AD ADA4950-2 Low power, selectable gain differential adc driver, g = 1, 2, 3 Datasheet

Low Power, Selectable Gain
Differential ADC Driver, G = 1, 2, 3
ADA4950-1/ADA4950-2
+INB 1
Differential gain configurations of 1, 2, and 3 are easily realized
with internal feedback networks that are connected externally
to set the closed-loop gain of the amplifier.
The ADA4950-1/ADA4950-2 are fabricated using the Analog
Devices, Inc., proprietary silicon-germanium (SiGe) complementary
bipolar process, enabling them to achieve low levels of distortion and
noise at low power consumption. The low offset and excellent
dynamic performance of the ADA4950-x make it well suited for
a wide variety of data acquisition and signal processing applications.
13 –VS
07957-001
+VS 8
+VS 7
9 VOCM
+VS 5
10 +OUT
+INB 4
+INA1
+INB1
–VS1
–VS1
PD1
–OUT1
24
23
22
21
20
19
ADA4950-2
18
17
16
15
14
13
+OUT1
VOCM1
–VS2
–VS2
PD2
–OUT2
–INA2
–INB2
+VS2
+VS2
VOCM2
+OUT2
07957-002
7
8
9
10
11
12
1
2
3
4
5
6
Figure 2. ADA4950-2
–40
VOUT, dm = 2V p-p
–50
–60
–70
–80
HD2, ±5V
HD3, ±5V
HD2, ±2.5V
HD3, ±2.5V
–90
–100
–110
–120
–130
–140
0.1
1
10
FREQUENCY (MHz)
100
07957-025
The ADA4950-1/ADA4950-2 are gain-selectable versions of the
ADA4932-1/ADA4932-2 with on-chip feedback and gain resistors.
They are ideal choices for driving high performance ADCs as singleended-to-differential or differential-to-differential amplifiers. The
output common-mode voltage is user adjustable by means of an
internal common-mode feedback loop, allowing the ADA4950-1/
ADA4950-2 output to match the input of the ADC. The internal
feedback loop also provides exceptional output balance as well
as suppression of even-order harmonic distortion products.
11 –OUT
–INA 3
–INA1
–INB1
+VS1
+VS1
+INB2
+INA2
HARMONIC DISTORTION (dBc)
GENERAL DESCRIPTION
12 PD
+INA 2
Figure 1. ADA4950-1
APPLICATIONS
ADC drivers
Single-ended-to-differential converters
IF and baseband gain blocks
Differential buffers
Line drivers
ADA4950-1
+VS 6
High performance at low power
High speed
−3 dB bandwidth of 750 MHz, G = 1
0.1 dB flatness to 210 MHz, VOUT, dm = 2 V p-p, RL, dm = 200 Ω
Slew rate: 2900 V/μs, 25% to 75%
Fast 0.1% settling time of 9 ns
Low power: 9.5 mA per amplifier
Low harmonic distortion
108 dB SFDR @ 10 MHz
98 dB SFDR @ 20 MHz
Low output voltage noise: 9.2 nV/√Hz, G = 1, RTO
±0.2 mV typical input offset voltage
Selectable differential gains of 1, 2, and 3
Differential-to-differential or single-ended-to-differential
operation
Adjustable output common-mode voltage
Input common-mode range shifted down by 1 VBE
Wide supply range: +3 V to ±5 V
Available in 16-lead and 24-lead LFCSP packages
14 –VS
FUNCTIONAL BLOCK DIAGRAMS
16 –VS
15 –VS
FEATURES
Figure 3. Harmonic Distortion vs. Frequency at Various Supplies
The ADA4950-x is available in a Pb-free, 3 mm × 3 mm, 16-lead
LFCSP (ADA4950-1, single) or a Pb-free, 4 mm × 4 mm, 24-lead
LFCSP (ADA4950-2, dual). The pinout has been optimized to
facilitate PCB layout and minimize distortion. The ADA4950-1/
ADA4950-2 are specified to operate over the −40°C to +105°C
temperature range; both operate on supplies from +3 V to ±5 V.
Rev. 0
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. Specifications subject to change without notice. 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: 781.461.3113
©2009 Analog Devices, Inc. All rights reserved.
ADA4950-1/ADA4950-2
TABLE OF CONTENTS
Features .............................................................................................. 1
Theory of Operation ...................................................................... 18
Applications ....................................................................................... 1
Applications Information .............................................................. 19
General Description ......................................................................... 1
Analyzing an Application Circuit ............................................ 19
Functional Block Diagrams ............................................................. 1
Selecting the Closed-Loop Gain............................................... 19
Revision History ............................................................................... 2
Estimating the Output Noise Voltage ...................................... 19
Specifications..................................................................................... 3
±5 V Operation ............................................................................. 3
Calculating the Input Impedance for an Application Circuit
....................................................................................................... 20
5 V Operation ............................................................................... 5
Input Common-Mode Voltage Range ..................................... 22
Absolute Maximum Ratings............................................................ 7
Input and Output Capacitive AC Coupling ............................ 22
Thermal Resistance ...................................................................... 7
Input Signal Swing Considerations .......................................... 22
Maximum Power Dissipation ..................................................... 7
Setting the Output Common-Mode Voltage .......................... 22
ESD Caution .................................................................................. 7
Layout, Grounding, and Bypassing .............................................. 23
Pin Configurations and Function Descriptions ........................... 8
High Performance ADC Driving ................................................. 24
Typical Performance Characteristics ............................................. 9
Outline Dimensions ....................................................................... 25
Test Circuits ..................................................................................... 16
Ordering Guide .......................................................................... 25
Terminology .................................................................................... 17
REVISION HISTORY
5/09—Revision 0: Initial Version
Rev. 0 | Page 2 of 28
ADA4950-1/ADA4950-2
SPECIFICATIONS
±5 V OPERATION
TA = 25°C, +VS = 5 V, −VS = −5 V, VOCM = 0 V, G = 1, RT = 53.6 Ω (when used), RL, dm = 1 kΩ, unless otherwise noted. All specifications
refer to single-ended input and differential outputs, unless otherwise noted. Refer to Figure 52 for signal definitions.
Differential Inputs to VOUT, dm Performance
Table 1.
Parameter
DYNAMIC PERFORMANCE
−3 dB Small-Signal Bandwidth
−3 dB Large-Signal Bandwidth
Bandwidth for 0.1 dB Flatness
ADA4950-1
ADA4950-2
Slew Rate
Settling Time to 0.1%
Overdrive Recovery Time
NOISE/HARMONIC PERFORMANCE
Second Harmonic
Third Harmonic
IMD3
Voltage Noise (Referred to Output)
Crosstalk (ADA4950-2)
INPUT CHARACTERISTICS
Offset Voltage (Referred to Input)
Input Capacitance
Input Common-Mode Voltage Range
CMRR
Open-Loop Gain
OUTPUT CHARACTERISTICS
Output Voltage Swing
Linear Output Current
Output Balance Error
Gain Error
Test Conditions/Comments
Min
VOUT, dm = 0.1 V p-p
VOUT, dm = 2.0 V p-p
VOUT, dm = 2.0 V p-p, RL = 200 Ω
VOUT, dm = 2 V p-p, 25% to 75%
VOUT, dm = 2 V step
VIN = 0 V to 5 V ramp, G = 2
See Figure 51 for distortion test circuit
VOUT, dm = 2 V p-p
1 MHz
10 MHz
20 MHz
50 MHz
VOUT, dm = 2 V p-p
1 MHz
10 MHz
20 MHz
50 MHz
f1 = 30 MHz, f2 = 30.1 MHz, VOUT, dm = 2 V p-p
f = 1 MHz
Gain = 1
Gain = 2
Gain = 3
f = 10 MHz; Channel 2 active, Channel 1
output
V+DIN = V−DIN = VOCM = 0 V
TMIN to TMAX variation
Single-ended at package pin
Directly at internal amplifier inputs, not
external input terminals
DC, ∆VOUT, dm/∆VIN, cm, ∆VIN, cm = ±1 V
−2.5
64
Maximum ∆VOUT, single-ended output,
RL = 1 kΩ
200 kHz, RL, dm = 10 Ω, SFDR = 69 dB
∆VOUT, cm/∆VOUT, dm, ∆VOUT, dm = 2 V p-p, 1 MHz;
see Figure 50 for output balance test circuit
Gain = 1
Gain = 2
Gain = 3
Rev. 0 | Page 3 of 28
–VS + 1.4 to
+VS – 1.4
Typ
Max
Unit
750
350
MHz
MHz
210
230
2900
9
20
MHz
MHz
V/μs
ns
ns
−108
−107
−98
−80
dBc
dBc
dBc
dBc
−126
−105
−99
−84
−94
dBc
dBc
dBc
dBc
dBc
9.2
12.5
16.6
−87
nV/√Hz
nV/√Hz
nV/√Hz
dB
±0.2
–3.7
0.5
−VS + 0.2 to
+VS − 1.8
−64
66
+2.5
mV
μV/°C
pF
V
−49
dB
dB
V
−VS + 1.2 to
+VS − 1.2
114
−62
0.5
1.0
0.8
mA peak
dB
1.2
1.9
1.7
%
%
%
ADA4950-1/ADA4950-2
VOCM to VOUT, cm Performance
Table 2.
