AD AD605B Dual, low noise, single-supply variable gain amplifier Datasheet

a
FEATURES
Two Independent Linear-in-dB Channels
Input Noise at Maximum Gain: 1.8 nV/√Hz, 2.7 pA/√Hz
Bandwidth: 40 MHz (–3 dB)
Differential Input
Absolute Gain Range Programmable:
–14 dB to +34 dB (FBK Shorted to OUT) Through
0 dB to +48 dB (FBK Open)
Variable Gain Scaling: 20 dB/V Through 40 dB/V
Stable Gain with Temperature and Supply Variations
Single-Ended Unipolar Gain Control
Output Common-Mode Independently Set
Power Shutdown at Lower End of Gain Control
Single 5 V Supply
Low Power: 90 mW/Channel
Drives A/D Converters Directly
Dual, Low Noise, Single-Supply
Variable Gain Amplifier
AD605
FUNCTIONAL BLOCK DIAGRAM
VGN
VREF
GAIN
CONTROL
AND
SCALING
PRECISION PASSIVE
INPUT ATTENUATOR
FIXED GAIN
AMPLIFIER
+34.4dB
OUT
FBK
VOCM
+IN
–IN
DIFFERENTIAL
ATTENUATOR
0 TO –48.4dB
AD605
APPLICATIONS
Ultrasound and Sonar Time-Gain Control
High Performance AGC Systems
Signal Measurement
GENERAL DESCRIPTION
The AD605 is a low noise, accurate, dual channel, linear-in-dB
variable gain amplifier, which is optimized for any application
requiring high performance, wide bandwidth variable gain control. Operating from a single 5 V supply, the AD605 provides
differential inputs and unipolar gain control for ease of use.
Added flexibility is achieved with a user-determined gain range
and an external reference input which provides user-determined
gain scaling (dB/V).
The high performance linear-in-dB response of the AD605 is
achieved with the differential input, single supply, exponential
amplifier (DSX-AMP) architecture. Each of the DSX-AMPs
comprise a variable attenuator of 0 dB to –48.4 dB followed by
a high speed fixed gain amplifier. The attenuator is based on a
7-stage R-1.5R ladder network. The attenuation between tap
points is 6.908 dB, and 48.360 dB for the entire ladder network.
The DSX-AMP architecture results in 1.8 nV/√Hz input noise
spectral density and will accept a ± 2.0 V input signal when
VOCM is biased at VP/2.
Each independent channel of the AD605 provides a gain range
of 48 dB which can be optimized for the application. Gain
ranges between –14 dB to +34 dB and 0 dB to +48 dB can be
selected by a single resistor between pins FBK and OUT. The
lower and upper gain ranges are determined by shorting pin
FBK to OUT, or leaving pin FBK unconnected, respectively.
The two channels of the AD605 can be cascaded to provide 96
dB of very accurate gain range in a monolithic package.
The gain control interface provides an input resistance of approximately 2 MΩ and scale factors from 20 dB/V to 30 dB/V for a
VREF input voltage of 2.5 V to 1.67 V, respectively. Note that
scale factors up to 40 dB/V are achievable with reduced accuracy for scales above 30 dB/V. The gain scales linearly in dB
with control voltages (VGN) of 0.4 V to 2.4 V for the 20 dB/V
scale and 0.20 V to 1.20 V for the 40 dB/V scale. When VGN
is <50 mV the amplifier is powered down to draw 1.9 mA.
Under normal operation, the quiescent supply current of each
amplifier channel is only 18 mA.
The AD605 is available in 16-lead PDIP and SOIC, and is
guaranteed for operation over the –40°C to +85°C temperature range.
REV. C
Information furnished by Analog Devices is believed to be accurate and
reliable. However, no responsibility is assumed by Analog Devices for its
use, nor for any infringements of patents or other rights of third parties that
may result from its use. No license is granted by implication or otherwise
under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
www.analog.com
Fax: 781/326-8703
© 2004 Analog Devices, Inc. All rights reserved.
(Each channel @ TA = 25ⴗC, VS = 5 V, RS = 50 ⍀, RL = 500 ⍀, CL = 5 pF, VREF = 2.5 V
AD605–SPECIFICATIONS (Scaling = 20 dB/V), –14 dB to +34 dB gain range, unless otherwise noted.)
