CNTL

= 3.0V, H2

VCA

CNTL

= 3.0V, H3

HARMONIC DISTORTION vs FREQUENCY

(Single-Ended, 1Vp-p, MGS = 000)

Frequency (MHz)

0.1 1 10

Harmonic Distortion (dBc)

–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90

VCA

CNTL

= 0.2V, H2

VCA

CNTL

= 0.2V, H3

VCA

CNTL

= 3.0V, H2

VCA

CNTL

= 3.0V, H3

HARMONIC DISTORTION vs FREQUENCY

(Single-Ended, 1Vp-p, MGS = 011)

Frequency (MHz)

0.1 1 10

Harmonic Distortion (dBc)

–40
–45
–50
–55
–60
–65
–70
–75
–80
–85
–90

VCA

CNTL

= 0.2V, H2

VCA

CNTL

= 0.2V, H3

VCA

CNTL

= 3.0V, H2

VCA

CNTL

= 3.0V, H3

HARMONIC DISTORTION vs FREQUENCY

(Single-Ended, 1Vp-p, MGS = 111)

Frequency (MHz)

0.1 1 10

Harmonic Distortion (dBc)

–30
–35
–40
–45
–50
–55
–60
–65
–70
–75
–80
–85

VCA

CNTL

= 0.2V, H2

VCA

CNTL

= 0.2V, H3

VCA

CNTL

= 3.0V, H2

VCA

CNTL

= 3.0V, H3

9

®

VCA2612

INTERMODULATION DISTORTION

(Differential, 2Vp-p, f = 10MHz)

Frequency (MHz)

9.989.96 10 10.2 10.4

Power (dBm)

–5
–15
–25
–35
–45
–55
–65
–75
–85
–95

–105

TYPICAL PERFORMANCE CURVES (Cont.)

At TA = +25°C, V

DD

= 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.

–1dB COMPRESSION vs VCA

CNTL

VCA

CNTL

(V)

0.2 1.0 1.20.4 0.6 0.8 1.6 1.8 2.0 2.21.4 2.4 2.6 3.02.8

P

IN

(dBm)

0

–5
–10
–15
–20
–25
–30
–35
–40

3rd-ORDER INTERCEPT vs VCA

CNTL

VCA

CNTL

(V)

0.2 1.0 1.20.4 0.6 0.8 1.6 1.8 2.0 2.21.4 2.4 2.6 3.02.8

IP3 (dBm)

0

–5
–10
–15
–20
–25
–30
–35
–40
–45
–50

INTERMODULATION DISTORTION

(Single-Ended, 1Vp-p, f = 10MHz)

Frequency (MHz)

9.989.96 10 10.2 10.4

Power (dBm)

–5
–15
–25
–35
–45
–55
–65
–75
–85
–95

–105

HARMONIC DISTORTION vs VCA

CNTL

(Differential, 2Vp-p)

VCA

CNTL

(V)

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

Harmonic Distortion (dBc)

–45

–50

–55

–60

–65

–70

–75

–80

MGS = 000, H2
MGS = 011, H2
MGS = 111, H2
MGS = 000, H3
MGS = 011, H3
MGS = 111, H3

HARMONIC DISTORTION vs VCA

CNTL

(Single-Ended, 1Vp-p)

VCA

CNTL

(V)

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

Harmonic Distortion (dBc)

–45

–50

–55

–60

–65

–70

–75

–80

MGS = 000, H2
MGS = 011, H2
MGS = 111, H2
MGS = 000, H3
MGS = 011, H3
MGS = 111, H3

10

®

VCA2612

TYPICAL PERFORMANCE CURVES (Cont.)

At TA = +25°C, V

DD

= 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.

CMRR vs FREQUENCY

(LNP only)

Frequency (MHz)

0.1 1 10 100

CMRR (dB)

0
–10
–20
–30
–40
–50
–60
–70
–80

PULSE RESPONSE (BURSTS)

(Differential, VCA

CNTL

= 3.0V, MGS = 111)

200ns/div

Output

500mV/div

Input

10V/div

OVERLOAD RECOVERY

(Differential, VCA

CNTL

= 3.0V, MGS = 111)

Input

1V/div

Output

1V/div

200ns/div

GAIN RESPONSE

(Differential, VCA

CNTL

Pulsed, MGS = 111)

Output

500mV/div

Input

2V/div

100ns/div

CMRR vs FREQUENCY

(VCA only)

Frequency (MHz)

0.1 1 10 100

CMRR (dB)

0
–10
–20
–30
–40
–50
–60
–70
–80
–90

VCA

CNTL

= 0.2V

VCA

CNTL

= 1.4V

VCA

CNTL

= 3.0V

CROSS TALK vs FREQUENCY

(Single-Ended, 1Vp-p, MGS = 011)

Frequency (MHz)

0.1 1 10

Cross Talk (dB)

0
–10
–20
–30
–40
–50
–60
–70
–80
–90

VCA

CNTRL

= 0V

VCA

CNTRL

= 1.5V

VCA

CNTRL

= 3.0V

11

®

VCA2612

TYPICAL PERFORMANCE CURVES (Cont.)