Parameter
VOCM DYNAMIC PERFORMANCE
−3 dB Small-Signal Bandwidth
−3 dB Large-Signal Bandwidth
Slew Rate
Input Voltage Noise (Referred to Input)
VOCM INPUT CHARACTERISTICS
Input Voltage Range
Input Resistance
Input Offset Voltage
VOCM CMRR
Gain
Test Conditions/Comments
Min
Typ
Max
Unit
VOUT, cm = 100 mV p-p
VOUT, cm = 2 V p-p
VIN = 1.5 V to 3.5 V, 25% to 75%
f = 1 MHz
250
105
430
9.8
MHz
MHz
V/μs
nV/√Hz
V
V+DIN = V−DIN = 0 V
ΔVOUT, dm/ΔVOCM, ΔVOCM = ±1 V
ΔVOUT, cm/ΔVOCM, ΔVOCM = ±1 V
0.98
–VS + 1.2 to
+VS – 1.2
26
+0.8
−60
1.0
32
+6
−49
1.01
kΩ
mV
dB
V/V
Min
Typ
Max
Unit
11
10.1
V
mA
μA/°C
mA
dB
22
−6
General Performance
Table 3.
Parameter
POWER SUPPLY
Operating Range
Quiescent Current per Amplifier
Power Supply Rejection Ratio
POWER-DOWN (PD)
PD Input Voltage
Turn-Off Time
Turn-On Time
PD Pin Bias Current per Amplifier
Enabled
Disabled
Test Conditions/Comments
3.0
8.8
TMIN to TMAX variation
Powered down
ΔVOUT, dm/ΔVS, ΔVS = 1 V p-p
Powered down
Enabled
9.5
31
0.7
−96
1.0
−84
≤(+VS – 2.5)
≥(+VS – 1.8)
600
28
−1.0
−250
PD = 5 V
PD = 0 V
OPERATING TEMPERATURE RANGE
−40
Rev. 0 | Page 4 of 28
+0.2
−180
V
V
ns
ns
+1.0
−140
μA
μA
+105
°C
ADA4950-1/ADA4950-2
5 V OPERATION
TA = 25°C, +VS = 5 V, −VS = 0 V, VOCM = 2.5 V, G = 1, RT = 53.6 Ω (when used), RL, dm = 1 kΩ, unless otherwise noted. All specifications
refer to single-ended input and differential outputs, unless otherwise noted. Refer to Figure 52 for signal definitions.
Differential Inputs to VOUT, dm Performance
Table 4.
Parameter
DYNAMIC PERFORMANCE
−3 dB Small-Signal Bandwidth
−3 dB Large-Signal Bandwidth
Bandwidth for 0.1 dB Flatness
ADA4950-1
ADA4950-2
Slew Rate
Settling Time to 0.1%
Overdrive Recovery Time
NOISE/HARMONIC PERFORMANCE
Second Harmonic
Third Harmonic
IMD3
Voltage Noise (Referred to Input)
Crosstalk (ADA4950-2)
INPUT CHARACTERISTICS
Offset Voltage (Referred to Input)
Input Capacitance
Input Common-Mode Voltage Range
CMRR
Open-Loop Gain
OUTPUT CHARACTERISTICS
Output Voltage Swing
Linear Output Current
Output Balance Error
Gain Error
Test Conditions/Comments
Min
VOUT, dm = 0.1 V p-p
VOUT, dm = 2.0 V p-p
VOUT, dm = 2.0 V p-p, RL = 200 Ω
VOUT, dm = 2 V p-p, 25% to 75%
VOUT, dm = 2 V step
VIN = 0 V to 2.5 V ramp, G = 2
See Figure 51 for distortion test circuit
VOUT, dm = 2 V p-p
1 MHz
10 MHz
20 MHz
50 MHz
VOUT, dm = 2 V p-p
1 MHz
10 MHz
20 MHz
50 MHz
f1 = 30 MHz, f2 = 30.1 MHz, VOUT, dm = 2 V p-p
f = 1 MHz
Gain = 1
Gain = 2
Gain = 3
f = 10 MHz; Channel 2 active, Channel 1
output
V+DIN = V−DIN = VOCM = 2.5 V
TMIN to TMAX variation
Single-ended at package pin
Directly at internal amplifier inputs, not
external input terminals
DC, ∆VOUT, dm/∆VIN, cm, ∆VIN, cm = ±1 V
−4
64
Maximum ∆VOUT, single-ended output,
RL = 1 kΩ
200 kHz, RL, dm = 10 Ω, SFDR = 67 dB
∆VOUT, cm/∆VOUT, dm, ∆VOUT, dm = 1 V p-p, 1 MHz;
see Figure 50 for output balance test circuit
Gain = 1
Gain = 2
Gain = 3
Rev. 0 | Page 5 of 28
–VS + 1.2 to
+VS – 1.2
Typ
Max
Unit
770
320
MHz
MHz
220
160
2200
10
19
MHz
MHz
V/μs
ns
ns
−108
−107
−98
−82
dBc
dBc
dBc
dBc
−124
−114
−99
−83
−94
dBc
dBc
dBc
dBc
dBc
9.2
12.5
16.6
−87
nV/√Hz
nV/√Hz
nV/√Hz
dB
±0.4
−3.7
0.5
–VS + 0.2 to
+VS – 1.8
−64
66
+4
mV
μV/°C
pF
V
−49
dB
dB
V
–VS + 1.1 to
+VS – 1.1
70
−62
0.5
1.0
0.8
mA peak
dB
1.2
1.9
1.7
%
%
%
ADA4950-1/ADA4950-2
VOCM to VOUT, cm Performance
Table 5.
Parameter
VOCM DYNAMIC PERFORMANCE
−3 dB Small-Signal Bandwidth
−3 dB Large-Signal Bandwidth
Slew Rate
Input Voltage Noise (Referred to Input)
VOCM INPUT CHARACTERISTICS
Input Voltage Range
Input Resistance
Input Offset Voltage
VOCM CMRR
Gain
Test Conditions/Comments
Min
Typ
Max
Unit
VOUT, cm = 100 mV p-p
VOUT, cm = 2 V p-p
VIN = 1.5 V to 3.5 V, 25% to 75%
f = 1 MHz
240
90
380
9.8
MHz
MHz
V/μs
nV/√Hz
V
V+DIN = V−DIN = 2.5 V
ΔVOUT, dm/ΔVOCM, ΔVOCM = ±1 V
ΔVOUT, cm/ΔVOCM, ΔVOCM = ±1 V
0.98
–VS + 1.2 to
+VS – 1.2
26
+1.0
−60
1.0
32
+6.5
−49
1.01
kΩ
mV
dB
V/V
Min
Typ
Max
Unit
11
9.6
V
mA
μA/°C
mA
dB
22
−6.5
General Performance
Table 6.
Parameter
POWER SUPPLY
Operating Range
Quiescent Current per Amplifier
Power Supply Rejection Ratio
POWER-DOWN (PD)
PD Input Voltage
Turn-Off Time
Turn-On Time
PD Pin Bias Current per Amplifier
Enabled
Disabled
Test Conditions/Comments
3.0
8.4
TMIN to TMAX variation
Powered down
ΔVOUT, dm/ΔVS, ΔVS = 1 V p-p
Powered down
Enabled
8.9
31
0.6
−96
0.9
−84
≤(+VS – 2.5)
≥(+VS – 1.8)
600
29
−1.0
−100
PD = 5 V
PD = 0 V
OPERATING TEMPERATURE RANGE
−40
Rev. 0 | Page 6 of 28
+0.2
−65
V
V
ns
ns
+1.0
−40
μA
μA
+105
°C
ADA4950-1/ADA4950-2
ABSOLUTE MAXIMUM RATINGS
Table 7.
Parameter
Supply Voltage
Power Dissipation
Input Current, +INx, −INx, PD
Storage Temperature Range
Operating Temperature Range
ADA4950-1
ADA4950-2
Lead Temperature (Soldering, 10 sec)
Junction Temperature
Rating
11 V
See Figure 4
±5 mA
−65°C to +125°C
−40°C to +105°C
−40°C to +105°C
300°C
150°C
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.