Model
Parameter
INPUT CHARACTERISTICS
Input Resistance
Input Capacitance
Peak Input Voltage
Input Voltage Noise
Input Current Noise
Noise Figure
Common-Mode Rejection Ratio
OUTPUT CHARACTERISTICS
–3 dB Bandwidth
Slew Rate
Output Signal Range
Output Impedance
Output Short-Circuit Current
Harmonic Distortion
HD2
HD3
HD2
HD3
Two-Tone Intermodulation
Distortion (IMD)
1 dB Compression Point
Third Order Intercept
Channel-to-Channel Crosstalk
Group Delay Variation
VOCM Input Resistance
ACCURACY
Absolute Gain Error
–14 dB to –11 dB
–11 dB to +29 dB
+29 dB to +34 dB
Gain Scaling Error
Output Offset Voltage
Output Offset Variation
GAIN CONTROL INTERFACE
Gain Scaling Factor
Gain Range
Input Voltage (VGN) Range
Input Bias Current
Input Resistance
Response Time
POWER SUPPLY
Supply Voltage
Power Dissipation
VREF Input Resistance
Quiescent Supply Current
Power Down
Power-Up Response Time
Power-Down Response Time
AD605A
Min Typ
Max
Conditions
At Minimum Gain
VGN = 2.9 V
VGN = 2.9 V
RS = 50 Ω, f = 10 MHz, VGN = 2.9 V
RS = 200 Ω, f = 10 MHz, VGN = 2.9 V
f = 1 MHz, VGN = 2.65 V
Constant with Gain
VGN = 1.5 V, Output = 1 V Step
RL ≥ 500 Ω
f = 10 MHz
VGN = 1 V, VOUT = 1 V p-p,
f = 1 MHz
f = 1 MHz
f = 10 MHz
f = 10 MHz
R S = 0 Ω, VGN = 2.9 V, VOUT = 1 V p-p
f = 1 MHz
f = 10 MHz
f = 10 MHz, VGN = 2.9 V, Output Referred
f = 10 MHz, VGN = 2.9 V, VOUT = 1 V p-p,
Input Referred
Ch1: VGN = 2.65 V, Inputs Shorted,
Ch2: VGN = 1.5 V (Mid Gain), f = 1 MHz,
VOUT = 1 V p-p
1 MHz < f < 10 MHz, Full Gain Range
175 ± 40
3.0
2.5 ± 2.5
1.8
2.7
8.4
12
–20
Ω
pF
V
nV/√Hz
pA/√Hz
dB
dB
dB
40
170
2.5 ± 1.5
2
± 40
40
170
2.5 ± 1.5
2
± 40
MHz
V/µs
V
Ω
mA
–64
–68
–51
–53
–64
–68
–51
–53
dBc
dBc
dBc
dBc
–72
–60
+15
–1
–72
–60
+15
–1
dBc
dBc
dBm
dBm
–70
–70
dB
± 2.0
45
± 2.0
45
ns
kΩ
–1.2
–1.0
–3.5
VREF = 2.5 V, 0.4 V < VGN < 2.4 V
VREF = 1.67 V
FBK Short to OUT
FBK Open
20 dB/V, VREF = 2.5 V
19
20
21
30
–14 – +34
0 – +48
0.1 – 2.9
–0.4
2
0.2
4.5
5.0
90
10
18
1.9
0.6
0.4
48 dB Gain Change
VPOS
VPOS, VGN < 50 mV
48 dB Gain, VOUT = 2 V p-p
–2–
Max Unit
175 ± 40
3.0
2.5 ± 2.5
1.8
2.7
8.4
12
–20
0.25 V < VGN < 0.40 V
0.40 V < VGN < 2.40 V
2.40 V < VGN < 2.65 V
0.4 V < VGN < 2.4 V
VREF = 2.500 V, VOCM = 2.500 V
VREF = 2.500 V, VOCM = 2.500 V
–50
AD605B
Min Typ
+1.0
± 0.3
–1.25
± 0.25
± 30
30
+3.0
+1.0
+1.2
50
95
5.5
23
3.0
–1.2 +0.75
–1.0 ± 0.2
–3.5 –1.25
± 0.25
–50 ± 30
30
+3.0
+1.0
+1.2
50
50
dB
dB
dB
dB/V
mV
mV
19
20
21
30
–14 – +34
0 – +48
0.1 – 2.9
–0.4
2
0.2
dB/V
dB/V
dB
dB
V
µA
MΩ
µs
4.5
5.0
90
10
18
1.9
0.6
0.4
V
mW
kΩ
mA
mA
µs
µs
5.5
23
3.0
REV. C
AD605
ABSOLUTE MAXIMUM RATINGS*
PIN CONFIGURATION
Supply Voltage +VS
Pins 12, 13 (with Pins 4, 5 = 0 V) . . . . . . . . . . . . . . . 6.5 V
Input Voltage Pins 1–3, 6–9, 16 . . . . . . . . . . . . . . . . VPOS, 0
Internal Power Dissipation
Plastic (N) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 W
Small Outline (R) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 W
Operating Temperature Range . . . . . . . . . . . –40°C to +85°C
Storage Temperature Range . . . . . . . . . . . . –65°C to +150°C
Lead Temperature, Soldering 60 seconds . . . . . . . . . . 300°C
VGN1 1
16 VREF
–IN1 2
15 OUT1
+IN1 3
GND1 4
14 FBK1
AD605
13 VPOS
TOP VIEW
GND2 5 (Not to Scale) 12 VPOS
*Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the
device at these or any other conditions above those indicated in the operational
section of this specification is not implied. Exposure to absolute maximum rating
conditions for extended periods may affect device reliability.
+IN2 6
11 FBK2
–IN2 7
10 OUT2
VGN2 8
9 VOCM
ORDERING GUIDE
Model
Temperature Range
Package Description
Package Option
␪JA
AD605AN
AD605AR
AD605AR-REEL
AD605AR-REEL7
AD605BN
AD605BR
AD605BR-REEL
AD605BR-REEL7
AD605ACHIPS
AD605-EB
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
–40°C to +85°C
PDIP
SOIC
SOIC 13" Reel
SOIC 7" Reel
PDIP
SOIC
SOIC 13" Reel
SOIC 7" Reel
DIE
Evaluation Board
N-16
R-16
R-16
R-16
N-16
R-16
R-16
R-16
85°C/W
100°C/W
100°C/W
100°C/W
85°C/W
100°C/W
100°C/W
100°C/W
PIN FUNCTION DESCRIPTIONS
16-Lead Package for Dual Channel AD605
Pin No.
