AD AD8005 270 mhz, 400 ua current feedback amplifier Datasheet

a
270 MHz, 400 ␮A
Current Feedback Amplifier
AD8005
FUNCTIONAL BLOCK DIAGRAM
FEATURES
Ultralow Power
400 ␮A Power Supply Current (4 mW on ⴞ5 VS)
Specified for Single Supply Operation
High Speed
270 MHz, –3 dB Bandwidth (G = +1)
170 MHz, –3 dB Bandwidth (G = +2)
280 V/␮s Slew Rate (G = +2)
28 ns Settling Time to 0.1%, 2 V Step (G = +2)
Low Distortion/Noise
–63 dBc @ 1 MHz, V O = 2 V p-p
–50 dBc @ 10 MHz, VO = 2 V p-p
4.0 nV/√Hz Input Voltage Noise @ 10 MHz
Good Video Specifications (RL = 1 k⍀, G = +2)
Gain Flatness 0.1 dB to 30 MHz
0.11% Differential Gain Error
0.4ⴗ Differential Phase Error
8-Lead Plastic DIP and SOIC
NC 1
PRODUCT DESCRIPTION
The AD8005 is an ultralow power, high-speed amplifier with a
wide signal bandwidth of 170 MHz and slew rate of 280 V/µs.
This performance is achieved while consuming only 400 µA of
quiescent supply current. These features increase the operating
time of high-speed battery-powered systems without reducing
dynamic performance.
6 OUT
–VS 4
5 NC
5-Lead SOT-23
5 +VS
OUT 1
–VS 2
+IN
4 –IN
3
The current feedback design results in gain flatness of 0.1 dB
to 30 MHz while offering differential gain and phase errors of
0.11% and 0.4°. Harmonic distortion is low over a wide
bandwidth with THDs of –63 dBc at 1 MHz and –50 dBc at
10 MHz. Ideal features for a signal conditioning amplifier or
buffer to a high-speed A-to-D converter in portable video,
medical or communication systems.
The AD8005 is characterized for +5 V and ± 5 V supplies and
will operate over the industrial temperature range of –40°C to
+85°C. The amplifier is supplied in 8-lead plastic DIP, 8-lead
SOIC and 5-lead SOT-23 packages.
–40
G = +2
VOUT = 200mV p-p
RL = 1kV
2ND
G = +2
VOUT = 2V p-p
RL = 1kV
–50
3RD
0
–1
DISTORTION – dBc
NORMALIZED GAIN – dB
7 +VS
+IN 3
NC = NO CONNECT
3
1
8 NC
AD8005
APPLICATIONS
Signal Conditioning
A/D Buffer
Power-Sensitive, High-Speed Systems
Battery Powered Equipment
Loop/Remote Power Systems
Communication or Video Test Systems
Portable Medical Instruments
2
AD8005
–IN 2
VS = ±5V
–2
–3
–60
3RD
–70
2ND
–80
VS = +5V
–4
–90
–5
–6
0.1
1
10
FREQUENCY – MHz
100
500
Figure 1. Frequency Response; G = +2, VS = +5 V or ± 5 V
–100
1
10
20
FREQUENCY – MHz
Figure 2. Distortion vs. Frequency; VS = ± 5 V
REV. A
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
which may result from its use. No license is granted by implication or
otherwise under any patent or patent rights of Analog Devices.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781/329-4700
World Wide Web Site: http://www.analog.com
Fax: 781/326-8703
© Analog Devices, Inc., 1999
AD8005–SPECIFICATIONS
