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®
OPA687
OPA687
®
©
1998 Burr-Brown Corporation PDS-1372C Printed in U.S.A. May, 2000
Wideband, Ultra-Low Noise,
Voltage Feedback OPERATIONAL AMPLIFIER
With Power Down
FEATURES
● HIGH GAIN BANDWIDTH: 3.8GHz
● LOW INPUT VOLTAGE NOISE: 0.95nV/√Hz
● VERY LOW DISTORTION: –95dBc (5MHz)
● LOW DISABLED POWER: 2mW
● VERY HIGH SLEW RATE: 900V/µs
● STABLE FOR G ≥ 12
DESCRIPTION
The OPA687 combines a very high gain bandwidth and
large signal performance with an ultra-low input noise
voltage (0.95nV/√Hz) while dissipating only 18mA supply current. Where power savings is paramount, the
OPA687 also includes an optional power down pin that,
when pulled low, will disable the amplifier and decrease
the quiescent current to only 1% of its powered up value.
This optional feature may be left disconnected to insure
normal amplifier operation when no power-down is required.
The combination of low input voltage and current noise,
along with a 3.8GHz gain bandwidth product, make the
OPA687 an ideal amplifier for wideband transimpedance
APPLICA TIONS
● LOW DISTORTION ADC DRIVER
● OC-3 FIBER OPTIC RECEIVER
● LOW NOISE DIFFERENTIAL AMPLIFIERS
● EQUALIZING RECEIVERS
● ULTRASOUND CHANNEL AMPLIFIERS
● IMPROVED REPLACEMENT FOR THE
CLC425
TM
stages. As a voltage gain stage, the OPA687 is optimized for a flat frequency response at a gain of +20 and
is guaranteed stable down to gains of +12. New external
compensation techniques allows the OPA687 to be used
at any inverting gain with excellent frequency response
control. Using this compensation can give an extremely
high dynamic range ADC driver to support > 40MSPS
12- and 14-bit converters.
Ultra-High Dynamic Range
Differential Input ADC Driver
+5V
–5V
OPA687
+5V
+5V
–5V
850Ω
39pF
OPA687
1.7pF
80pF
V
IN
+
V
IN
–
80pF
1.7pF
850Ω
39pF
100Ω
20Ω
20Ω
1:2
50Ω Source
< 6dB
Noise
Figure
ADS852
14-Bit
65MSPS
100Ω
V
CM
V
CM
OPA687
IInternational Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111
Twx: 910-952-1111 • Internet: http://www.burr-brown.com/ • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product In
Center Frequency (MHz)
3rd-Order Spurious (dBc)
0
–60
–65
–70
–75
–80
–85
5101520253035404550
4Vp-p
2Vp-p
Measured 2-Tone, 3rd-Order Distortion for
Differential ADC Driver.
OPA687 RELATED PRODUCTS
INPUT NOISE GAIN BANDWIDTH
SINGLES DUAL VOLTAGE (nV/√Hz) PRODUCT (MHz)
OPA642 2.7 210
OPA643 2.3 800
OPA686 OPA2686 1.3 1600
For most current data sheet and other product
information, visit www.burr-brown.com
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®
OPA687
OPA687U, N
TYP GUARANTEED
0°C to –40°C to
MIN/
TEST
PARAMETER CONDITIONS +25°C +25°C
(2)
70°C
(3)
+85°C
(3)
UNITS MAX
LEVEL
(1)
SPECIFICATIONS: VS = ±5V
RL = 100Ω, RF = 750Ω, and RG = 39.2Ω, G = +20 (Figure 1 for AC performance only), unless otherwise noted.
NOTES: (1) Test Levels: (A) 100% tested at 25°C. Over temperature limits by characterization and simulation. (B) Limits set by characterization and simulation.
(C) Typical value only for information. (2) Junction temperature = ambient for +25°C guaranteed specifications. (3) Junction temperature = ambient at low temperature
limit: junction temperature = ambient +23°C at high temperature limit for over temperature guaranteed specifications. (4) Current is considered positive out of node. V
CM
is the input common-mode voltage. (5) Tested <3dB below minimum specified CMRR at ±CMIR limits.