Mnemonic
Description
1
VGN1
2
3
4
5
6
7
8
–IN1
+IN1
GND1
GND2
+IN2
–IN2
VGN2
9
10
11
12
13
14
15
16
VOCM
OUT2
FBK2
VPOS
VPOS
FBK1
OUT1
VREF
CH1 Gain-Control Input and Power-Down Pin. If grounded, device is off, otherwise positive voltage
increases gain.
CH1 Negative Input.
CH1 Positive Input.
Ground.
Ground.
CH2 Positive Input.
CH2 Negative Input.
CH2 Gain-Control Input and Power-Down Pin. If grounded, device is off, otherwise positive voltage
increases gain.
Input to this pin defines common-mode voltage for OUT1 and OUT2.
CH2 Output.
Feedback Pin that Selects Gain Range of CH2.
Positive Supply.
Positive Supply.
Feedback Pin that Selects Gain Range of CH1.
CH1 Output.
Input to this pin sets gain-scaling for both channels: 2.5 V = 20 dB/V, 1.67 V = 30 dB/V.
CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily
accumulate on the human body and test equipment and can discharge without detection. Although
the AD605 features proprietary ESD protection circuitry, permanent damage may occur on devices
subjected to high-energy electrostatic discharges. Therefore, proper ESD precautions are
recommended to avoid performance degradation or loss of functionality.
REV. C
–3–
AD605–Typical Performance Characteristics (per Channel)
(VREF = 2.5 V (20 dB/V Scaling), f = 1 MHz, RL = 500 ⍀, CL = 5 pF, TA = 25ⴗC, VSS = 5 V)
50
40
–40 C, +25 C, +85 C
40
40
30
30
FBK (OPEN)
GAIN (dB)
GAIN (dB)
20
10
20
FBK (SHORT)
10
0
0.9
0.5
1.3 1.7
VGN (V)
2.1
2.5
–20
0.1
2.9
0.5
0.9
1.3 1.7
VGN (V)
2.1
2.5
–20
0.1
2.9
TPC 2. Gain vs. VGN for Different
Gain Ranges
TPC 1. Gain vs. VGN
0.5
0.9
1.3 1.7
VGN (V)
2.1
2.5
2.9
TPC 3. Gain vs. VGN for Different
Gain Scalings
2.0
3.0
40.0
20dB/V
(VREF = 2.50V)
10
–10
–10
–20
0.1
ACTUAL
0
0
–10
ACTUAL
30dB/V
(VREF = 1.67V)
20
GAIN (dB)
30
2.5
37.5
1.5
27.5
25.0
GAIN ERROR (dB)
GAIN ERROR (dB)
ACTUAL
30.0
1.0
1.0
–40 C
0.5
0.0
–0.5
+25 C
+85 C
–1.0
–1.5
f = 1MHz
0.5
0.0
f = 5MHz
–0.5
f = 10MHz
–1.0
–2.0
22.5
20.0
1.25
–1.5
–2.5
1.50
1.75
2.00
VREF (V)
2.25
2.50
TPC 4. Gain Scaling vs. VREF
–3.0
0.2
0.7
1.2
1.7
VGN (V)
2.2
2.7
–2.0
0.2
N = 50
⌬G(dB) =
G(CH1) – G(CH2)
18
16
18
16
2.7
2.2
N = 50
⌬G(dB) =
G(CH1) – G(CH2)
14
PERCENTAGE
14
12
10
8
6
12
10
8
6
4
4
2
2
–0.8 –0.6 –0.4 –0.2
0.0
0.2
0.4
0.6
0.8
DELTA GAIN (dB)
TPC 7. Gain Error vs. VGN for
Different Gain Scalings
1.2
1.7
VGN (V)
20
20
0
0.7
TPC 6. Gain Error vs. VGN at
Different Frequencies
TPC 5. Gain Error vs. VGN at
Different Temperatures
PERCENTAGE
GAIN SCALING (dBV)
35.0
32.5
1.5
2.0
THEORETICAL
TPC 8. Gain Match, VGN1 = VGN2 =
1.0 V
–4–
0
–0.8 –0.6 –0.4 –0.2
0.0
0.2
0.4
0.6
0.8
DELTA GAIN (dB)
TPC 9. Gain Match, VGN1 = VGN2 =
2.50 V
REV. C
AD605
60
2.525
VGN = 2.9V (FBK = OPEN)
+85 C
2.515
VGN = 1.5V (FBK = OPEN)
120
+25 C
VOS (V)
VGN = 1.5V (FBK = SHORT)
VGN = 0.1V (FBK = OPEN)
0
VGN = 0.1V (FBK = SHORT)
–20
NOISE (nV/ Hz)
2.510
20
GAIN (dB)
125
–40 C
VGN = 2.9V (FBK = SHORT)
40
130
VOCM = 2.50V
2.520
2.505
2.500
2.495
+85 C
2.490
VGN = 0.0V
110
95
2.480
–60
100k
10M
1M
FREQUENCY (Hz)
2.475
100M
0
TPC 10. AC Response
0.5
1.0
1.5
2.0
VGN (V)
2.5
90
3.0
TPC 11. Output Offset vs. VGN
1000
–40 C
105
100
2.485
–40
+25 C
115
0
0.5
1.0
1.5
2.0
VGN (V)
2.5
3.0
TPC 12. Output Referred Noise vs.