ⴞ5 V SUPPLIES (@ T = +25ⴗC, V = ⴞ5 V, R = 1 k⍀ unless otherwise noted)
A
S
L
Parameter
Conditions
DYNAMIC PERFORMANCE
RF = 3.01 kΩ for “N” Package or
RF = 2.49 kΩ for “R” Package or
RF = 2.10 kΩ for “RT” Package
G = +1, VO = 0.2 V p-p
G = +2, VO = 0.2 V p-p
G = +2, VO = 0.2 V p-p
G = +10, VO = 4 V p-p, RF = 499 Ω
G = +2, VO = 4 V Step
G = –1, VO = 4 V Step, RF = 1.5 kΩ
G = +2, VO = 2 V Step
–3 dB Small Signal Bandwidth
Bandwidth for 0.1 dB Flatness
Large Signal Bandwidth
Slew Rate (Rising Edge)
Settling Time to 0.1%
DISTORTION/NOISE PERFORMANCE
Total Harmonic Distortion
Differential Gain
Differential Phase
Input Voltage Noise
Input Current Noise
Min
225
140
10
RF = 3.01 kΩ for “N” Package or
RF = 2.49 kΩ for “R” Package or
RF = 2.10 kΩ for “RT” Package
fC = 1 MHz, VO = 2 V p-p, G = +2
fC = 10 MHz, VO = 2 V p-p, G = +2
NTSC, G = +2
NTSC, G = +2
f = 10 MHz
f = 10 MHz, +IIN
–IIN
DC PERFORMANCE
Input Offset Voltage
AD8005A
Typ
Max
270
170
30
40
280
1500
28
MHz
MHz
MHz
MHz
V/µs
V/µs
ns
–63
–50
0.11
0.4
4.0
1.1
9.1
dBc
dBc
%
Degrees
nV/√Hz
pA/√Hz
pA/√Hz
400
6
1000
± mV
± mV
µV/°C
±µA
±µA
±µA
±µA
nA/°C
kΩ
VCM = ± 2.5 V
46
90
260
1.6
3.8
54
MΩ
Ω
pF
±V
dB
Positive
Negative
RL = 50 Ω
+3.7
5
TMIN to TMAX
Offset Drift
+Input Bias Current
40
0.5
TMIN to TMAX
–Input Bias Current
5
TMIN to TMAX
Input Bias Current Drift (± )
Open-Loop Transimpedance
INPUT CHARACTERISTICS
Input Resistance
Input Capacitance
Input Common-Mode Voltage Range
Common-Mode Rejection Ratio
OUTPUT CHARACTERISTICS
Output Voltage Swing
Output Current
Short Circuit Current
+Input
–Input
+Input
POWER SUPPLY
Quiescent Current
Power Supply Rejection Ratio
Units
+3.90
–3.90
10
60
400
TMIN to TMAX
VS = ± 4 V to ± 6 V
OPERATING TEMPERATURE RANGE
56
–40
30
50
1
2
10
12
–3.7
V
V
mA
mA
475
560
µA
µA
dB
+85
°C
66
Specifications subject to change without notice.
–2–
REV. A
AD8005
+5 V SUPPLY (@ T = +25ⴗC, V = +5 V, R = 1 k⍀ to 2.5 V unless otherwise noted)
A
S
L
Parameter
Conditions
DYNAMIC PERFORMANCE
RF = 3.01 kΩ for “N” Package or
RF = 2.49 kΩ for “R” Package or
RF = 2.10 kΩ for “RT” Package
G = +1, VO = 0.2 V p-p
G = +2, VO = 0.2 V p-p
G = +2, VO = 0.2 V p-p
G = +10, VO = 2 V p-p, RF = 499 Ω
G = +2, VO = 2 V Step
G = –1, VO = 2 V Step, RF = 1.5 kΩ
G = +2, VO = 2 V Step
–3 dB Small Signal Bandwidth
Bandwidth for 0.1 dB Flatness
Large Signal Bandwidth
Slew Rate (Rising Edge)
Settling Time to 0.1%
DISTORTION/NOISE PERFORMANCE
Total Harmonic Distortion
Differential Gain
Differential Phase
Input Voltage Noise
Input Current Noise
Min
190
110
10
RF = 3.01 kΩ for “N” Package or
RF = 2.49 kΩ for “R” Package or
RF = 2.10 kΩ for “RT” Package
fC = 1 MHz, VO = 2 V p-p, G = +2
fC = 10 MHz, VO = 2 V p-p, G = +2
NTSC, G = +2, RL to 1.5 V