At TA = +25°C, V

DD

= 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.

80

79.5
79

78.5
78

77.5
77

76.5
76

I

CC

vs TEMPERATURE

Temperature (°C)

–40 –10 5–25 35 5020 65

80 95

I

CC

(mA)

GROUP DELAY vs FREQUENCY

Frequency (MHz)

1 10 100

Group Delay (ns)

15
14
13
12
11
10

9
8
7
6
5
4
3
2
1
0

VCA

CNTL

= 3.0V

VCA

CNTL

= 0.2V

–45
–40
–35
–30
–25
–20
–15
–10

–5

0
5

10

PSRR vs FREQUENCY

Frequency (Hz)

1.E+01 1.E+031.E+02 1.E+05 1.E+061.E+04 1.E+07

PSRR (dB)

12

®

VCA2612

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 (VCA

CNTL

) 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 VCA

CNTL

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.

VCALNP

Channel A

Input

VCA

Control

PGA

Channel A

Output

External

In

A

Maximum

Gain

Select

MGS

Analog
Control

VCALNP

Channel B

Input

PGA

Channel B

Output

External

In

B

FIGURE 1. Simplified Block Diagram of the VCA2612.

FIGURE 2. Recommended Signal Coupling.

0

–24

VCA Attenuation (dB)

–45

Control Voltage

Maximum Attenuation

Maximum Attenuation

FIGURE 3. Swept Attenuator Characteristic.

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.

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

op amp. The “VCM” node shown in the drawing is the V

CM

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.

V

CM

1kΩ

1kΩ

47nF

To VCA

Input

Signal

13

®

VCA2612

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):

R

L

93Ω

R

S1

105Ω

Q

3

Q

4

Q

5

Q

2

Q

1

R

S2

34Ω

R

S3

17Ω

LNPG

S2

+IN –IN

To Bias

Circuitry

LNPG

S1

LNPG

S3

R

L

93Ω

V

DD

–Out

Buffer Buffer

+Out

Q

8

Q

7

To Bias

Circuitry

Q

6

Q

9

Q

10

FIGURE 4. Schematic of the Low Noise Pre-Amplifier (LNP).

Gain

R

R

L
S

=•











2

(1)

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 (24, 27, 30, 33, 36,

39. 42, 45) dB.
The “GAIN vs VCA

CNTL

” curve on page 5 shows the
composite gain control characteristic of the entire VCA2612.
Setting VCA

CNTL

to 3.0V causes the digital MGS gain

control to step in 3dB increments. Setting VCA

CNTL

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.

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 (46dB) to
59µs at the lowest gain (25dB).

14

®

VCA2612

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.)

Figure 5 shows 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):

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

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).

where R

EXT

is the externally selected resistor value needed
to achieve the desired gain setting, RS1 is the fixed parallel
resistor in Figure 4, and R

FIX

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 R

S

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., VCA

CNTL

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.

NOISE (nv/√Hz)

LNP GAIN (dB) Input-Referred Output-Referred

25 1.54 2260
22 1.59 1650
17 1.82 1060

5 4.07 597

T ABLE II. Noise Performance for MGS = 11 1 and VCA

CNTL

= 3.0V .

LNP PIN STRAPPING LNP GAIN (dB)

LNPG

S1

, LNPGS2, LNPGS3 Connected Together 25

LNPGS1 Connected to LNPG

S3

22

LNPG

S1

Connected to LNPG

S2

17

All Pins Open 5

TABLE I. Pin Strappings of the LNP for Various Gains.

(3)

(2)

R

R

A

IN

F

=

+

()

1

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 R

S

combinations are brought out to device pins LNPGS1, LNPG

S2

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.

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.

R

R R R R Gain R R

Gain R R

EXT

S L FIX L S FIX

SL

=

+•

•

22

2

11

1

–

–

15

®

VCA2612

R

F

A

R

IN

R

IN

=

R

S

R

S

R

S

= R

S

R

F

1 + A

Active Feedback

A

Conventional Cable Termination

FIGURE 5. Configurations for Active Feedback and Con-

ventional Cable Termination.

VCA NOISE = 3.8nV√Hz, LNP GAIN = 20dB

Source Impedance (Ω)

0 300100 200 500400 600 700 800

Noise Figure (dB)

9
8
7
6
5
4
3
2
1
0

♦

×

6.0E-10

8.0E-10

1.0E-09

1.2E-09

1.4E-09

1.6E-09

1.8E-09

2.0E-09

LNP Noise

nV/√Hz

×

♦

Source Impedance (Ω)

0 300100 200 500400 600 700 800

Noise Figure (dB)

14

12

10

8

6

4

2

0

6.0E-10

8.0E-10

1.0E-09

1.2E-09

1.4E-09

1.6E-09

1.8E-09

2.0E-09

LNP Noise

nV/√Hz

×

♦

♦

×

VCA NOISE = 3.8nV√Hz, LNP GAIN = 20dB

R

S

1MΩ

C

L

V

CM

R

F

44pF

Buffer

1MΩ

V

CM

44pF

Gain

Stage

Buffer

FIGURE 6. Noise Figure for Active Termination.