The power dissipated in the package (PD) is the sum of the quiescent power dissipation and the power dissipated in the package
due to the load drive. The quiescent power is the voltage between
the supply pins (VS) times the quiescent current (IS). The power
dissipated due to the load drive depends upon the particular
application. The power dissipated due to the load drive is calculated by multiplying the load current by the associated voltage
drop across the device. RMS voltages and currents must be used
in these calculations.
Airflow increases heat dissipation, effectively reducing θJA. In
addition, more metal directly in contact with the package leads/
exposed pad from metal traces, through holes, ground, and
power planes reduces θJA.
Figure 4 shows the maximum safe power dissipation in the
package vs. the ambient temperature for the single 16-lead
LFCSP (91°C/W) and the dual 24-lead LFCSP (65°C/W) on
a JEDEC standard 4-layer board with the exposed pad soldered
to a PCB pad that is connected to a solid plane.
3.5
θJA is specified for the device (including exposed pad) soldered
to a high thermal conductivity 2s2p printed circuit board, as
described in EIA/JESD51-7.
Table 8. Thermal Resistance
Package Type
ADA4950-1, 16-Lead LFCSP (Exposed Pad)
ADA4950-2, 24-Lead LFCSP (Exposed Pad)
θJA
91
65
θJC
28
16
Unit
°C/W
°C/W
3.0
2.5
2.0
ADA4950-2
1.5
ADA4950-1
1.0
0.5
07957-004
MAXIMUM POWER DISSIPATION (W)
THERMAL RESISTANCE
MAXIMUM POWER DISSIPATION
The maximum safe power dissipation in the ADA4950-x package
is limited by the associated rise in junction temperature (TJ) on
the die. At approximately 150°C, which is the glass transition
temperature, the plastic changes its properties. Even temporarily
exceeding this temperature limit can change the stresses that the
package exerts on the die, permanently shifting the parametric
performance of the ADA4950-x. Exceeding a junction temperature of 150°C for an extended period can result in changes in
the silicon devices, potentially causing failure.
0
–40
–20
60
0
20
40
AMBIENT TEMPERATURE (°C)
80
100
Figure 4. Maximum Power Dissipation vs. Ambient Temperature
for a 4-Layer Board
ESD CAUTION
Rev. 0 | Page 7 of 28
ADA4950-1/ADA4950-2
+INA1
+INB1
–VS1
–VS1
PD1
–OUT1
24
23
22
21
20
19
12 PD
11 –OUT
TOP VIEW
(Not to Scale)
10 +OUT
1
2
3
4
5
6
PIN 1
INDICATOR
ADA4950-2
TOP VIEW
(Not to Scale)
18
17
16
15
14
13
+OUT1
VOCM1
–VS2
–VS2
PD2
–OUT2
7
8
9
10
11
12
9 VOCM
–INA2
–INB2
+VS2
+VS2
VOCM2
+OUT2
+VS 8
+VS 5
+VS 7
ADA4950-1
–INA 3
+VS 6
+INA 2
–INB 4
–INA1
–INB1
+VS1
+VS1
+INB2
+INA2
NOTES
1. SOLDER THE EXPOSED PADDLE ON THE BACK OF
THE PACKAGE TO A GROUND PLANE OR TO A
POWER PLANE.
NOTES
1. SOLDER THE EXPOSED PADDLE ON THE BACK OF
THE PACKAGE TO A GROUND PLANE OR TO A
POWER PLANE.
Figure 5. ADA4950-1 Pin Configuration
07957-006
14 –VS
PIN 1
INDICATOR
07957-005
+INB 1
13 –VS
16 –VS
15 –VS
PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS
Figure 6. ADA4950-2 Pin Configuration
Table 9. ADA4950-1 Pin Function Descriptions
Pin No.
1
2
3
4
5 to 8
9
10
11
12
13 to 16
17 (EPAD)
Mnemonic
+INB
+INA
−INA
−INB
+VS
VOCM
+OUT
−OUT
PD
−VS
Exposed Paddle (EPAD)
Description
Positive Input B, 250 Ω Input. Use alone for G = 2 or tie to +INA for G = 3.
Positive Input A, 500 Ω Input. Use alone for G = 1 or tie to +INB for G = 3.
Negative Input A, 500 Ω Input. Use alone for G = 1 or tie to −INB for G = 3.
Negative Input B, 250 Ω Input. Use alone for G = 2 or tie to −INA for G = 3.
Positive Supply Voltage.
Output Common-Mode Voltage.
Positive Output.
Negative Output.
Power-Down Pin.
Negative Supply Voltage.
Solder the exposed paddle on the back of the package to a ground plane or to a power plane.
Table 10. ADA4950-2 Pin Function Descriptions
Pin No.
1
2
3, 4
5
6
7
8
9, 10
11
12
13
14
15, 16
17
18
19
20
21, 22
23
24
25 (EPAD)
Mnemonic
−INA1
−INB1
+VS1
+INB2
+INA2
−INA2
−INB2
+VS2
VOCM2
+OUT2
−OUT2
PD2
−VS2
VOCM1
+OUT1
−OUT1
PD1
−VS1
+INB1
+INA1
Exposed Paddle (EPAD)
Description
Negative Input A, Amplifier 1, 500 Ω Input. Use alone for G = 1 or tie to –INB1 for G = 3.
Negative Input B, Amplifier 1, 250 Ω Input. Use alone for G = 2 or tie to –INA1 for G = 3.
Positive Supply Voltage, Amplifier 1.
Positive Input B, Amplifier 2, 250 Ω Input. Use alone for G = 2 or tie to +INA2 for G = 3.
Positive Input A, Amplifier 2, 500 Ω Input. Use alone for G = 1 or tie to +INB2 for G = 3.
Negative Input A, Amplifier 2, 500 Ω Input. Use alone for G = 1 or tie to –INB2 for G = 3.
Negative Input B, Amplifier 2, 250 Ω Input. Use alone for G = 2 or tie to –INA2 for G = 3.
Positive Supply Voltage, Amplifier 2.
Output Common-Mode Voltage, Amplifier 2.
Positive Output, Amplifier 2.
Negative Output, Amplifier 2.
Power-Down Pin, Amplifier 2.
Negative Supply Voltage, Amplifier 2.
Output Common-Mode Voltage, Amplifier 1.
Positive Output, Amplifier 1.
Negative Output, Amplifier 1.
Power-Down Pin, Amplifier 1.
Negative Supply Voltage, Amplifier 1.
Positive Input B, Amplifier 1, 250 Ω Input. Use alone for G = 2 or tie to +INA1 for G = 3.
Positive Input A, Amplifier 1, 500 Ω Input. Use alone for G = 1 or tie to +INB1 for G = 3.
Solder the exposed paddle on the back of the package to a ground plane or to a power plane.
Rev. 0 | Page 8 of 28
ADA4950-1/ADA4950-2
TYPICAL PERFORMANCE CHARACTERISTICS
TA = 25°C, +VS = 5 V, −VS = −5 V, VOCM = 0 V, G = 1, RT = 53.6 Ω (when used), RL, dm = 1 kΩ, unless otherwise noted. Refer to Figure 49
for test setup. Refer to Figure 52 for signal definitions.