NTSC, G = +2, RL to 1.5 V
f = 10 MHz
f = 10 MHz, +IIN
–IIN
DC PERFORMANCE
Input Offset Voltage
AD8005A
Typ
MHz
MHz
MHz
MHz
V/µs
V/µs
ns
–60
–50
0.14
0.70
4.0
1.1
9.1
dBc
dBc
%
Degrees
nV/√Hz
pA/√Hz
pA/√Hz
50
8
500
± mV
± mV
µV/°C
±µA
±µA
±µA
±µA
nA/°C
kΩ
48
120
300
1.6
1.5 to 3.5
54
MΩ
Ω
pF
V
dB
0.95 to 4.05
10
30
V
mA
mA
5
Offset Drift
+Input Bias Current
40
0.5
TMIN to TMAX
–Input Bias Current
5
TMIN to TMAX
INPUT CHARACTERISTICS
Input Resistance
Input Capacitance
Input Common-Mode Voltage Range
Common-Mode Rejection Ratio
OUTPUT CHARACTERISTICS
Output Voltage Swing
Output Current
Short Circuit Current
+Input
–Input
+Input
VCM = 1.5 V to 3.5 V
1.1 to 3.9
RL = 50 Ω
POWER SUPPLY
Quiescent Current
Power Supply Rejection Ratio
350
TMIN to TMAX
VS = +4 V to +6 V
56
OPERATING TEMPERATURE RANGE
–40
Specifications subject to change without notice.
REV. A
–3–
Units
225
130
30
45
260
775
30
TMIN to TMAX
Input Bias Current Drift (± )
Open-Loop Transimpedance
Max
35
50
1
2
10
11
425
475
µA
µA
dB
+85
°C
66
AD8005
ABSOLUTE MAXIMUM RATINGS 1
MAXIMUM POWER DISSIPATION
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.6 V
Internal Power Dissipation2
Plastic DIP Package (N) . . . . . . . . . . . . . . . . . . . . 1.3 Watts
Small Outline Package (R) . . . . . . . . . . . . . . . . . . 0.75 Watts
SOT-23-5 Package (RT) . . . . . . . . . . . . . . . . . . . 0.5 Watts
Input Voltage (Common Mode) . . . . . . . . . . . . . . . ± VS ± 1 V
Differential Input Voltage . . . . . . . . . . . . . . . . . . . . . . . ± 3.5 V
Output Short Circuit Duration
. . . . . . . . . . . . . . . . . . . . . . . Observe Power Derating Curves
Storage Temperature Range
N, R & RT Package . . . . . . . . . . . . . . . . . –65°C to +125°C
Operating Temperature Range (A Grade) . . . –40°C to +85°C
Lead Temperature Range (Soldering 10 sec) . . . . . . . . +300°C
The maximum power that can be safely dissipated by the
AD8005 is limited by the associated rise in junction temperature. The maximum safe junction temperature for plastic
encapsulated devices is determined by the glass transition temperature of the plastic, approximately +150°C. Exceeding this
limit temporarily may cause a shift in parametric performance
due to a change in the stresses exerted on the die by the package.
Exceeding a junction temperature of +175°C for an extended
period can result in device failure.
While the AD8005 is internally short circuit protected, this may
not be sufficient to guarantee that the maximum junction temperature (+150°C) is not exceeded under all conditions. To
ensure proper operation, it is necessary to observe the maximum
power derating curves shown in Figure 3.
NOTES
1
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.