AC PERFORMANCE (Figure 1)
Closed-Loop Bandwidth G = +12, R
G
= 39.2Ω, VO = 200mVp-p 600 MHz typ C
G = +20, R
G
= 39.2Ω, VO = 200mVp-p 290 180 160 140 MHz min B
G = +50, R
G
= 39.2Ω, VO = 200mVp-p 75 60 54 48 MHz min B
Gain Bandwidth Product G ≥ +50 3800 3000 2700 2400 MHz min B
Bandwidth for 0.1dB Gain Flatness G = +20, R
L
= 100Ω 35 24 20 18 MHz min B
Peaking at a Gain of +12 3 8 10 14 dB max B
Harmonic Distortion G = +20, f = 5MHz, V
O
= 2Vp-p
2nd Harmonic R
L
= 100Ω –74 –70 –68 –65 dBc max B
R
L
= 500Ω –95 –90 –88 –85 dBc max B
3rd Harmonic R
L
= 100Ω –108 –95 –90 –85 dBc max B
R
L
= 500Ω –110 –105 –100 –95 dBc max B
Two-Tone, 3rd-Order Intercept G = +20, f = 20MHz 43 40 39 37 dBm min B
Input Voltage Noise Density f > 1MHz 0.95 1.1 1.15 1.3 nV/√Hz max B
Input Current Noise Density f > 1MHz 2.5 3.2 3.3 3.5 pA/√Hz max B
Pulse Response
Rise/Fall Time 0.2V Step 1.2 2.0 2.2 2.5 ns max B
Slew Rate 2V Step 900 675 550 450 V/µs min B
Settling Time to 0.01% 2V Step 17 ns typ C
0.1% 2V Step 15 18 20 25 ns max B
1% 2V Step 8 11 13 17 ns max B
DC PERFORMANCE
(4)
Open-Loop Voltage Gain (AOL)V
O
= 0V 85 78 75 70 dB min A
Input Offset Voltage V
CM
= 0V ±0.1 ±1 ±1.2 ±1.6 mV max A
Average Offset Voltage Drift V
CM
= 0V 5 10 µV/°C max B
Input Bias Current V
CM
= 0V –20 –33 –36 –40 µA max A
Input Bias Current Drift (magnitude) V
CM
= 0V –50 –100 nA/°C max B
Input Offset Current V
CM
= 0V ±0.2 ±1.0 ±1.5 ±1.8 µA max A
Input Offset Current Drift V
CM
= 0V ±12 ±15 nA/°C max B
INPUT
Common-Mode Input Range (CMIR)
(5)
±3.2 ±3.0 ±2.9 ±2.8 V min A
Common-Mode Rejection Ratio (CMRR) V
CM
= ±0.5V, Input Referred 100 88 83 78 dB min A
Input Impedance
Differential V
CM
= 0V 2.5 || 2.5 kΩ || pF typ C
Common-Mode V
CM
= 0V 1.0 || 1.2 MΩ || pF typ C
OUTPUT
Output Voltage Swing ≥ 400Ω Load ±3.6
±3.3 ±3.1 ±3.0 V min A
100Ω Load ±3.5
±3.2 ±2.9 ±2.8 V min A
Current Output, Sourcing V
O
= 0V 80 60 50 40 mA min A
Current Output, Sinking V
O
= 0V –80 –60 –50 –40 mA min A
Closed-Loop Output Impedance G = +20, f = < 100kHz 0.006 Ω typ C
POWER SUPPLY
Specified Operating Voltage ±5 V typ C
Maximum Operating Voltage ±6
±6 ±6 ±6 V max A
Quiescent Current, max V
S
= ±5V 18.5 19 19.5 20.5 mA max A
Quiescent Current, min V
S
= ±5V 18.5 18 17.5 16 mA min A
Power Supply Rejection Ratio
+PSRR, –PSRR |V
S
| = 4.5V to 5.5V, Input Referred 85 80 78 75 dB min A
POWER-DOWN (Disabled Low) (Pin 8 SO-8; Pin 5 on SOT23-6)
Power-Down Quiescent Current (+V
S
) –225 –300 –350 –400 µA max A
On Voltage (Enabled High or Floated) 3.3 3.5 3.6 3.7 V min A
Off Voltage (Disabled Asserted Low) 1.8 1.7 1.6 1.5 V max A
Power-Down Pin Input Bias Current (V
DIS
= 0) 100 160 160 160 µA max A
Power-Down Time 200 ns typ C
Power-Up Time 60 ns typ C
Off Isolation 5MHz, Input to Output 70 dB typ C
THERMAL
Specification U, N
–40 to +85
°C typ C
Thermal Resistance,
θ
JA
Junction to Ambient
U 8-Pin, SO-8 125 °C/W typ C
N 6-Pin, SOT23 150 °C/W typ C
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®
OPA687
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes no responsibility for the use of this
information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject to change without notice. No patent rights or licenses to any of the circuits described
herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant any BURR-BROWN product for use in life support devices and/or systems.