2
0
–1
G = 1, RT = 53.6Ω
G = 2, RT = 57.6Ω
G = 3, RT = 61.9Ω
–2
–3
–4
–5
–6
–7
1
10
100
FREQUENCY (MHz)
–5
–6
–7
1
10
100
FREQUENCY (MHz)
1000
VOUT, dm = 2V p-p
1
0
CLOSED-LOOP GAIN (dB)
0
VS = ±5V
VS = ±2.5V
–1
–2
–3
–4
–5
VS = ±5V
VS = ±2.5V
–2
–3
–4
–5
–6
07957-008
–6
–1
–7
1
10
100
FREQUENCY (MHz)
07957-011
CLOSED-LOOP GAIN (dB)
–4
2
VOUT, dm = 100mV p-p
1
–7
–8
1000
Figure 8. Small-Signal Frequency Response for Various Supplies
1
10
100
FREQUENCY (MHz)
1000
Figure 11. Large-Signal Frequency Response for Various Supplies
2
2
VOUT, dm = 100mV p-p
1
VOUT, dm = 2V p-p
1
0
–1
CLOSED-LOOP GAIN (dB)
0
CLOSED-LOOP GAIN (dB)
–3
Figure 10. Large-Signal Frequency Response for Various Gains
2
TA = –40°C
TA = +25°C
TA = +105°C
–2
–3
–4
–5
TA = –40°C
TA = +25°C
TA = +105°C
–1
–2
–3
–4
–5
–6
07957-009
–6
–7
–8
G = 1, RT = 53.6Ω
G = 2, RT = 57.6Ω
G = 3, RT = 61.9Ω
–2
–8
1000
Figure 7. Small-Signal Frequency Response for Various Gains
–8
0
–1
1
10
100
FREQUENCY (MHz)
–7
–8
1000
Figure 9. Small-Signal Frequency Response for Various Temperatures
07957-012
–8
VOUT, dm = 2V p-p
1
07957-010
NORMALIZED CLOSED-LOOP GAIN (dB)
VOUT, dm = 100mV p-p
1
07957-007
NORMALIZED CLOSED-LOOP GAIN (dB)
2
1
10
100
FREQUENCY (MHz)
1000
Figure 12. Large-Signal Frequency Response for Various Temperatures
Rev. 0 | Page 9 of 28
ADA4950-1/ADA4950-2
2
2
VOUT, dm = 100mV p-p
1
CLOSED-LOOP GAIN (dB)
RL = 1kΩ
RL = 200Ω
–2
–3
–4
–5
–4
–5
1
10
100
FREQUENCY (MHz)
–7
–8
1000
Figure 13. Small-Signal Frequency Response at Various Loads
1
10
100
FREQUENCY (MHz)
1000
Figure 16. Large-Signal Frequency Response at Various Loads
2
2
VOUT, dm = 100mV p-p
1
VOUT, dm = 2V p-p
1
0
–1
CLOSED-LOOP GAIN (dB)
0
CLOSED-LOOP GAIN (dB)
–3
07957-016
–7
VOCM = –2.5VDC
VOCM = 0V
VOCM = +2.5VDC
–2
–3
–4
–5
–1
VOCM = –2.5VDC
VOCM = 0V
VOCM = +2.5VDC
–2
–3
–4
–5
–6
07957-014
–6
–7
–8
RL = 1kΩ
RL = 200Ω
–2
–6
07957-013
–6
–1
1
10
100
FREQUENCY (MHz)
07957-017
CLOSED-LOOP GAIN (dB)
0
–1
–8
VOUT, dm = 2V p-p
1
0
–7
–8
1000
Figure 14. Small-Signal Frequency Response for Various VOCM Levels
1
10
100
FREQUENCY (MHz)
1000
Figure 17. Large-Signal Frequency Response for Various VOCM Levels
4
4
VOUT, dm = 100mV p-p
0
CL = 0pF
CL = 0.9pF
CL = 1.8pF
CL = 2.7pF
–2
–4
0
CL = 0pF
CL = 0.9pF
CL = 1.8pF
CL = 2.7pF
–2
–4
1
10
100
FREQUENCY (MHz)
–8
1000
Figure 15. Small-Signal Frequency Response at Various Capacitive Loads
07957-018
–6
07957-015
–6
–8
VOUT, dm = 2V p-p
2
CLOSED-LOOP GAIN (dB)
CLOSED-LOOP GAIN (dB)
2
1
10
100
FREQUENCY (MHz)
1000
Figure 18. Large-Signal Frequency Response at Various Capacitive Loads
Rev. 0 | Page 10 of 28
ADA4950-1/ADA4950-2
0.5
0.5
VOUT, dm = 100mV p-p
0.4
0.3
CLOSED-LOOP GAIN (dB)
0.1
0
–0.1
–0.3
–0.4
RL = 1kΩ
R L = 200Ω
AMP 1, RL = 1kΩ
AMP 1, RL = 200Ω
AMP 2, RL = 1kΩ
AMP 2, RL = 200Ω
10
100
FREQUENCY (MHz)
0
–0.1
–0.4
1
2
1000
VOCM (AC) = 2V p-p
1
VOCM (AC) = 100mV p-p
0
0
–1
VOCM GAIN (dB)
VOCM = –2.5VDC
VOCM = 0V
VOCM = +2.5VDC
–2
–3
–4
–3
–4
–5
–6
–6
07957-020
–5
–7
1
10
100
FREQUENCY (MHz)
–7
–8
1000
Figure 20. VOCM Small-Signal Frequency Response at Various DC Levels
–40
–40
VOUT, dm = 2V p-p
–80
HD2, RL, dm = 1kΩ
HD3, RL, dm = 1kΩ
HD2, RL, dm = 200Ω
HD3, RL, dm = 200Ω
–90
–100
–110
–120
–130
1000
VOUT, dm = 2V p-p
–60
–70
–80
–90
HD2,
HD3,
HD2,
HD3,
HD2,
HD3,
G
G
G
G
G
G
=1
=1
=2
=2
=3
=3
–100
–110
–120
–130
1
10
FREQUENCY (MHz)
100
07957-021
–140
0.1
10
100
FREQUENCY (MHz)
–50
HARMONIC DISTORTION (dBc)
–70
1
Figure 23. VOCM Large-Signal Frequency Response at Various DC Levels
–50
–60
VOCM = –2.5VDC
VOCM = 0V
VOCM = +2.5VDC
–2
07957-023
–1
VOCM GAIN (dB)
10
100
FREQUENCY (MHz)
Figure 22. 0.1 dB Flatness, Large-Signal Frequency Response for Various Loads
2
1
RL = 1kΩ
R L = 200Ω
AMP 1, RL = 1kΩ
AMP 1, RL = 200Ω
AMP 2, RL = 1kΩ
AMP 2, RL = 200Ω
–0.5
1000
Figure 19. 0.1 dB Flatness, Small-Signal Frequency Response for Various
Loads
HARMONIC DISTORTION (dBc)
ADA4950-1,
ADA4950-1,
ADA4950-2,
ADA4950-2,
ADA4950-2,
ADA4950-2,
–0.2
–0.3
–0.5
1
0.1
07957-019
ADA4950-1,
ADA4950-1,
ADA4950-2,
ADA4950-2,
ADA4950-2,
ADA4950-2,
–0.2
0.2
07957-022
0.2
Figure 21. Harmonic Distortion vs. Frequency at Various Loads
–140
0.1
1
10
FREQUENCY (MHz)
100
Figure 24. Harmonic Distortion vs. Frequency at Various Gains
Rev. 0 | Page 11 of 28
07957-024
CLOSED-LOOP GAIN (dB)
0.3
–8
VOUT, dm = 2V p-p
0.4
ADA4950-1/ADA4950-2
–40
VOUT, dm = 2V p-p
–50
–50
–60
–60
HARMONIC DISTORTION (dBc)
HD2, ±5V
HD3, ±5V
HD2, ±2.5V
HD3, ±2.5V
–70
–80
–90
–100
–110
–120
–130
–80
–100
–110
–120
100
–140
0
HARMONIC DISTORTION (dBc)
5
8
9
6
7
VOUT, dm (V p-p)
10
11
12
13
14
10MHz
10MHz
30MHz
30MHz
–80
–90
–100
–110
HD2 AT
HD3 AT
HD2 AT
HD3 AT
–60
–70
10MHz
10MHz
30MHz
30MHz
–80
–90
–100
–110
–120
–2
–1
0
1
2
3
4
VOCM (V)
1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8
VOCM (V)
07957-026
–3
Figure 26. Harmonic Distortion vs. VOCM at Various Frequencies, ±5 V Supplies
Figure 29. Harmonic Distortion vs. VOCM at Various Frequencies, 5 V Supply
–40
–50
SPURIOUS-FREE DYNAMIC RANGE (dBc)
–50
HD2, VOUT, dm = 2V p-p
HD3, VOUT, dm = 2V p-p
HD2, VOUT, dm = 4V p-p
HD3, VOUT, dm = 4V p-p
–90
–100
–110
–120
1
10
FREQUENCY (MHz)
100
Figure 27. Harmonic Distortion vs. Frequency at Various VOUT, dm
VOUT, dm = 2V p-p
–60
–70
–80
–90
RL, dm = 200Ω
–100
RL, dm = 1kΩ
–110
–120
–130
–140
0.1
07957-027
–130
07957-029
HARMONIC DISTORTION (dBc)
HD2 AT
HD3 AT
HD2 AT
HD3 AT
–120
HARMONIC DISTORTION (dBc)
4
VOUT, dm = 2V p-p
–50
–70
–140
0.1
3
–40
VOUT, dm = 2V p-p
–60
–80