2
Specification is for device in free air:
8-Lead Plastic DIP Package: θJA = 90°C/W
8-Lead SOIC Package: θJA = 155°C/W
5-Lead SOT-23 Package: θJA = 240°C/W
MAXIMUM POWER DISSIPATION – Watts
2.0
TJ = +150°C
8-LEAD PLASTIC-DIP PACKAGE
1.5
8-LEAD SOIC PACKAGE
1.0
0.5
5-LEAD SOT-23 PACKAGE
0
–50 –40 –30 –20 –10 0 10 20 30 40 50 60
AMBIENT TEMPERATURE – °C
70 80 90
Figure 3. Maximum Power Dissipation vs. Temperature
ORDERING GUIDE
Model
Temperature
Range
Package
Description
Package
Option
AD8005AN
AD8005AR
AD8005AR-REEL
AD8005ART-REEL
AD8005AR-REEL7
AD8005ART-REEL7
–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
8-Lead Plastic DIP
8-Lead Plastic SOIC
13" Tape and Reel
13" Tape and Reel
7" Tape and Reel
7" Tape and Reel
N-8
SO-8
SO-8
RT-5
SO-8
RT-5
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 AD8005 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.
–4–
Brand
Code
H1A
H1A
WARNING!
ESD SENSITIVE DEVICE
REV. A
Typical Characteristics–AD8005
5
NORMALIZED GAIN – dB
3
2
5
3
1
0
G = +2
–1
G = +10
RF = 499V
–2
2
1
0
–2
–3
–4
–4
10
FREQUENCY – MHz
100
–5
500
Figure 4. Frequency Response; G = +1, +2, +10; VS = ± 5 V
G = –1
RF = 1.5kV
–1
–3
–5
1
VS = 65V
VOUT = 200mV p-p
RL = 1kV
4
G = +1
VS = 65V
VOUT = 200mV p-p
RL = 1kV
NORMALIZED GAIN – dB
4
G = –10
RF = 1kV
1
10
FREQUENCY – MHz
100
500
Figure 7. Frequency Response; G = –1, –10; VS = ± 5 V
6.2
6.1
140
0
120
–40
6.0
G = +2
VOUT = 200mV p-p
RL = 1kV
5.5
–80
–120
80
GAIN
60
–160
40
–200
20
–240
PHASE – Degrees
5.6
GAIN – dB
GAIN – dB
5.8
5.7
PHASE
100
5.9
5.4
5.3
5.2
0.1
1
10
FREQUENCY – MHz
100
0
1k
500
Figure 5. Gain Flatness; G = +2; VS = ± 5 V or +5 V
10
6
9
PEAK-TO-PEAK OUTPUT VOLTAGE
( 1%THD) – Volts
7
GAIN – dB
4
VS = 65V
VOUT = 4V p-p
3
VS = 65V
VOUT = 2V p-p
2
1
0
–1
–2
100k
1M
10M
FREQUENCY – Hz
100M
–280
1G
Figure 8. Transimpedance Gain and Phase vs. Frequency
5
8
7
6
5
4
VS = +5V
G = +2
RL = 1kV
3
2
1
1
10
100
FREQUENCY – MHz
0
0.5
500
1
10
FREQUENCY – MHz
100
Figure 9. Output Swing vs. Frequency; VS = ± 5 V
Figure 6. Large Signal Frequency Response;
G = +2, RL = 1 kΩ
REV. A
10k
–5–
AD8005–Typical Characteristics
–40
–40
–50
3RD
–60
3RD
–70
2ND
–80
–90
3RD
–60
3RD
–70
2ND
–80
–90
–100
10
1
–100
20
10
1
FREQUENCY – MHz
Figure 13. Distortion vs. Frequency VS = +5 V
MIN = –0.06 MAX = 0.03 p-p/MAX = 0.09
MIN = –0.08 MAX = 0.04 p-p/MAX = 0.12
0.10
VS = ±5V
RL = 1kV
G = +2
0.05
DIFF GAIN – %
DIFF GAIN – %
0.10
0.00
–0.05
–0.10
VS = +5V
RL = 1kV TO +1.5V
G = +2
0.05
0.00
–0.05
–0.10
MIN = –0.01 MAX = 0.39 p-p = 0.40
MIN = 0.00 MAX = 0.70 p-p = 0.70
DIFF PHASE – Degrees
0.06
0.04
0.02