VGN
2.00
1.90
VGN = 2.9V
VGN = 2.9V
1.95
1.85
10
NOISE (nV/ Hz)
NOISE (nV/ Hz)
NOISE (nV/ Hz)
1.90
100
1.85
1.80
1.75
1.80
1.75
1.70
1.70
1.65
1.65
1
0.1
0.5
0.9
1.3 1.7
VGN (V)
2.1
2.5
2.9
TPC 13. Input Referred Noise vs.
VGN
1.60
–40
–20
0
20
40
60
TEMPERATURE ( C)
80 90
TPC 14. Input Referred Noise vs.
Temperature
100
1.60
100k
60
RS = 50⍀
VGN = 2.9V
50
NOISE FIGURE (dB)
RSOURCE ALONE
NOISE FIGURE (dB)
NOISE (nV/ Hz)
25
10
10M
TPC 15. Input Referred Noise vs.
Frequency
30
VGN = 2.9V
1
1M
FREQUENCY (Hz)
20
15
40
30
20
10
10
0.1
1
10
100
Frequency (⍀)
1k
TPC 16. Input Referred Noise vs.
RSOURCE
REV. C
5
1
10
100
RSOURCE (⍀)
TPC 17. Noise TPC vs. RSOURCE
–5–
1k
0
0.1
0.5
0.9
1.3 1.7
VGN (V)
2.1
2.5
TPC 18. Noise TPC vs. VGN
2.9
AD605
–35
–40
–45
–50
HD3
–55
HD2
–60
–65
–70
100k
10M
1M
FREQUENCY (Hz)
–20
–40
–30
–50 HD2
(1MHz)
f = 10MHz
VO = 1V p-p
VGN = 1.0V
–40
HD3
(10MHz)
–45
–50
HD2
(10MHz)
–55
–60
–60
–70
–80
–90
–65
–100
–70
–75
0.5
100M
TPC 19. Harmonic Distortion vs.
Frequency
–35
POUT (dBm)
VO = 1V p-p
VGN = 1.0V
HARMONIC DISTORTION (dBc)
HARMONIC DISTORTION (dBc)
–30
HD3
(1MHz)
0.8
–110
1.1
1.4
1.7 2.0
VGN (V)
2.3
2.6
TPC 20. Harmonic Distortion vs. VGN
15
–120
2.9
9.92
9.96
10
10.02
FREQUENCY (MHz)
TPC 21. Intermodulation Distortion
2V
35
VO = 1V p-p
–5
–10
20
f = 10MHz
15
10
5
FREQ = 10MHz
FREQ = 1MHz
–15
–20
0.1
f = 1MHz
0.5
0.9
1.3 1.7
VGN (V)
400mV / DIV
0
25
INPUT
GENERATOR
LIMIT = 21 dBm
INTERCEPT (dBm)
PIN (dBm)
5
VO = 2V p-p
VGN = 1.5V
30
10
10.04
0
2.1
2.5
–5
0.6
2.9
TPC 22. 1 dB Compression vs. VGN
1
1.4
1.8
2.2
VGN (V)
2.6
–2V
253ns
3
TPC 23. Third Order Intercept vs. VGN
1.253␮s
100ns / DIV
TPC 24. Large Signal Pulse Response
200
500mV
2.9V
500mV
2.9V
100
90
10
10
0%
TRIG'D
–200
253ns
0.0V
100ns / DIV
100
90
VGN(V)
40mV / DIV
VGN (V)
VO = 200mV p-p
VGN = 1.5V
0.1V
500mV
200ns
0%
500mV
100ns
1.253␮s
TPC 25. Small Signal Pulse Response
TPC 26. Power-Up/Down Response
–6–
TPC 27. Gain Response
REV. C
AD605
–30
180
VIN = 0dBm
VGN = 2.9V
175
–10
–20
VGN2 = 2.9V
–60
VGN2 = 2.5V
–70
VGN2 = 2.0V
–80
–90
100k
INPUT IMPEDANCE (⍀)
VGN = 2.9V
–50
CMRR (dB)
CROSSTALK (dB)
–40
0
VGN1 = 1V
VOUT1 = 1V p-p
VIN2 = GND
VGN = 2.5V
–30
VGN = 2.0V
–40
VGN = 0.1V
–50
170
165
160
155
150
145
VGN2 = 0.1V
1M
10M
FREQUENCY (Hz)
100M
TPC 28. Crosstalk (CH1 to CH2) vs.
Frequency
–60
100k
10M
1M
FREQUENCY (Hz)
100M
TPC 29. Common-Mode Rejection vs.
Frequency
25
140
100k
1M
10M
FREQUENCY (Hz)
TPC 30. Input Impedance vs.