2
Figure 28. Harmonic Distortion vs. VOUT, dm, f = 10 MHz
–50
–70
1
07957-028
10
FREQUENCY (MHz)
07957-025
1
–30
–60
±5V
±5V
±2.5V
±2.5V
–90
Figure 25. Harmonic Distortion vs. Frequency at Various Supplies
–130
–4
HD2,
HD3,
HD2,
HD3,
–70
–130
–140
0.1
–40
VOCM = 0V
1
10
FREQUENCY (MHz)
100
07957-030
HARMONIC DISTORTION (dBc)
–40
Figure 30. Spurious-Free Dynamic Range vs. Frequency at Various Loads
Rev. 0 | Page 12 of 28
ADA4950-1/ADA4950-2
10
90
–20
60
45
40
0
GAIN
–30
20
GAIN (dB)
–40
–50
–60
–70
–45
0
–90
PHASE
–20
–135
–40
–180
–60
–225
–80
–90
–100
PHASE (Degrees)
–10
NORMALIZED SPECTRUM (dB)
80
VOUT, dm = 2V p-p
0
29.7
29.8
29.9 30.0 30.1 30.2
FREQUENCY (MHz)
30.3
30.4
30.5
–80
1k
10k
100M
1G
–270
10G
0
RL, dm = 200Ω
VIN, dm = 100mV p-p
RL, dm = 200Ω
VIN = 2V p-p
–47
–20
–49
–51
–40
–53
PSRR (dB)
–55
–57
PSRR+
–60
PSRR–
–80
–59
–61
07957-032
–100
–63
1
10
100
FREQUENCY (MHz)
–120
1000
07957-035
CMRR (dB)
10M
Figure 34. Open-Loop Gain and Phase vs. Frequency
–45
1
1000
0
0
RL, dm = 200Ω
VIN, dm = 2V p-p
VOUT, dm = 2V p-p
–20
–20
–40
–30
–40
–60
AMPLIFIER 2 TO
AMPLIFIER 1
–80
–100
–60
–120
–70
10M
100M
FREQUENCY (Hz)
1G
07957-033
–50
–140
AMPLIFIER 1 TO
AMPLIFIER 2
07957-036
CROSSTALK (dB)
–10
1M
10
100
FREQUENCY (MHz)
Figure 35. PSRR vs. Frequency
Figure 32. CMRR vs. Frequency
OUTPUT BALANCE (dB)
1M
FREQUENCY (Hz)
Figure 31. 30 MHz Intermodulation Distortion
–65
100k
07957-240
–120
29.6
07957-031
–110
1
10
100
FREQUENCY (MHz)
Figure 36. Crosstalk vs. Frequency, ADA4950-2
Figure 33. Output Balance vs. Frequency
Rev. 0 | Page 13 of 28
1000
ADA4950-1/ADA4950-2
–10
INPUT SINGLE-ENDED, 50Ω LOAD TERMINATION
OUTPUT DIFFERENTIAL, 100Ω SOURCE TERMINATION
S11: SINGLE-ENDED-TO-SINGLE-ENDED
S22: DIFFERENTIAL-TO-DIFFERENTIAL
–20
RL, dm = 200Ω
VIN, dm = 100mV p-p
1k
100
–30
S11
–40
S22
–60
07957-037
–50
1
10
100
FREQUENCY (MHz)
+OUT
–OUT
VOUT, dm
1
0.1
0.1
1000
1
10
FREQUENCY (MHz)
100
1k
Figure 40. Closed-Loop Output Impedance Magnitude vs. Frequency, G = 1
Figure 37. Return Loss (S11, S22) vs. Frequency
1000
15
10
5
VOLTAGE (V)
G=2
G=3
G=1
0
VOUT, dm
–5
10
–10
1
1
10
100
1k
10k
100k
1M
–15
10M
2 × VIN
0
0.1
0.2
0.3
0.4
0.5
0.6
07957-041
100
07957-038
OUTPUT VOLTAGE NOISE DENSITY (nV/√Hz)
10
07957-040
CLOSED-LOOP OUTPUT
IMPEDANCE MAGNITUDE (Ω)
S-PARAMETERS (dB)
0
0.7
0.8
0.9
1.0
TIME (µs)
FREQUENCY (Hz)
Figure 41. Overdrive Recovery, G = 2
Figure 38. Voltage Noise Spectral Density for Various Gains,
Referred to Output
0.06
1.5
G=3
0.02
0
–0.02
–0.04
–0.06
0
5
10
15
TIME (ns)
20
25
30
Figure 39. Small-Signal Pulse Response for Various Gains
1.0
G=1
G=2
0.5
G=3
0
–0.5
–1.0
–1.5
07957-042
G=2
NORMALIZED OUTPUT VOLTAGE (V)
0.04
07957-039
NORMALIZED OUTPUT VOLTAGE (V)
G=1
0
5
10
15
TIME (ns)
20
25
Figure 42. Large-Signal Pulse Response for Various Gains
Rev. 0 | Page 14 of 28
30
ADA4950-1/ADA4950-2
2.0
0.10
1.5
OUTPUT VOLTAGE (V)
0
CL = 0pF
CL = 0.9pF
CL = 1.8pF
CL = 2.7pF
5
10
0
–1.5
15
TIME (ns)
20
25
–2.0
30
Figure 43. Small-Signal Pulse Response for Various Capacitive Loads
0
5
0
–0.02
07957-044
–0.04
–0.06
25
30
5
10
15
TIME (ns)
20
25
1.0
0.5
0
–0.5
–1.0
–1.5
30
0
5
Figure 44. VOCM Small-Signal Pulse Response
10
15
TIME (ns)
20
25
30
Figure 47. VOCM Large-Signal Pulse Response
1.5
6
0.5
0.4
1.0
1.2
VOCM = +1V DC
5
1.0
4
0.8
3
0.6
2
0.4
ERROR
0.1
0
INPUT
–0.1
–0.5
ERROR (%)
0.2
0.5
–0.2
OUTPUT
PD PIN VOLTAGE (V)
0.3
1
–0.4
–5
0
5
10
15
20
TIME (ns)
25
30
35
40
–0.5
0
07957-045
–1.0
Figure 45. Settling Time
–1
0.2
PD PIN INPUT
(SHOWN INVERTED
FOR CLARITY)
–0.3
0
1
2
3
4
TIME (ms)
5
0
6
Figure 48. PD Response Time
Rev. 0 | Page 15 of 28
7
8
–0.2
07957-048
0
VOLTAGE (V)
20
07957-047
OUTPUT COMMON-MODE VOLTAGE (V)
0.02
–1.5
15
TIME (ns)
1.5
0.04
0
10
Figure 46. Large-Signal Pulse Response for Various Capacitive Loads
0.06
OUTPUT COMMON-MODE VOLTAGE (V)
CL = 0pF
CL = 0.9pF
CL = 1.8pF
CL = 2.7pF
–0.5
07957-046
–0.10
0
0.5
–1.0
07957-043
–0.05
1.0
NONINVERTING OUTPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
0.05
ADA4950-1/ADA4950-2
TEST CIRCUITS
+5V
NC
DC-COUPLED
SOURCE
500Ω
250Ω
500Ω
50Ω
53.6Ω
VIN
VOCM
ADA4950-x
1kΩ
500Ω
0.1µF
NC
250Ω
500Ω
07957-049
25.5Ω
–5V
Figure 49. Equivalent Basic Test Circuit, G = 1
DIFFERENTIAL NETWORK
ANALYZER SOURCE
DIFFERENTIAL NETWORK
ANALYZER RECEIVER
+5V
49.9Ω
250Ω
NC
56.2Ω
500Ω
49.9Ω
50Ω
500Ω
VOCM
ADA4950-x
500Ω
50Ω
56.2Ω
NC
500Ω
49.9Ω
07957-051
49.9Ω
250Ω
–5V
Figure 50. Test Circuit for Output Balance, CMRR
+5V
NC
DC-COUPLED
SOURCE
LOW-PASS
FILTER
VIN
500Ω
0.1µF
500Ω
53.6Ω
VOCM
ADA4950-x
261Ω
2:1
DUAL
FILTER
CT
0.1µF 442Ω
500Ω
25.5Ω
50Ω
200Ω
442Ω
0.1µF
NC
250Ω
500Ω
–5V
Figure 51. Test Circuit for Distortion Measurements
Rev. 0 | Page 16 of 28
07957-252
50Ω
250Ω
ADA4950-1/ADA4950-2
TERMINOLOGY
+INA
VOCM
–INA
–INB
RGB
RF
250Ω
500Ω
RGA
+IN
500Ω
500Ω
RGB
–OUT
ADA4950-x
RGA
Common-Mode Voltage
Common-mode voltage refers to the average of two node
voltages with respect to the local ground reference. The output
common-mode voltage is defined as
–IN
250Ω
RL, dm VOUT, dm
+OUT
RF
500Ω
VOUT, cm = (V+OUT + V−OUT)/2
07957-152
+INB
Figure 52. Signal and Circuit Definitions
Differential Voltage
Differential voltage refers to the difference between two node
voltages. For example, the output differential voltage (or equivalently, output differential node voltage) is defined as
VOUT, dm = (V+OUT − V−OUT)
Output Balance
Output balance is a measure of how close the output differential
signals are to being equal in amplitude and opposite in phase.