0.00
VS = ±5V
RL = 1kV
G = +2
–0.02
–0.04
–0.06
1st 2nd
3rd 4th 5th 6th 7th 8th 9th 10th 11th
MODULATING RAMP LEVEL – IRE
Figure 11. Differential Gain and Phase, VS = ± 5 V
0.5
0.0
–1.0
8
8
VS = 65V
6
5
4
3
VS = +5V
7
2nd 3rd 4th 5th 6th 7th 8th 9th 10th 11th
MODULATING RAMP LEVEL – IRE
5
4
3
2
1
0
10
0
3
10k
Figure 12. Output Voltage Swing vs. Load
f = 5MHz
G = +2
RL = 1kV
6
1
100
1k
LOAD RESISTANCE – V
1st
Figure 14. Differential Gain and Phase, VS = +5 V
9
2
VS = +5V
RL = 1kV TO +1.5V
G = +2
–0.5
PEAK-TO-PEAK OUTPUT
AT 5MHz ( 0.5% THD) – Volts
SWING – V p-p
1.0
9
7
20
FREQUENCY – MHz
Figure 10. Distortion vs. Frequency; VS = ± 5 V
DIFF PHASE – Degrees
2ND
G = +2
VOUT = 2V p-p
RL = 1kV
–50
DISTORTION – dBc
DISTORTION – dBc
2ND
G = +2
VOUT = 2V p-p
RL = 1kV
4
5
6
7
8
9
TOTAL SUPPLY VOLTAGE – Volts
10
11
Figure 15. Output Swing vs. Supply
–6–
REV. A
AD8005
–5
12.5
–15
INPUT VOLTAGE NOISE – nV/ Hz
VS = +5V OR 65V
G = +2
RL = 1kV
–10
CMRR – dB
–20
–25
–30
–35
–40
–45
10.0
7.5
5.0
2.5
–50
–55
0.03
1
10
FREQUENCY – MHz
0.1
0
100
Figure 16. CMRR vs. Frequency; VS = +5 V or ± 5 V
10
100
1k
10k
100k
FREQUENCY – Hz
1M
10M
Figure 19. Noise vs. Frequency; VS = +5 V or ± 5 V
62.5
10
INPUT CURRENT NOISE – pA/ Hz
OUTPUT RESISTANCE – V
100
VS = +5V AND 65V
RL = 1kV
G = +2
VS = +5V
VS = 65V
1
0.03
50.0
37.5
25.0
12.5
INVERTING CURRENT
NONINVERTING CURRENT
0.1
1
10
FREQUENCY – MHz
100
0
500
10
100
1k
10k
100k
FREQUENCY – Hz
1M
10M
Figure 20. Noise vs. Frequency; VS = +5 V or ± 5 V
Figure 17. Output Resistance vs. Frequency;
VS = ± 5 V and +5 V
10
0
–10
VS = +5V OR 65V
G = +2
RL = 1kV
–PSRR
–20
PSRR – dB
VOUT
100
90
+PSRR
VIN
–30
VS = 65V
G = +6
RL = 1kV
–40
–50
10
–60
0%
–70
–80
0.03
1V
0.1
1
10
FREQUENCY – MHz
100
150ns
500
Figure 18. PSRR vs. Frequency; VS = +5 V or ± 5 V
REV. A
2V
Figure 21. ± Overdrive Recovery, VS = ± 5 V, VIN = 2 V Step
–7–
AD8005–Typical Characteristics
RG
RF
RL
1kV
CPROBE
VIN
1.5kV
VOUT
1.5kV
VIN
51.1V
RL
1kV
CPROBE
+VS
50V
0.01mF
10mF
0.01mF
10mF
VOUT
+VS
0.01mF
10mF
0.01mF
10mF
–VS
–VS
PROBE : TEK P6137
CLOAD = 10pF NOMINAL
PROBE : TEK P6137
CLOAD = 10pF NOMINAL
Figure 22. Test Circuit; G = +2; RF = RG = 3.01 kΩ for
N Package; RF = RG = 2.49 kΩ for R and RT Packages
Figure 25. Test Circuit; G = –1, RF = RG = 1.5 kΩ for
N, R and RT Packages
100
90
100
90
10
10
0%
0%
50mV
10ns
50mV
Figure 23. 200 mV Step Response; G = +2, VS = ± 2.5 V
or ± 5 V
10ns
Figure 26. 200 mV Step Response; G = –1, VS = ± 2.5 V
or ± 5 V
100
100
90
90
10
10
0%
0%
1V
1V
10ns
Figure 24. Step Response; G = +2, VS = ± 5 V
10ns
Figure 27. Step Response; G = –1, VS = ± 5 V