Frequency
16
+IS (AD605)
14
12
15
DELAY (ns)
SUPPLY CURRENT (mA)
20
10
VGN = 0.1V
5
+IS (VGN = 0)
0
–40
6
VGN = 2.9V
–20
0
20
40
60
TEMPERATURE ( C)
4
100k
80 90
10M
1M
FREQUENCY (Hz)
100M
TPC 32. Group Delay vs. Frequency
TPC 31. Supply Current (One
Channel) vs. Temperature
REV. C
10
8
–7–
100M
AD605
VREF
VGN
GAIN
CONTROL
DISTRIBUTED GM
175⍀
C1
+IN
DIFFERENTIAL
ATTENUATOR
EXT
C2
G1
–IN
VPOS
Ao
G2
R3
200k⍀
R2
20⍀
VOCM
C3
OUT
175⍀
3.36k⍀
R1
820⍀
FBK
R4
200k⍀
EXT
Figure 1. Simplified Block Diagram of a Single Channel of the AD605
determined by the midpoint between +VCC and GND, so care
should be taken to control the supply voltage to 5 V. The input
resistance looking into the VREF pin is 10 kΩ ± 20%.
THEORY OF OPERATION
The AD605 is a dual channel, low noise variable gain amplifier.
Figure 1 shows the simplified block diagram of one channel.
Each channel consists of a single-supply X-AMP® (hereafter
called DSX, differential single-supply X-AMP) comprised of
The AD605 is a single-supply circuit and the VOCM pin is used
to establish the dc level of the midpoint of this portion of the
circuit. VOCM needs only an external decoupling capacitor to
ground to center the midpoint between the supply voltages (5 V,
GND); however if the dc level of the output is important to the
user (see Applications section for the AD9050 data sheet example),
then VOCM can be specifically set. The input resistance looking into the VOCM pin is 45 kΩ ± 20%.
(a) precision passive attenuator (differential ladder)
(b) gain control block
(c) VOCM buffer with supply splitting resistors R3 and R4
(d) active feedback amplifier1 (AFA) with gain setting
resistors R1 and R2
Differential Ladder (Attenuator)
The linear-in-dB gain response of the AD605 can generally be
described by Equation 1.
G (dB) = (Gain Scaling (dB/V)) × (Gain Control (V)) –
(19 dB – (14 dB) × (FB))
The attenuator before the fixed gain amplifier is realized by a
differential 7-stage R-1.5R resistive ladder network with an
untrimmed input resistance of 175 Ω single-ended or 350 Ω
differentially. The signal applied at the input of the ladder
network (Figure 2) is attenuated by 6.908 dB per tap; thus, the
attenuation at the first tap is 6.908 dB, at the second, 13.816 dB,
and so on all the way to the last tap where the attenuation is
48.356 dB. A unique circuit technique is used to interpolate
continuously between the tap points, thereby providing continuous attenuation from 0 dB to –48.36 dB. One can think of the
ladder network together with the interpolation mechanism as a
voltage-controlled potentiometer.
(1)
where FB = 0 if FBK-to-OUT are shorted,
FB = 1 if FBK-to-OUT is open.
Each channel provides between –14 dB to +34.4 dB through
0 dB to +48.4 dB of gain depending on the value of the resistance
connected between pin FBK and OUT. The center 40 dB of
gain is exactly linear-in-dB while the gain error increases at the
top and bottom of the range. The gain is set by the gain control
voltage (VGN). The VREF input establishes the gain scaling.
The useful gain scaling range is between 20 dB/V and 40 dB/V for
a VREF voltage of 2.5 V and 1.25 V, respectively. For example,
if FBK to OUT were shorted and VREF were set to 2.50 V (to
establish a gain scaling of 20 dB/V), the gain equation would
simplify to
G (dB) = (20 (dB/V )) × (VGN (V )) – 19 dB
Since the DSX is a single-supply circuit, some means of biasing
its inputs must be provided. Node MID together with the
VOCM buffer performs this function. Without internal biasing,
external biasing would be required. If not done carefully, the
biasing network can introduce additional noise and offsets. By
providing internal biasing, the user is relieved of this task and
only needs to ac couple the signal into the DSX. It should be
made clear again that the input to the DSX is still fully differential if driven differentially, i.e., pins +IN and –IN see the same
signal but with opposite polarity. What changes is the load as
seen by the driver; it is 175 Ω when each input is driven singleended, but 350 Ω when driven differentially. This can be easily
explained when thinking of the ladder network as just two 175 Ω
resistors connected back-to-back with the middle node, MID,
being biased by the VOCM buffer. A differential signal applied
between nodes +IN and –IN will result in zero current into
node MID, but a single-ended signal applied to either input
+IN or –IN while the other input is ac grounded will cause the
current delivered by the source to flow into the VOCM buffer
via node MID.
(2)
The desired gain can then be achieved by setting the unipolar
gain control (VGN) to a voltage within its nominal operating
range of 0.25 V to 2.65 V (for 20 dB/V gain scaling). The gain is
monotonic for a complete gain control range of 0.1 V to 2.9 V.
Maximum gain can be achieved at a VGN of 2.9 V.
Since the two channels are identical, only Channel 1 will be
used to describe their operation. VREF and VOCM are the only
inputs that are shared by the two channels, and since they are
normally ac grounds, crosstalk between the two channels is
minimized. For highest gain scaling accuracy, VREF should
have an external low impedance voltage source. For low accuracy 20 dB/V applications, the VREF input can be decoupled with
a capacitor to ground. In this mode the gain scaling will be
1
To understand the active-feedback amplifier topology, refer to the AD830 data
sheet. The AD830 is a practical implementation of the idea.