Any imbalances in amplitude or phase produce an undesired
common-mode signal at the amplifier output. Output balance
error is defined as the magnitude of the output common-mode
voltage divided by the magnitude of the output differential
mode voltage.
where V+OUT and V−OUT refer to the voltages at the +OUT and
−OUT output terminals with respect to a common ground
reference.
The input differential voltage is defined in different ways,
depending upon the selected gain.
For G = 1
VIN, dm = (+INA − (−INA))
where +INA and −INA refer to the voltages at the +INA and
−INA input terminals with respect to a common ground
reference (input terminals +INB and −INB are floating).
For G = 2
VIN, dm = (+INB − (−INB))
where +INB and −INB refer to the voltages at the +INB and
−INB input terminals with respect to a common ground
reference (input terminals +INA and −INA are floating).
For G = 3, input terminals +INA and +INB are connected
together, and input terminals −INA and −INB are connected
together.
VIN, dm = (+INAB − (−INAB))
where +INAB and −INAB refer to the voltages at the connection
of input terminals +INA and +INB and at the connection of
input terminals −INA and −INB with respect to a common
ground reference.
Rev. 0 | Page 17 of 28
Output Balance Error =
ΔVOUT , cm
ΔVOUT , dm
ADA4950-1/ADA4950-2
THEORY OF OPERATION
The ADA4950-x differs from conventional op amps in that it has
two outputs whose voltages move in opposite directions and an
additional input, VOCM. Like an op amp, it relies on high open-loop
gain and negative feedback to force these outputs to the desired
voltages. The ADA4950-x behaves much like a standard voltage
feedback op amp and facilitates single-ended-to-differential
conversions, common-mode level shifting, and amplifications
of differential signals. Like an op amp, the ADA4950-x has high
input impedance at its internal input terminals (to the right of
the internal gain resistors) and low output impedance. Because
it uses voltage feedback, the ADA4950-x manifests a nominally
constant gain bandwidth product.
Two feedback loops are used to control the differential and
common-mode output voltages. The differential feedback loop,
set with on-chip feedback and gain resistors, controls only the
differential output voltage. The common-mode feedback loop is
internal to the actual amplifier and controls only the commonmode output voltage. This architecture makes it easy to set the
output common-mode level to any arbitrary value within the
specified limits. The output common-mode voltage is forced, by
the internal common-mode feedback loop, to be equal to the
voltage applied to the VOCM input.
The internal common-mode feedback loop produces outputs
that are highly balanced over a wide frequency range without
requiring tightly matched external components. This results in
differential outputs that are very close to the ideal of being
identical in amplitude and that are exactly 180° apart in phase.
Rev. 0 | Page 18 of 28
ADA4950-1/ADA4950-2
APPLICATIONS INFORMATION
ANALYZING AN APPLICATION CIRCUIT
The ADA4950-x uses high open-loop gain and negative feedback
to force its differential and common-mode output voltages in
such a way as to minimize the differential and common-mode
error voltages. The differential error voltage is defined as the
voltage between the differential inputs labeled +INx and −INx
(see Figure 52). For most purposes, this voltage can be assumed
to be 0. Similarly, the difference between the actual output
common-mode voltage and the voltage applied to VOCM can
also be assumed to be 0. Starting from these principles, any
application circuit can be analyzed.
SELECTING THE CLOSED-LOOP GAIN
Using the approach described in the Analyzing an Application
Circuit section, the differential gain of the circuit in Figure 52
can be determined by
V IN , dm
=
RF
RG
where the input resistors (RG) and the feedback resistors (RF) on
each side are equal.
For G = 1, the +INA and −INA inputs are used, and the +INB
and −INB inputs are left floating. The differential gain in this
case is calculated as follows:
G=
G=
500 Ω
RF
=
=3
RG 500 Ω || 250 Ω
ESTIMATING THE OUTPUT NOISE VOLTAGE
The differential output noise of the ADA4950-x can be estimated
using the noise model in Figure 53. The values of RG depend on
the selected gain. The input-referred noise voltage density, vnIN,
is modeled as a differential input, and the noise currents, inIN− and
inIN+, appear between each input and ground. The output voltage
due to vnIN is obtained by multiplying vnIN by the noise gain, GN
(defined in the GN equation that follows Table 13). The noise
currents are uncorrelated with the same mean-square value,
and each produces an output voltage that is equal to the noise
current multiplied by the associated feedback resistance. The
noise voltage density at the VOCM pin is vnCM. When the feedback
networks have the same feedback factor, as is true in most cases,
the output noise due to vnCM is common mode. Each of the four
resistors contributes (4kTRxx)1/2. The noise from the feedback
resistors appears directly at the output, and the noise from the
gain resistors appears at the output multiplied by RF/RG. Table 11
summarizes the input noise sources, the multiplication factors,
and the output-referred noise density terms.
R F 500 Ω
=
=1
RG 500 Ω
vnRG1
vnRF1
RF1
inIN+
For G = 2, the +INB and −INB inputs are used, and the +INA
and −INA inputs are left floating. The differential gain in this
case is calculated as follows:
G=
RG1
+
inIN–
R F 500 Ω
=
=2
RG 250 Ω
vnIN
ADA4950-x
vnOD
VOCM
vnRG2
RG2
RF2
vnCM
vnRF2
07957-053
VOUT , dm
For G = 3, the +INA and +INB inputs are connected together,
and the −INA and −INB inputs are connected together. The
differential gain in this case is calculated as follows:
Figure 53. Noise Model
Table 11. Output Noise Voltage Density Calculations for Matched Feedback Networks
Input Noise Contribution
Differential Input
Inverting Input
Noninverting Input
VOCM Input
Gain Resistor, RG1
Gain Resistor, RG2
Feedback Resistor, RF1
Feedback Resistor, RF2
Input Noise Term
vnIN
inIN−
inIN+
vnCM
vnRG1
vnRG2
vnRF1
vnRF2
Input Noise
Voltage Density
vnIN
inIN− × (RF2)
inIN+ × (RF1)
vnCM
(4kTRG1)1/2
(4kTRG2)1/2
(4kTRF1)1/2
(4kTRF2)1/2
Rev. 0 | Page 19 of 28
Output
Multiplication Factor
GN
1
1
0
RF1/RG1
RF2/RG2
1
1
Differential Output Noise
Voltage Density Term
vnO1 = GN(vnIN)
vnO2 = (inIN−)(RF2)
vnO3 = (inIN+)(RF1)
vnO4 = 0 V
vnO5 = (RF1/RG1)(4kTRG1)1/2
vnO6 = (RF2/RG2)(4kTRG2)1/2
vnO7 = (4kTRF1)1/2
vnO8 = (4kTRF2)1/2
ADA4950-1/ADA4950-2
Table 12. Differential Input, DC-Coupled
Nominal Linear Gain
1
2
3
RF (Ω)
500
500
500
RG (Ω)
500
250
250||500
RIN, dm (Ω)
1000
500
333
Differential Output Noise Density (nV/√Hz)
9.25
12.9
16.6
Table 13. Single-Ended, Ground-Referenced Input, DC-Coupled, RS = 50 Ω
Nominal Linear Gain
1
2
3
RG1 (Ω)
500
250
250||500
RT (Ω) (Std 1%)
53.6
57.6
61.9
RIN, se (Ω)
667
375
267
RG2 (Ω)1
526
277
194
Differential Output Noise Density (nV/√Hz)
9.07
12.2
15.0
RG2 = RG1 + (RS||RT).
RF
Similar to the case of a conventional op amp, the output noise
voltage densities can be estimated by multiplying the inputreferred terms at +INx and −INx by the appropriate output
factor, where:
+VS
β1 =
RG
R IN , se
Note that the output noise from VOCM goes to 0 in this case. The
total differential output noise density, vnOD, is the root-sumsquare of the individual output noise terms.
⎛
⎞
⎜
⎟
R
G
⎜
⎟
=
⎜
⎟
RF
1
−
⎜
⎟
(
)
2
R
R
×
+
F ⎠
G
⎝
RF
8
2
∑ v nOi
+VS
RIN, se
i =1
RG
Table 12 and Table 13 list the three available gain settings,
associated resistor values, input impedance, and output noise
density for both balanced and unbalanced input configurations.
VOCM
ADA4950-x
–VS
RF
The effective input impedance of a circuit depends on whether
the amplifier is being driven by a single-ended or differential
signal source. For balanced differential input signals, as shown
in Figure 54, the input impedance (RIN, dm) is
RL
VOUT, dm
RG
CALCULATING THE INPUT IMPEDANCE FOR AN
APPLICATION CIRCUIT
The value of RG depends on the selected gain.