–8–
REV. A
AD8005
Single-Supply Level Shifter
APPLICATIONS
Driving Capacitive Loads
Capacitive loads interact with an op amp’s output impedance
to create an extra delay in the feedback path. This reduces
circuit stability, and can cause unwanted ringing and oscillation. A given value of capacitance causes much less ringing
when the amplifier is used with a higher noise gain.
The capacitive load drive of the AD8005 can be increased by
adding a low valued resistor in series with the capacitive load.
Introducing a series resistor tends to isolate the capacitive load
from the feedback loop thereby diminishing its influence. Figure 29 shows the effects of a series resistor on capacitive drive
for varying voltage gains. As the closed-loop gain is increased,
the larger phase margin allows for larger capacitive loads with
less overshoot. Adding a series resistor at lower closed-loop
gains accomplishes the same effect. For large capacitive loads,
the frequency response of the amplifier will be dominated by
the roll-off of the series resistor and capacitive load.
In addition to providing buffering, many systems require that an
op amp provide level shifting. A common example is the level
shifting that is required to move a bipolar signal into the unipolar range of many modern analog-to-digital converters (ADCs). In
general, single supply ADCs have input ranges that are referenced neither to ground nor supply. Instead the reference level
is some point in between, usually halfway between ground and
supply (+2.5 V for a single supply 5 V ADC). Because highspeed ADCs typically have input voltage ranges of 1 V to 2 V,
the op amp driving it must be single supply but not necessarily
rail-to-rail.
R2
1.5kV
+5V
R1
1.5kV
0.01mF
10mF
VIN
AD8005
R3
30.1kV
RG
VOUT
VREF
+5V
RF
AD8005
R4
10kV
RS
RL
1kV
0.1mF
CL
Figure 30. Bipolar to Unipolar Level Shifter
Figure 30 shows a level shifter circuit that can move a bipolar
signal into a unipolar range. A positive reference voltage, derived
from the +5 V supply, sets a bias level of +1.25 V at the noninverting terminal of the op amp. In ac applications, the accuracy of
this voltage level is not important. Noise is however a serious
consideration. A 0.1 µF capacitor provides useful decoupling of
this noise.
Figure 28. Driving Capacitive Loads
80
VS = 65V
2V OUTPUT STEP
WITH 30% OVERSHOOT
CAPACITIVE LOAD – pF
70
60
RS = 10V
The bias level on the noninverting terminal sets the input commonmode voltage to +1.25 V. Because the output will always be
positive, the op amp may therefore be powered with a single
+5 V power supply.