–8–
REV. C
AD605
R
–6.908dB
R
+IN
–13.82dB
R
–20.72dB
1.5R
1.5R
R
–27.63dB
1.5R
R
–34.54dB
R
1.5R
1.5R
–41.45dB
R
–48.36dB
1.5R
1.5R
175⍀
1.5R
175⍀
MID
1.5R
R
1.5R
R
1.5R
1.5R
R
R
1.5R
1.5R
R
R
R
–IN
NOTE: R = 96⍀
1.5R = 144⍀
Figure 2. R-1.5R Dual Ladder Network
One feature of the X-AMP architecture is that the output referred
noise is constant versus gain over most of the gain range. This
can be easily explained by looking at Figure 2 and observing
that the tap resistance is equal for all taps after only a few taps
away from the inputs. The resistance seen looking into each tap is
54.4 Ω which makes 0.95 nV/√Hz of Johnson noise spectral
density. Since there are two attenuators, the overall noise
contribution of the ladder network is √2 times 0.95 nV/√Hz
or 1.34 nV/√Hz, a large fraction of the total DSX noise. The rest
of the DSX circuit components contribute another 1.20 nV/√Hz
which together with the attenuator produces 1.8 nV/√Hz of
total DSX input referred noise.
From these equations one can see that all gain curves intercept
at the same –19 dB point; this intercept will be 14 dB higher
(–5 dB) if the FBK to OUT connection is left open. Outside
of the central linear range, the gain starts to deviate from the
ideal control law but still provides another 8.4 dB of range.
For a given gain scaling one can calculate VREF as shown in
Equation 6.
V REF =
40dB/V
Gain Control Interface
The gain control interface provides an input resistance of
approximately 2 MΩ at pin VGN1 and gain scaling factors
from 20 dB/V to 40 dB/V for VREF input voltages of 2.5 V to
1.25 V, respectively. The gain varies linearly-in-dB for the center 40 dB of gain range, that is for VGN equal to 0.4 V to 2.4 V
for the 20 dB/V scale, and 0.25 V to 1.25 V for the 40 dB/V
scale. Figure 3 shows the ideal gain curves when the FBK to
OUT connection is shorted as described by the following
equations:
G (20 dB/V ) = 20 × VGN – 19, VREF = 2.500 V
(3)
G (30 dB/V ) = 30 × VGN – 19, VREF = 1.6666 V
(4)
G (40 dB/V ) = 40 × VGN – 19, VREF = 1.250 V
(5)
REV. C
20dB/V
30
The DSX is a single, single-supply circuit and therefore its
inputs need to be ac-coupled to accommodate ground-based
signals. External capacitors C1 and C2 in Figure 1 level shift
the input signal from ground to the dc value established by
VOCM (nominal 2.5 V). C1 and C2, together with the 175 Ω
looking into each of DSX inputs (+IN and –IN), will act as
high-pass filters with corner frequencies depending on the
values chosen for C1 and C2. For example, if C1 and C2 are
0.1 µF, then together with the 175 Ω input resistance of each
side of the differential ladder of the DSX, a –3 dB high-pass
corner at 9.1 kHz is formed.
The choice for all three of these coupling capacitors depends on
the application. They should allow the signals of interest to pass
unattenuated, while at the same time they can be used to limit
the low frequency noise in the system.
30dB/V
(6)
35
AC Coupling
If the DSX output needs to be ground referenced, then another
ac-coupling capacitor will be required for level shifting. This
capacitor will also eliminate any dc offsets contributed by the
DSX. With a nominal load of 500 Ω and a 0.1 µF coupling
capacitor, this adds a high-pass filter with –3 dB corner frequency at about 3.2 kHz.
2.500V × 20 dB /V
Gain Scale
25
20
GAIN (dB)
15
LINEAR-IN-dB RANGE
OF AD605
10
5
0
0.5
–5
1.0
1.5
2.0
2.5
3.0
GAIN CONTROL VOLTAGE
–10
–15
–20
Figure 3. Ideal Gain Curves vs. VREF
Usable gain control voltage ranges are 0.1 V to 2.9 V for 20 dB/V
scale and 0.1 V to 1.45 V for the 40 dB/V scale. VGN voltages
of less than 0.1 V are not used for gain control since below
50 mV the channel is powered down. This can be used to conserve power and at the same time gate-off the signal. The supply
current for a powered-down channel is 1.9 mA, the response
time to power the device on or off is less than 1 µs.
Active Feedback Amplifier (Fixed Gain Amp)
To achieve single-supply operation and a fully differential input
to the DSX, an active feedback amplifier (AFA) was utilized.
The AFA is basically an op amp with two gm stages; one of the
active stages is used in the feedback path (therefore the name),
while the other is used as a differential input. Note that the
differential input is an open-loop gm stage which requires that it
be highly linear over the expected input signal range. In this
design, the gm stage that senses the voltages on the attenuator is
a distributed one; for example, there are as many gm stages as
there are taps on the ladder network. Only a few of them are on
at any one time, depending on the gain control voltage.