–IN
For an unbalanced, single-ended input signal (see Figure 55),
the input impedance is
1
R
=1+ F
β
RG
RIN, dm = (RG + RG) = 2 × RG
VOUT, dm
Figure 54. ADA4950-x Configured for Balanced (Differential) Inputs
When the feedback factors are matched, RF1/RG1 = RF2/RG2,
β1 = β2 = β, and the noise gain becomes
v nOD =
ADA4950-x
–VS
RF
RG1
RG2
and β2 =
are the feedback factors.
RF1 + RG1
RF2 + RG2
GN =
VOCM
VIN, dm
2
GN =
is the circuit noise gain.
( β1 + β 2 )
+IN
07957-054
RG
07957-055
1
RF (Ω)
500
500
500
Figure 55. ADA4950-x with Unbalanced (Single-Ended) Input
The input impedance of the circuit is effectively higher than it
is for a conventional op amp connected as an inverter because a
fraction of the differential output voltage appears at the inputs
as a common-mode signal, partially bootstrapping the voltage
across the input resistor, RG. The common-mode voltage at the
amplifier input terminals can be easily determined by noting
that the voltage at the inverting input is equal to the noninverting
output voltage divided down by the voltage divider that is formed
by RF and RG in the lower loop. This voltage is present at both
input terminals due to negative voltage feedback and is in phase
with the input signal, thus reducing the effective voltage across
RG in the upper loop and partially bootstrapping RG.
Rev. 0 | Page 20 of 28
ADA4950-1/ADA4950-2
Terminating a Single-Ended Input
Figure 57 shows that the effective RG in the upper feedback
loop is now greater than the RG in the lower loop due to the
addition of the termination resistors. To compensate for the
imbalance of the gain resistors, add a correction resistor (RTS)
in series with RG in the lower loop. RTS is the Thevenin
equivalent of the source resistance, RS, and the termination
resistance, RT, and is equal to RS||RT.
3.
This section describes how to properly terminate a single-ended
input to the ADA4950-x with a gain of 1, RF = 500 Ω, and RG =
500 Ω. An example using an input source with a terminated output
voltage of 1 V p-p and source resistance of 50 Ω illustrates the
steps that must be followed. Note that because the terminated
output voltage of the source is 1 V p-p, the open-circuit output
voltage of the source is 2 V p-p. The source shown in Figure 56
indicates this open-circuit voltage.
RS
The input impedance is calculated using the following
formula:
VS
2V p-p
⎛
⎞ ⎛
⎞
⎜
⎟ ⎜
⎟
RG
500
⎜
⎟
⎜
⎟ = 667 Ω
=
RIN , se =
⎜
⎟ ⎜
500
⎟
RF
⎜ 1 − 2 × ( R + R ) ⎟ ⎜ 1 − 2 × ( 500 + 500) ⎟
⎠
⎝
⎠ ⎝
G
F
+VS
RG
50Ω
500Ω
VOCM
RF
500Ω
+VS
ADA4950-x
RL VOUT, dm
RG
VTH
1.03V p-p
500Ω
RTH
RG
25.5Ω
500Ω
VOCM
–VS
07957-156
RF
500Ω
RTS
25.5Ω
500Ω
Figure 59. Thevenin Equivalent and Matched Gain Resistors
Figure 59 presents a tractable circuit with matched feedback
loops that can be easily evaluated.
RF
RS
500Ω
+VS
RG
RT
53.6Ω
500Ω
VOCM
ADA4950-x
RL
VOUT, dm
RG
500Ω
–VS
RF
500Ω
Figure 57. Adding Termination Resistor, RT
07957-157
VS
2V p-p
50Ω
500Ω
RF
To match the 50 Ω source resistance, calculate the
termination resistor, RT, using RT||667 Ω = 50 Ω. The
closest standard 1% value for RT is 53.6 Ω.
RIN, se
50Ω
RL VOUT, dm
–VS
Figure 56. Calculating Single-Ended Input Impedance, RIN
2.
ADA4950-x
RG
07957-059
RS
VTH
1.03V p-p
25.9Ω
Note that VTH is greater than 1 V p-p, which was obtained
with RT = 50 Ω. The modified circuit with the Thevenin
equivalent (closest 1% value used for RTH) of the terminated
source and RTS in the lower feedback loop is shown in
Figure 59.
RF
VS
2V p-p
RT
53.6Ω
Figure 58. Calculating the Thevenin Equivalent
500Ω
RIN, se
667Ω
RTH
50Ω
07957-052
1.
RTS = RTH = RS||RT = 25.9 Ω
It is useful to point out two effects that occur with a terminated
input. The first is that the value of RG is increased in both loops,
lowering the overall closed-loop gain. The second is that VTH is
a little larger than 1 V p-p, as it would be if RT = 50 Ω. These two
effects have opposite impacts on the output voltage, and for
large resistor values in the feedback loops (~1 kΩ), the effects
essentially cancel each other out. For small RF and RG, or high
gains, however, the diminished closed-loop gain is not canceled
completely by the increased VTH. This can be seen by evaluating
Figure 59.
The desired differential output in this example is 1 V p-p
because the terminated input signal is 1 V p-p and the closedloop gain = 1. The actual differential output voltage, however, is
equal to (1.03 V p-p)(500/525.5) = 0.98 V p-p.
Rev. 0 | Page 21 of 28
ADA4950-1/ADA4950-2
+VS
INPUT COMMON-MODE VOLTAGE RANGE
The ADA4950-x input common-mode voltage range is shifted
down by approximately one VBE, in contrast to other ADC
drivers with centered input ranges such as the ADA4939-x. The
downward-shifted input common-mode range is especially
suited to dc-coupled, single-ended-to-differential, and singlesupply applications.
×1
250Ω
500Ω
×4
×1
For ±5 V operation, the input common-mode voltage range at the
summing nodes of the amplifier is specified as −4.8 V to +3.2 V.
With a 5 V supply, the input common-mode voltage range at the
summing nodes of the amplifier is specified as +0.2 V to +3.2 V.
To avoid nonlinearities, the voltage swing at the +INx and −INx
terminals must be confined to these ranges.
×4
×1
×1
×1
500Ω
VOCM
ADA4950-x
500Ω
×4
×1
250Ω
500Ω
×4
Although the ADA4950-x is well suited to dc-coupled applications, it is nonetheless possible to use it in ac-coupled circuits.
Input ac coupling capacitors can be inserted between the source
and RG. This ac coupling blocks the flow of the dc commonmode feedback current and causes the ADA4950-x dc input
common-mode voltage to equal the dc output common-mode
voltage. The ac coupling capacitors must be placed in both
loops to keep the feedback factors matched. Output ac coupling
capacitors can be placed in series between each output and its
respective load.
INPUT SIGNAL SWING CONSIDERATIONS
The input terminals of fully differential amplifiers with external
gain and feedback resistors connect directly to the amplifier
summing nodes; the common-mode voltage swing at these
terminals is generally smaller than the input and output swings.
In most linear applications, the summing node voltages do not
approach levels that result in the forward-biasing of the internal
ESD protection diodes on the amplifier inputs.
Signals at the inputs of the ADA4950-x are applied to the input
side of the gain resistors, and, if caution is not exercised, these
signals can be large enough to forward-bias the ESD protection
diodes. The four inputs that make up the differential signal paths
each have four ESD diodes in series to the negative supply and
one diode to the positive supply; the VOCM input has one ESD
diode to each supply. Figure 60 illustrates the ESD protection
circuitry.
–VS
07957-253
INPUT AND OUTPUT CAPACITIVE AC COUPLING
Figure 60. Input ESD Protection Circuitry
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The VOCM pin of the ADA4950-x is internally biased with a voltage divider comprising two 50 kΩ resistors across the supplies,
with a tap at a voltage approximately equal to the midsupply
point, [(+VS) + (−VS)]/2. Because of this internal divider, the
VOCM pin sources and sinks current, depending on the externally
applied voltage and its associated source resistance. Relying on
the internal bias results in an output common-mode voltage
that is within approximately 100 mV of the expected value.
In cases where more accurate control of the output commonmode level is required, it is recommended that an external
source or resistor divider be used with source resistance less
than 100 Ω. If an external voltage divider consisting of equal
resistor values is used to set VOCM to midsupply with greater
accuracy than produced internally, higher values can be used
because the external resistors are placed in parallel with the
internal resistors. The input VOCM offset listed in the Specifications
section assumes that the VOCM input is driven by a low impedance
voltage source.
It is also possible to connect the VOCM input to a common-mode
level (CML) output of an ADC; however, care must be taken to
ensure that the output has sufficient drive capability. The input
impedance of the VOCM pin is approximately 10 kΩ to a voltage
of nominally midsupply. If multiple ADA4950-x devices share
one ADC reference output, a buffer may be necessary to drive
the parallel inputs.