50
RS = 5V
40
30
RS = 0V
The overall gain function is given by the equation:
20
 R2
 R4   R2
V OUT = –   V IN + 
 1+  V REF
 R1
 R3 + R4  R1
10
0
1
2
3
4
CLOSED-LOOP GAIN – V/V
5
In the above example, the equation simplifies to
V OUT = –V IN + 2.5V
Figure 29. Capacitive Load Drive vs. Closed-Loop Gain
REV. A
–9–
AD8005
Single-Ended-to-Differential Conversion
RF
RG
VOUT
RT
+5V
2.49kV
BIPOLAR
SIGNAL
60.5V
0.1mF
+5V
0.1mF
RIN
1kV
RO
VIN
Many single supply ADCs have differential inputs. In such cases,
the ideal common-mode operating point is usually halfway
between supply and ground. Figure 31 shows how to convert a
single-ended bipolar signal into a differential signal with a
common-mode level of 2.5 V.
C1
0.01mF
C3
10mF
C2
0.01mF
C4
10mF
+VS
–VS
INVERTING CONFIGURATION
RG
RF
RO
VOUT
AD8005
2.49kV
RF1
2.49kV
RG
619V
RF2
3.09kV
VIN
RT
VOUT
+5V
C1
0.01mF
C3
10mF
C2
0.01mF
C4
10mF
–VS
0.1mF
NONINVERTING CONFIGURATION
+5V
Figure 32. Inverting and Noninverting Configurations
AD8005
2.49kV
2.49kV
+VS
Chip capacitors have low parasitic resistance and inductance
and are suitable for supply bypassing (see Figure 32). Make sure
that one end of the capacitor is within 1/8 inch of each power
pin with the other end connected to the ground plane. An
additional large (0.47 µF–10 µF) tantalum electrolytic capacitor
should also be connected in parallel. This capacitor supplies
current for fast, large signal changes at the output. It must not
necessarily be as close to the power pin as the smaller capacitor.
0.1mF
Figure 31. Single-Ended-to-Differential Converter
Amp 1 has its +input driven with the ac-coupled input signal
while the +input of Amp 2 is connected to a bias level of +2.5 V.
Thus the –input of Amp 2 is driven to virtual +2.5 V by its
output. Therefore, Amp 1 is configured for a noninverting gain
of five, (1 + RF1/RG), because RG is connected to the virtual
+2.5 V of Amp 2’s –input.
When the +input of Amp 1 is driven with a signal, the same
signal appears at the –input of Amp 1. This signal serves as an
input to Amp 2 configured for a gain of –5, (–RF2/RG). Thus the
two outputs move in opposite directions with the same gain and
create a balanced differential signal.
This circuit can be simplified to create a bipolar in/bipolar out
single-ended to differential converter. Obviously, a single supply
is no longer adequate and the –VS pins must now be powered
with –5 V. The +input to Amp 2 is tied to ground. The ac
coupling on the +input of Amp 1 is removed and the signal can
be fed directly into Amp 1.
Locate the feedback resistor close to the inverting input pin in
order to keep the stray capacitance at this node to a minimum.
Capacitance variations of less than 1.5 pF at the inverting input
will significantly affect high-speed performance.
Use stripline design techniques for long signal traces (i.e., greater
than about 1 inch). Striplines should have a characteristic
impedance of either 50 Ω or 75 Ω. For the Stripline to be
effective, correct termination at both ends of the line is necessary.
Table I. Typical Bandwidth vs. Gain Setting Resistors
Gain
RF
RG
RT
Small Signal –3 dB
BW (MHz),
VS = ⴞ5 V
–1
–10
+1
+2
+10
1.49 kΩ
1 kΩ
2.49 kΩ
2.49 kΩ
499 Ω
1.49 kΩ
100 Ω
ⴥ
2.49 kΩ
56.2 Ω
52.3
100 Ω
49.9 Ω
49.9 Ω
49.9 Ω
120 MHz
60 MHz
270 MHz
170 MHz
40 MHz
Layout Considerations
In order to achieve the specified high-speed performance of the
AD8005 you must be attentive to board layout and component
selection. Proper RF design techniques and selection of components with low parasitics are necessary.