PACKAGE SPECIFIED
DRAWING TEMPERATURE PACKAGE ORDERING TRANSPORT
PRODUCT PACKAGE NUMBER
(1)
RANGE MARKING NUMBER
(2)
MEDIA
OPA687U SO-8 Surface-Mount 182 –40°C to +85°C OPA687U OPA687U Rails
"""""OPA687U/2K5 Tape and Reel
OPA687N 6-Lead SOT23-6 332 –40°C to +85°C A87 OPA687N/250 Tape and Reel
"""""OPA687N/3K Tape and Reel
NOTES: (1) For detailed drawing and dimension table, please see end of data sheet, or Appendix C of Burr-Brown IC Data Book. (2) Models with a slash (/ ) are
available only in Tape and Reel in the quantities indicated (e.g., /2K5 indicates 2500 devices per reel). Ordering 2500 pieces of “OPA687U/2K5” will get a single
2500-piece Tape and Reel. For detailed Tape and Reel mechanical information, refer to Appendix B of Burr-Brown IC Data Book.
PACKAGE/ORDERING INFORMATION
ELECTROSTATIC
DISCHARGE SENSITIVITY
This integrated circuit can be damaged by ESD. Burr-Brown
recommends that all integrated circuits be handled with
appropriate precautions. Failure to observe proper handling
and installation procedures can cause damage.
ESD damage can range from subtle performance degradation to complete device failure. Precision integrated circuits
may be more susceptible to damage because very small
parametric changes could cause the device not to meet its
published specifications.
ABSOLUTE MAXIMUM RATINGS
Power Supply................................................................................ ±6.5V
DC
Internal Power Dissipation ...................................... See Thermal Analysis
Differential Input Voltage .................................................................. ±1.2V
Input Voltage Range ............................................................................ ±V
S
Storage Temperature Range: U, N ................................. –40°C to +125°C
Lead Temperature (soldering, 10s) ............................................... +300°C
Junction Temperature (T
J
) ............................................................. +175°C
PIN CONFIGURATION
Top View SO-8
Top View SOT23-6
1
2
3
6
5
4
Output
–V
S
Noninverting Input
+V
S
DIS
Inverting Input
OPA687
123
654
A87
Pin Orientation/Package Marking
8
7
6
5
1
2
3
4
OPA687
NC: No Connection
NC
Inverting Input
Noninverting Input
–V
S
DIS
+V
S
Output
NC
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®
OPA687
TYPICAL PERFORMANCE CURVES: VS = ±5V
RF = 750Ω, RG = 39.2Ω, G = +20 and RL = 100Ω, unless otherwise noted.