Rev. 0 | Page 22 of 28
ADA4950-1/ADA4950-2
LAYOUT, GROUNDING, AND BYPASSING
Signal routing should be short and direct to avoid parasitic
effects. Wherever complementary signals exist, provide a
symmetrical layout to maximize balanced performance. When
routing differential signals over a long distance, keep PCB
traces close together, and twist any differential wiring to
minimize loop area. Doing this reduces radiated energy and
makes the circuit less susceptible to interference.
As a high speed device, the ADA4950-x is sensitive to the
PCB environment in which it operates. Realizing its superior
performance requires attention to the details of high speed
PCB design.
The first requirement is a solid ground plane that covers as
much of the board area around the ADA4950-x as possible. The
thermal resistance, θJA, is specified for the device, including the
exposed pad, soldered to a high thermal conductivity 4-layer
circuit board, as described in EIA/JESD51-7.
1.30
0.80
Bypass the power supply pins as close to the device as possible
and directly to a nearby ground plane. Use high frequency
ceramic chip capacitors. It is recommended that two parallel
bypass capacitors (1000 pF and 0.1 μF) be used for each supply.
Place the 1000 pF capacitor closer to the device. Farther away,
provide low frequency bulk bypassing using 10 μF tantalum
capacitors from each supply to ground.
07957-056
1.30 0.80
Figure 61. Recommended PCB Thermal Attach Pad (Dimensions in Millimeters)
1.30
TOP METAL
GROUND PLANE
0.30
PLATED
VIA HOLE
07957-057
POWER PLANE
BOTTOM METAL
Figure 62. Cross-Section of 4-Layer PCB Showing Thermal Via Connection to Buried Ground Plane (Dimensions in Millimeters)
Rev. 0 | Page 23 of 28
ADA4950-1/ADA4950-2
HIGH PERFORMANCE ADC DRIVING
Because the inputs are dc-coupled, dc common-mode current
flows in the feedback loops, and a nominal dc level of 0.76 V is
present at the amplifier input terminals. A fraction of the output
signal is also present at the input terminals as a common-mode
signal; its level is equal to the ac output swing at the noninverting
output, divided down by the feedback factor of the lower loop. In
this example, this ripple is 0.5 V p-p × [276.7/(276.7 + 500)] =
0.18 V p-p. This ac signal is riding on the 0.76 V dc level, producing
a voltage swing between 0.67 V and 0.85 V at the input terminals.
This is well within the specified limits of 0.2 V to 1.5 V.
The ADA4950-x is ideally suited for broadband dc-coupled
applications. The circuit in Figure 63 shows a front-end
connection for an ADA4950-1 driving an AD9245 ADC,
with dc coupling on the ADA4950-1 input and output. (The
AD9245 achieves its optimum performance when driven
differentially.) The ADA4950-1 eliminates the need for a
transformer to drive the ADC and performs a single-ended-todifferential conversion and buffering of the driving signal.
The ADA4950-1 is configured with a single 3.3 V supply and a
gain of 2 for a single-ended input to differential output. The
57.6 Ω termination resistor, in parallel with the single-ended
input impedance of 375 Ω, provides a 50 Ω termination for the
source. The additional 26.7 Ω Thevenin resistance added to the
inverting input balances the parallel impedance of the 50 Ω
source and the termination resistor driving the noninverting
input. The required Thevenin bias voltage of 0.27 VDC applied
to the lower loop is obtained by scaling the VREF output of the
AD9245 and buffering it with the AD8031.
With an output common-mode voltage of 1.65 V, each ADA4950-1
output swings between 1.4 V and 1.9 V, opposite in phase, providing a gain of 2 and a 1 V p-p differential signal to the ADC input.
The differential RC section between the ADA4950-1 output and
the ADC provides single-pole low-pass filtering and extra buffering
for the current spikes that are output from the ADC input when
its SHA capacitors are discharged.
The AD9245 is configured for a 1 V p-p full-scale input by
connecting its SENSE pin to VREF, as shown in Figure 63.
In this example, the 50 Ω signal generator has a 1 V p-p unipolar
open-circuit output voltage, and 0.5 V p-p output voltage when
terminated in 50 Ω. The VOCM input is bypassed for noise reduction and set externally with 1% resistors to maximize output
dynamic range on the tight 3.3 V supply.
+3.3V
VOUT, dm = 1V p-p
VOUT, cm = +1.65V
1.0V
50Ω
0.5V
1.0V p-p
UNIPOLAR
SIGNAL
SOURCE
0.1µF
0.1µF
0.1µF
500Ω
57.6Ω
10kΩ
NC
500Ω
VOCM
10kΩ
NC
ADA4950-1
20pF
AD9245
33Ω
500Ω
AVDD
VIN–
33Ω
VIN+
VREF SENSE
250Ω
AGND
500Ω
0.1µF
26.7Ω
10µF
+
866Ω
0.1µF
0.1µF
10µF
+
0.1µF
1.0kΩ
AD8031
07957-254
0V
250Ω
Figure 63. ADA4950-1 Driving an AD9245 ADC with Unipolar DC-Coupled Input and Output, Gain = 2
Rev. 0 | Page 24 of 28
ADA4950-1/ADA4950-2
OUTLINE DIMENSIONS
3.00
BSC SQ
0.60 MAX
0.45
13
16
12 (BOTTOM VIEW) 1
2.75
BSC SQ
TOP
VIEW
EXPOSED
PAD
9
0.50
BSC
12° MAX
0.30
0.23
0.18
0.25 MIN
FOR PROPER CONNECTION OF
THE EXPOSED PAD, REFER TO
THE PIN CONFIGURATION AND
FUNCTION DESCRIPTIONS
SECTION OF THIS DATA SHEET.
0.05 MAX
0.02 NOM
SEATING
PLANE
4
5
1.50 REF
0.80 MAX
0.65 TYP
1.00
0.85
0.80
8
PIN 1
INDICATOR
*1.45
1.30 SQ
1.15
0.20 REF
072208-A
PIN 1
INDICATOR
0.50
0.40
0.30
*COMPLIANT TO JEDEC STANDARDS MO-220-VEED-2
EXCEPT FOR EXPOSED PAD DIMENSION.
Figure 64. 16-Lead Lead Frame Chip Scale Package [LFCSP_VQ]
3 mm × 3 mm Body, Very Thin Quad (CP-16-2)
Dimensions shown in millimeters
0.60 MAX
4.00
BSC SQ
TOP
VIEW
3.75
BSC SQ
0.50
BSC
0.50
0.40
0.30
1.00
0.85
0.80
12° MAX
0.80 MAX
0.65 TYP
0.30
0.23
0.18
SEATING
PLANE
PIN 1
INDICATOR
24 1
19
18
2.25
2.10 SQ
1.95
EXPOSED
PAD
(BOTTOM VIEW)
13
12
7
6
0.25 MIN
2.50 REF
0.05 MAX
0.02 NOM
COPLANARITY
0.08
0.20 REF
FOR PROPER CONNECTION OF
THE EXPOSED PAD, REFER TO
THE PIN CONFIGURATION AND
FUNCTION DESCRIPTIONS
SECTION OF THIS DATA SHEET.
COMPLIANT TO JEDEC STANDARDS MO-220-VGGD-2
072208-A
PIN 1
INDICATOR
0.60 MAX
Figure 65. 24-Lead Lead Frame Chip Scale Package [LFCSP_VQ]
4 mm × 4 mm Body, Very Thin Quad (CP-24-1)
Dimensions shown in millimeters
ORDERING GUIDE
Model
ADA4950-1YCPZ-R2 1
ADA4950-1YCPZ-RL1
ADA4950-1YCPZ-R71
ADA4950-2YCPZ-R21
ADA4950-2YCPZ-RL1
ADA4950-2YCPZ-R71
1
Temperature Range
−40°C to +105°C
−40°C to +105°C
−40°C to +105°C
−40°C to +105°C
−40°C to +105°C
−40°C to +105°C
Package Description
16-Lead LFCSP_VQ
16-Lead LFCSP_VQ
16-Lead LFCSP_VQ
24-Lead LFCSP_VQ
24-Lead LFCSP_VQ
24-Lead LFCSP_VQ
Z = RoHS Compliant Part.
Rev. 0 | Page 25 of 28
Package Option
CP-16-2
CP-16-2
CP-16-2
CP-24-1
CP-24-1
CP-24-1
Ordering Quantity
250
5,000
1,500
250
5,000
1,500
Branding
H1L
H1L
H1L
ADA4950-1/ADA4950-2
NOTES
Rev. 0 | Page 26 of 28
ADA4950-1/ADA4950-2
NOTES
Rev. 0 | Page 27 of 28
ADA4950-1/ADA4950-2
NOTES
©2009 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D07957-0-5/09(0)
Rev. 0 | Page 28 of 28