–9–
AD605
The AFA makes a differential input structure possible since one
of its inputs (G1) is fully differential; this input is made up of a
distributed gm stage. The second input (G2) is used for feedback. The output of G1 will be some function of the voltages
sensed on the attenuator taps which is applied to a high-gain
amplifier (A0). Because of negative feedback, the differential
input to the high-gain amplifier has to be zero; this in turn implies
that the differential input voltage to G2 times gm2 (the transconductance of G2) has to be equal to the differential input voltage
to G1 times gm1 (the transconductance of G1). Therefore the
overall gain function of the AFA is
VOUT
gm1 R1 × R2
=
×
VATTEN gm2
R2
VGN
0.1␮F
VIN
1 VGN1
VREF 16
2.500V
2 –IN1
OUT1 15
OUT
3 +IN1
0.1␮F
AD605
FBK1 14
4 GND1
VPOS 13
5 GND2
VPOS 12
6 +IN2
FBK2 11
7 –IN2
OUT2 10
8 VGN2
0.1␮F
5V
VOCM 9
0.1␮F
Figure 4. Basic Connections for a Single Channel
(7)
where VOUT is the output voltage, VATTEN is the effective voltage
sensed on the attenuator, (R1 + R2)/R2 = 42, and gm1/gm2 =
1.25; the overall gain is thus 52.5 (34.4 dB).
The AFA has additional features: (1) inverting the output signal
by switching the positive and negative input to the ladder network;
(2) the possibility of using the –IN input as a second signal input;
and (3) independent control of the DSX common-mode voltage.
Under normal operating conditions it is best to just connect a
decoupling capacitor to pin VOCM in which case the commonmode voltage of the DSX is half the supply voltage; this allows for
maximum signal swing. Nevertheless, the common-mode voltage
can be shifted up or down by directly applying a voltage to VOCM.
It can also be used as another signal input, the only limitation
being the rather low slew rate of the VOCM buffer.
If the dc level of the output signal is not critical, another coupling
capacitor is normally used at the output of the DSX; again this
is done for level shifting and to eliminate any dc offsets contributed by the DSX (see AC Coupling section).
The gain range of the DSX is programmable by a resistor connected between pins FBK and OUT. The possible ranges are
–14 dB to +34.4 dB when the pins are shorted together, to 0 dB
to +48.4 dB when FBK is left open. Note that for the higher
gain range, the bandwidth of the amplifier is reduced by a factor
of five to about 8 MHz since the gain increased by 14 dB. This
is the case for any constant gain bandwidth product amplifier
which includes the active feedback amplifier.
As shown here, the output is ac-coupled for optimum performance. In the case of connecting to the 10-bit 40 MSPS A/D
converter AD9050, ac coupling can be eliminated as long as
pin VOCM is biased by the same 3.3 V common-mode voltage
as the AD9050.
Pin VREF requires a voltage of 1.25 V to 2.5 V, with gain
scaling between 40 dB/V and 20 dB/V, respectively. Voltage
VGN controls the gain; its nominal operating range is from
0.25 V to 2.65 V for 20 dB/V gain scaling, and 0.125 V to
1.325 V for 40 dB/V scaling. When this pin is taken to ground,
the channel will power down and disable its output.
Connecting Two Amplifiers to Double the Gain Range
Figure 5 shows the two channels of the AD605 connected in
series to provide a total gain range of 96.8 dB. When R1 and R2
are shorts, the gain range will be from –28 dB to +68.8 dB with
a slightly reduced bandwidth of about 30 MHz. The reduction
in bandwidth is due to two identical low-pass circuits being
connected in series; in the case of two identical single-pole lowpass filters, the bandwidth would be reduced by exactly √2. If
R1 and R2 are replaced by open circuits, i.e., Pins FBK1 and
FBK2 are left unconnected, then the gain range will shift up by
28 dB to 0 dB to +96.8 dB. As noted earlier, the bandwidth of
each individual channel will be reduced by a factor of 5 to
about 8 MHz since the gain increased by 14 dB. In addition,
there is still the √2 reduction because of the series connection of
the two channels which results in a final bandwidth of the higher
gain version of about 6 MHz.
VGN
APPLICATIONS
The basic circuit in Figure 4 shows the connections for one channel of the AD605 with a gain range of –14 dB to +34.4 dB. The
signal is applied at Pin 3. The ac-coupling capacitors before pins
–IN1 and +IN1 should be selected according to the required lower
cutoff frequency. In this example, the 0.1 µF capacitors together with the 175 Ω of each of the DSX input pins provides a
–3 dB high-pass corner of about 9.1 kHz. The upper cutoff
frequency is determined by the amplifier and is 40 MHz.
C1
0.1␮F
VIN
1 VGN1
VREF 16
2 –IN1
OUT1 15
3 +IN1
C2
0.1␮F
C3
0.1␮F
C4
0.1␮F
AD605
FBK1 14
4 GND1
VPOS 13
5 GND2
VPOS 12
6 +IN2
FBK2 11
7 –IN2
OUT2 10
8 VGN2
VOCM 9
2.500V
R1
5V
R2
C5
0.1␮F
OUT
C6
0.1␮F
Figure 5. Doubling the Gain Range with Two Amplifiers
–10–
REV. C
AD605
Two other easy combinations are possible to provide a gain
range of –14 dB to +82.8 dB: (1) make R1 a short and R2 an
open; or (2) make R1 an open and R2 a short. The bandwidth
for both of these cases will be dominated by the channel that is
set to the higher gain and will be about 8 MHz. From a noise
standpoint, the second choice is the best since by increasing the
gain of the first amplifier, the second amplifier’s noise will have
less of an impact on the total output noise. One further observation regarding noise is that by increasing the gain the output
noise will increase proportionally; therefore, there is no increase
in signal-to-noise ratio. It will actually stay fixed.