The PCB should have a ground plane that covers all unused
portions of the component side of the board. This will provide a
low impedance path for signals flowing to ground. The ground
plane should be removed from the area under and around the
chip (leave about 2 mm between the pin contacts and the
ground plane). This helps to reduce stray capacitance. If both
signal tracks and the ground plane are on the same side of the
PCB, also leave a 2 mm gap between ground plane and track.
–10–
REV. A
AD8005
Increasing Feedback Resistors
Unlike conventional voltage feedback op amps, the choice of feedback resistor has a direct impact on the closed-loop bandwidth
and stability of a current feedback op amp circuit. Reducing the
resistance below the recommended value makes the amplifier
more unstable. Increasing the size of the feedback resistor
reduces the closed-loop bandwidth.
360mA (rms)
562V
4.99kV
+5V
AD8005
VIN
In power-critical applications where some bandwidth can be
sacrificed, increasing the size of the feedback resistor will yield
significant power savings. A good example of this is the gain of
+10 case. Operating from a bipolar supply (± 5 V), the quiescent
current is 475 µA (excluding the feedback network). The recommended feedback and gain resistors are 499 Ω and 56.2 Ω
respectively. In order to drive an rms output voltage of 2 V, the
output must deliver a current of 3.6 mA to the feedback network. Increasing the size of the resistor network by a factor of
10 as shown in Figure 33 will reduce this current to 360 µA.
The closed loop bandwidth will however decrease to 20 MHz.
VOUT
2V (rms)
QUIESCENT CURRENT
475mA (MAX)
0.2V (rms)
–5V
Figure 33. Saving Power by Increasing Feedback Resistor
Network
REV. A
–11–
AD8005
OUTLINE DIMENSIONS
Dimensions shown in inches and (mm).
8-Lead Plastic DIP
(N-8)
8
C2186a–0–8/99
0.430 (10.92)
0.348 (8.84)
5
0.280 (7.11)
0.240 (6.10)
1
4
0.060 (1.52)
0.015 (0.38)
PIN 1
0.210 (5.33)
MAX
0.325 (8.25)
0.300 (7.62)
0.195 (4.95)
0.115 (2.93)
0.130
(3.30)
MIN
0.160 (4.06)
0.115 (2.93)
0.015 (0.381)
0.008 (0.204)
SEATING
PLANE
0.022 (0.558) 0.100 0.070 (1.77)
0.014 (0.356) (2.54) 0.045 (1.15)
BSC
8-Lead Plastic SOIC
(SO-8)
0.1968 (5.00)
0.1890 (4.80)
0.1574 (4.00)
0.1497 (3.80)
8
5
1
4
PIN 1
0.2440 (6.20)
0.2284 (5.80)
0.0688 (1.75)
0.0532 (1.35)
0.0098 (0.25)
0.0040 (0.10)
0.0500 0.0192 (0.49)
(1.27) 0.0138 (0.35)
BSC
SEATING
PLANE
0.0196 (0.50)
x 45°
0.0099 (0.25)
0.0098 (0.25)
0.0075 (0.19)
8°
0°
0.0500 (1.27)
0.0160 (0.41)
5-Lead Plastic SOT-23
(RT-5)
0.1181 (3.00)
0.1102 (2.80)
0.0669 (1.70)
0.0590 (1.50)
3
2
1
4
0.1181 (3.00)
0.1024 (2.60)
5
0.0748 (1.90)
BSC
0.0512 (1.30)
0.0354 (0.90)
0.0059 (0.15)
0.0019 (0.05)
0.0079 (0.20)
0.0031 (0.08)
0.0571 (1.45)
0.0374 (0.95)
0.0197 (0.50)
0.0138 (0.35)
SEATING
PLANE
–12–
PRINTED IN U.S.A.
0.0374 (0.95) BSC
10°
0°
0.0217 (0.55)
0.0138 (0.35)
REV. A