FIGURE 7. Noise Figure for Conventional Termination.

FIGURE 8. Low Frequency LNP Time Constants.

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 FBSW

CNTL

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.

16

®

VCA2612

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 (non-inverting
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.

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 VCA

CNTL

control voltage. Figure 9
shows a block 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 (V

CM –VT

)
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 ad­justment in eight steps, via the three Maximum Gain Setting
(MGS) bits. These adjust the maximum achievable gain
(corresponding to minimum attenuation in the VCA, with
VCA

CNTL

= 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

9. In the simplified diagram of Figure 10, each shunt FET is
shown as two sub-elements, QNA and QNB. Selector switches,
driven by the MGS bits, activate either or both of the sub­element FETs to adjust the maximum RON and thus achieve
the stepped attenuation options.

R

S

Q

1A

A1

B1

VCM

INPUT

OUTPUT

PROGRAMMABLE ATTENUATOR SECTION

B2

Q

1B

Q

2A

A2

Q

2B

Q

3A

A3

Q

3B

Q

4A

A4

Q

4B

Q

5A

A5

Q

5B

FIGURE 10. Programmable Attenuator Section.

17

®

VCA2612

R

S

Attenuator

Input

Attenuator

Output

A1 - A10 Attenuator Stages

Control

Input

Q

1

V

CM

0dB

–4.5dB

Q

2

Q

3

C

1

V1

Q

4

Q

5

Q

S

C1 - C10 Clipping Amplifiers

Attenuation Characteristic of Individual FETs

Q

6

Q

7

Q

8

Q

9

Q

10

C

2

V2

VCM-V

T

0

V1 V2 V3 V4 V5 V6 V7 V8 V9 V10

Characteristic of Attenuator Control Stage Output

OVERALL CONTROL CHARACTERISTICS OF ATTENUATOR

–4.5dB

0dB

0.3V 3V

Control Signal

C

3

V3

C

4

V4

C

5

V5

C

6

V6

C

7

V7

C

8

V8

C

9

V9

C

10

V10

A1 A2 A3 A4 A5 A6 A7 A8 A9 A10

FIGURE 9. Piecewise Approximation to Logarithmic Control Characteristics.

18

®

VCA2612

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.

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 VCA

CNTL

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.

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

FIGURE 12. Overload Recovery Response For Minimum Gain. FIGURE 13. Overload Recovery Response For Maximum Gain.

VCA

CNTL

= 0.2V, DIFFERENTIAL, MGS = 100

200ns/div

1V/div

Output

Input

VCA

CNTL

= 3.0V, DIFFERENTIAL, MGS = 100

200ns/div

1V/div

Output

Input

FIGURE 11. Overload Protection Circuitry.

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

11. Both high and low overrange conditions are sensed and
corrected by this circuit.

Figures 12 and 13 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.

Comparators

E = Maximum Peak Amplitude
E
A

–

From VCA

Selection

Logic

PGA

Gain = A

Output

E
A

19

®

VCA2612

MGS VCA GAIN min to max DIFFERENTIAL VCA • PGA GAIN

SETTING VCA

CNTL

= 0V to 3V PGA GAIN min to max

000 –24dB to 0dB 24dB 0dB to 24dB
001 –27dB to 0dB 27dB 0dB to 27dB
010 –30dB to 0dB 30dB 0dB to 30dB
011 –33dB to 0dB 33dB 0dB to 33dB
100 –36dB to 0dB 36dB 0dB to 36dB
101 –39dB to 0dB 39dB 0dB to 39dB
110 –42dB to 0dB 42dB 0dB to 42dB
101 –45dB to 0dB 45dB 0dB to 45dB

TABLE III. MGS Settings.

PGA POST-AMPLIFIER—DETAIL

Figure 14 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
attenuation at that setting. Therefore, the VCA • PGA
overall gain will always be 0dB (unity) when the analog
VCA

CNTL

input is set to 0V (= maximum attenuation). For

VCA

CNTL

= 3V (no attenuation), the VCA • PGA gain will
be controlled by the programmed PGA gain (24 to 45 dB in
3dB steps).

For clarity, the gain and attenuation factors are detailed in
Table III.

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 gain-setting 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.

R

S1

R

L

R

S2

+Out

+In

Q

11

Q

3

Q

4

Q

5

Q

1

V

CM

Q

2

–Out

Q

9

Q

8

Q

13

Q

14

Q

7

Q

6

Q

12

V

DD

V

CM

R

L

Q

10

–In

To Bias

Circuitry

To Bias

Circuitry

FIGURE 14. Simplified Block Diagram of the PGA.