6
3
0
–3
–6
–9
–12
–15
–18
–21
–24
NONINVERTING SMALL-SIGNAL
FREQUENCY RESPONSE
Frequency (MHz)
Normalized Gain (3dB/div)
1 10 100 1000
RG = 39.2Ω
V
O
= 0.2Vp-p
G = +30
G = +50
See Figure 1
G = +12
G = +20
6
3
0
–3
–6
–9
–12
–15
–18
–21
–24
INVERTING SMALL-SIGNAL
FREQUENCY RESPONSE
Frequency (MHz)
Normalized Gain (3dB/div)
1 10 100 1000
RG = RS = 50Ω
V
O
= 0.2Vp-p
G = –40
G = –50
See Figure 2
G = –20
G = –30
32
29
26
23
20
17
14
11
8
5
2
NONINVERTING LARGE-SIGNAL
FREQUENCY RESPONSE
Frequency (MHz)
Gain (3dB/div)
1 10 100 1000
RG = 39.2Ω
G = +20V/V
VO = 2Vp-p
VO = 0.2Vp-p
VO = 5Vp-p
See Figure 1
VO = 1Vp-p
200
100
0
–100
–200
1.2
0.8
0.4
0
–0.4
–0.8
–1.2
NONINVERTING PULSE RESPONSE
Time (5ns/div)
Output Voltage (100mV/div)
Output Voltage (400mV/div)
G = +20V/V
See Figure 1
Large Signal ±1V
Small Signal ±100mV
Right Scale
Left Scale
38
35
32
29
26
23
20
18
14
13
10
INVERTING LARGE-SIGNAL
FREQUENCY RESPONSE
Frequency (MHz)
Gain (3dB/div)
1 10 100 1000
RG = RS = 50Ω
G = –40V/V
VO = 2Vp-p
VO = 0.2Vp-p
See Figure 2
VO = 1Vp-p
VO = 5Vp-p
200
100
0
–100
–200
1.2
0.8
0.4
0
–0.4
–0.8
–1.2
INVERTING PULSE RESPONSE
Time (5ns/div)
Output Voltage (100mV/div)
Output Voltage (400mV/div)
G = –40V/V
See Figure 2
Large Signal ±1V
Small Signal ±100mV
Right Scale
Left Scale
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®
OPA687
TYPICAL PERFORMANCE CURVES: VS = ±5V (CONT)
RF = 750Ω, RG = 39.2Ω, G = +20 and RL = 100Ω, unless otherwise noted (Figure 1).
–70
–80
–90
–100
–110
Output Voltage (Vp-p)
0.1 101
5MHz 2nd HARMONIC DISTORTION
vs OUTPUT VOLTAGE
2nd Harmonic Distortion (dBc)
RL = 200Ω
RL = 100Ω
RL = 500Ω
–70
–80
–90
–100
–110
Output Voltage (Vp-p)
0.1 101
5MHz 3rd HARMONIC DISTORTION
vs OUTPUT VOLTAGE
3rd Harmonic Distortion (dBc)
RL = 100Ω
RL = 500Ω
RL =200Ω
–60
–70
–80
–90
–100
Output Voltage (Vp-p)
0.1 101
10MHz 2nd HARMONIC DISTORTION
vs OUTPUT VOLTAGE
2nd Harmonic Distortion (dBc)
RL = 200Ω
RL = 100Ω
RL = 500Ω
–60
–70
–80
–90
–100
Output Voltage (Vp-p)
0.1 101
10MHz 3rd HARMONIC DISTORTION
vs OUTPUT VOLTAGE
3rd Harmonic Distortion (dBc)
RL = 200Ω
RL = 100Ω
RL = 500Ω
–50
–60
–70
–80
–90
Output Voltage (Vp-p)
0.1 101
20MHz 2nd HARMONIC DISTORTION
vs OUTPUT VOLTAGE
2nd Harmonic Distortion (dBc)
RL = 200Ω
RL = 100Ω
RL = 500Ω
–50
–60
–70
–80
–90
Output Voltage (Vp-p)
0.1 101
20MHz 3rd HARMONIC DISTORTION
vs OUTPUT VOLTAGE
3rd Harmonic Distortion (dBc)
RL = 200Ω
RL = 100Ω
RL = 500Ω
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®
OPA687
TYPICAL PERFORMANCE CURVES: VS = ±5V (CONT)
RF = 750Ω, RG = 39.2Ω, G = +20 and RL = 100Ω, unless otherwise noted (Figure 1).