80
60
f = 1MHz
50
ACTUAL
GAIN (dB)
40
30
20
10
0
–10
–20
It should be noted that by selecting the appropriate values of R1
and R2, any gain range between –28 dB to +68.8 dB and 0 dB
to +96.8 dB can be achieved with the circuit in Figure 5. When
using any value other than shorts and opens for R1 and R2, the
final value of the gain range will depend on external resistors
matching on-chip resistors. Since the internal resistors can vary
by as much as ± 20%, the actual values for a particular gain have
to be determined empirically. Note that the two channels within
one part will match quite well; therefore, R1 will track R2 in
Figure 5.
–30
–40
0.1
0.9
0.5
1.3
1.7
VGN (V)
2.1
2.5
2.9
Figure 6. Gain vs. VGN for the Circuit in Figure 5
4
f = 1MHz
3
GAIN ERROR (dB)
2
C3 is not required since the common-mode voltage at Pin OUT1
should be identical to the one at Pins +IN2 and –IN2. However,
since only 1 mV of offset at the output of the first DSX will
introduce an offset of 53 mV when the second DSX is set to the
maximum gain of the lowest gain range (34.4 dB), and 263 mV
when set to the maximum gain of the highest gain range (48.4 dB),
it is important to include ac coupling to get the maximum dynamic range at the output of the cascaded amplifiers. C5 is
necessary if the output signal needs to be referenced to any
common-mode level other than half of the supply as is provided
by Pin OUT2.
1
0
–1
–2
–3
–4
0.2
0.7
1.2
1.7
VGN (V)
2.2
2.7
Figure 7. Gain Error vs. VGN for the Circuit in Figure 5
Figure 6 shows the gain versus VGN for the circuit in Figure 5
at 1 MHz and the lowest gain range (–14 dB to +34.4 dB). Note
that the gain scaling is 40 dB/V, double the 20 dB/V of an individual DSX; this is the result of the parallel connection of the
gain control inputs, VGN1 and VGN2. One could of course
also sequentially increase the gain by first increasing the gain of
Channel 1 and then Channel 2. In that case VGN1 and VGN2
will have to be driven from separate voltage sources, for instance
two separate DACs. Figure 7 shows the gain error of Figure 5.
REV. C
THEORETICAL
70
–11–
AD605
OUTLINE DIMENSIONS
16-Lead Plastic Dual In-Line Package [PDIP]
(N-16)
0.785 (19.94)
0.765 (19.43)
0.745 (18.92)
16
9
1
8
C00541–0–7/04(C)
Dimensions shown in inches and (millimeters)
0.295 (7.49)
0.285 (7.24)
0.275 (6.99)
0.100 (2.54)
BSC
0.015 (0.38)
MIN
0.325 (8.26)
0.310 (7.87)
0.300 (7.62)
0.180 (4.57)
MAX
0.150 (3.81)
0.130 (3.30)
0.110 (2.79)
0.022 (0.56)
0.018 (0.46)
0.014 (0.36)
0.060 (1.52) SEATING
PLANE
0.050 (1.27)
0.045 (1.14)
0.150 (3.81)
0.135 (3.43)
0.120 (3.05)
0.015 (0.38)
0.010 (0.25)
0.008 (0.20)
COMPLIANT TO JEDEC STANDARDS MO-095AC
CONTROLLING DIMENSIONS ARE IN INCHES; MILLIMETER DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF INCH EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
16-Lead Standard Small Outline Package [SOIC]
Narrow Body
(R-16)
Dimensions shown in millimeters and (inches)
10.00 (0.3937)
9.80 (0.3858)
4.00 (0.1575)
3.80 (0.1496)
16
9
1
8
1.27 (0.0500)
BSC
6.20 (0.2441)
5.80 (0.2283)
1.75 (0.0689)
1.35 (0.0531)
0.25 (0.0098)
0.10 (0.0039)
COPLANARITY
0.10
0.50 (0.0197)
ⴛ 45ⴗ
0.25 (0.0098)
8ⴗ
0.51 (0.0201) SEATING
0.25 (0.0098) 0ⴗ 1.27 (0.0500)
0.31 (0.0122) PLANE
0.40 (0.0157)
0.17 (0.0067)
COMPLIANT TO JEDEC STANDARDS MS-012AC
CONTROLLING DIMENSIONS ARE IN MILLIMETERS; INCH DIMENSIONS
(IN PARENTHESES) ARE ROUNDED-OFF MILLIMETER EQUIVALENTS FOR
REFERENCE ONLY AND ARE NOT APPROPRIATE FOR USE IN DESIGN
Revision History
Location
Page
7/04—Data Sheet Changed from REV. B to REV. C.
Edits to GENERAL DESCRIPTION ..............................................................................................................................................1
Edits to SPECIFICATIONS ...........................................................................................................................................................2
Edits to ORDERING GUIDE .........................................................................................................................................................3
Change to TPC 22 ..........................................................................................................................................................................6
Updated OUTLINE DIMENSIONS.............................................................................................................................................12
–12–
REV. C