2nd HARMONIC DISTORTION vs FREQUENCY
–50
–60
–70
–80
–90
–100
2nd Harmonic Distortion (dBc)
VO = 2Vp-p
R
L
= 100Ω
Frequency (MHz)
11020
G = +51
G = +30
G = +20
10.0
1.0
0.10
INPUT VOLTAGE and CURRENT NOISE DENSITY
Frequency (Hz)
100 10M1k 10k 100k 1M
Current Noise (pA/√Hz)
Voltage Noise (nV/√Hz)
2.5pA/√Hz
0.95nV/√Hz
Current Noise
Voltage Noise
3rd HARMONIC DISTORTION vs FREQUENCY
–50
–60
–70
–80
–90
–100
3rd Harmonic Distortion (dBc)
VO = 2Vp-p
R
L
= 100Ω
Frequency (MHz)
11020
G = +51
G = +30
G = +20
TWO-TONE, 3rd-ORDER INTERMODULATION
INTERCEPT vs FREQUENCY
Frequency (MHz)
Intercept (dBm)
10
50
45
40
35
30
25
20
15
10
20 30 40 50 60 70 80 90 100
OPA687
50Ω
39.2Ω
750Ω
50Ω
50Ω
P
I
P
O
50
45
40
35
30
25
20
15
10
5
0
R
S
vs CAPACITIVE LOAD
Capacitive Load (pF)
1 10 100
R
S
(Ω)
28
27
26
25
24
23
22
21
20
19
18
Frequency (MHz)
FREQUENCY RESPONSE vs CAPACITIVE LOAD
1 10010 500
Gain to Capacitive Load (1dB/div)
CL = 10pF
CL = 50pF
CL = 20pF
CL = 100pF
OPA687
R
S
V
IN
V
O
C
L
1kΩ
750Ω
39.2Ω
1kΩ is optional
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®
OPA687
TYPICAL PERFORMANCE CURVES: VS = ±5V (CONT)
VS = ±5V, G = +20, RG = 39.2Ω, and RL = 100Ω, unless otherwise noted (Figure 1).
CMRR and PSRR
Frequency (Hz)
100 10M 100M1k 10k 100k 1M
Rejection Ratio (dB)
CMRR
+PSRR
–PSRR
120
100
80
60
40
20
0
CLOSED-LOOP OUTPUT IMPEDANCE vs FREQUENCY
Frequency (Hz)
10M 100M10k 100k 1M
Output Impedance (Ω)
10
1
0.1
0.01
0.001
OPA687
750Ω
39.2Ω
50Ω
Z
O
POWER SUPPLY and OUTPUT CURRENT
vs TEMPERATURE
Temperature (°C)
Power Supply Current (mA)
Output Current (mA)
–50
24
20
16
12
8
4
0
120
100
80
60
40
20
0
–25 0 25 50 75 100 125
Output Current Sourcing
Output Current Sinking
Power Supply Current
100
90
80
70
60
50
40
30
20
10
0
0
–30
–60
–90
–120
–150
–180
–210
–240
–270
–300
OPEN-LOOP GAIN and PHASE
Frequency (Hz)
100 10M 100M 1G 10G1k 10k 100k 1M
Open-Loop Gain (10dB/div)
Open-Loop Phase (30°/div)
| AOL|
∠ A
OL
INPUT DC ERRORS vs TEMPERATURE
Temperature (°C)
Input Offset Voltage (mV)
Input Bias and Offset Current (µA)
–50
1.1
0.9
0.7
0.5
0.3
0.1
–0.1
29
24
19
14
9
4
–1
–25 0 25 50 75 100 125
Input Bias Current
Input Offset Voltage
Input Offset Current (0.2µA)
1MΩ
100kΩ
10kΩ
1kΩ
COMMON-MODE and
DIFFERENTIAL INPUT IMPEDANCE
Frequency (Hz)
100 10M1k 10k 100k 1M
Impedance (Magnitude)
Common-Mode Input Impedance
Differential Input Impedance
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OPA687
APPLICATIONS INFORMATION
WIDEBAND, NON-INVERTING OPERATION
The OPA687 provides a unique combination of a very low
input voltage noise along with a very low distortion output
stage to give one of the highest dynamic range op amps
available. Its very high Gain Bandwidth Product (GBP) can
be used to either deliver high signal bandwidths at high
gains, or to deliver very low distortion signals at moderate
frequencies and lower gains. To achieve the full performance of the OPA687, careful attention to PC board layout
and component selection is required as discussed in the
remaining sections of this data sheet.
Figure 1 shows the non-inverting gain of +20 circuit used as
the basis for most of the Typical Performance Curves. Most
of the curves were characterized using signal sources with
50Ω driving impedance, and with measurement equipment
presenting a 50Ω load impedance. In Figure 1, the 50Ω
shunt resistor at the VI terminal matches the source impedance of the test generator, while the 50Ω series resistor at the
VO terminal provides a matching resistor for the measurement equipment load. Generally, data sheet voltage swing
specifications are at the output pin (VO in Figure 1), while
output power specifications are at the matched 50Ω load.
The total 100Ω load at the output, combined with the 790Ω
total feedback network load, presents the OPA687 with an
effective output load of 89Ω for the circuit of Figure 1.
Voltage feedback op amps, unlike current feedback designs,
can use a wide range of resistor values to set their gain. The
circuit of Figure 1, and the specifications at other gains, use
an RG set to 39.2Ω and RF adjusted to get the desired gain.
Using this guideline will guarantee that the noise added at the
output due to Johnson noise of the resistors will not significantly increase the total over that due to the 0.95nV/√Hz input
voltage noise for the op amp itself. This RG is suggested as a
good starting point for design. Other values are certainly
acceptable if required by the design.
OPA687
+5V
–5V
–V
S
+V
S
50Ω
V
O
V
I
50Ω
+
0.1µF
+
6.8µF
6.8µF
R
G
39.2Ω
R
F
750Ω
50Ω Source
50Ω Load
0.1µF
FIGURE 1. Non-Inverting G = +20 Specifications and Test
Circuit.
WIDEBAND, INVERTING GAIN OPERATION
There can be significant benefits to operating the OPA687 as
an inverting amplifier. This is particularly true when a
matched input impedance is required. Figure 2 shows the
inverting gain circuit used as a starting point for the Typical
Performance Curves showing inverting mode performance.
OPA687
+5V
–5V
+V
S
–V
S
95.3Ω
50ΩV
O
V
I
+
6.8µF0.1µF
+
6.8µF0.1µF
0.1µF
R
F
2kΩ
R
G
50Ω
50Ω Source
50Ω Load
Driving this circuit from a 50Ω source, and constraining the
gain resistor, RG, to equal 50Ω, will give both a signal
bandwidth and noise advantage. RG, in this case, is acting
as both the input termination resistor and the gain setting
resistor for the circuit. Although the signal gain for the
circuit of Figure 2 is double that for Figure 1, their noise
gains are equal when the 50Ω source resistor is included.
This has the interesting effect of doubling the equivalent
GBP for the amplifier. This can be seen in comparing the
G = +12 and G = –20 small-signal frequency response
curves. Both show approximately 500MHz bandwidth with
3dB peaking, but the inverting configuration of Figure 2 is
giving 4.4dB higher signal gain. The noise gains are approximately equal in this case. If the signal source is
actually the low impedance output of another amplifier, R
G
should be increased to be greater than the minimum value
allowed at the output of that amplifier and RF adjusted to
get the desired gain. It is critical for stable operation of the
OPA687 that this driving amplifier show a very low output
impedance through frequencies exceeding the expected
closed-loop bandwidth for the OPA687.
WIDEBAND, HIGH SENSITIVITY,
TRANSIMPEDANCE DESIGN
The high Gain Bandwidth Product (GBP) and low input voltage
and current noise for the OPA687 make it an ideal wideband
transimpedance amplifier for low to moderate transimpedance
gains. Very high transimpedance gains (> 100kΩ) will benefit
from the low input noise current of a FET-input op amp such
as the OPA655. Unity gain stability in the op amp is NOT
FIGURE 2. Inverting G = –40 Specifications and Test
Circuit.
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®
OPA687
required for application as a transimpedance amplifier. Figure 3 shows one possible transimpedance design example
that would be particularly suitable for the 155Mbit data rate
of an OC-3 receiver. Designs that require high bandwidth
from a large area detector with relatively low transimpedance
gain will benefit from the low input voltage noise for the
OPA687. The amplifier’s input voltage noise is peaked up,
at the output, over frequency by the diode source capacitance and can, in many cases, become the dominant output
noise contribution. The key elements to the design are the
expected diode capacitance (CD) with the reverse bias voltage (–VB) applied, the desired transimpedance gain, RF, and
the GBP for the OPA687 (3600MHz). With these three
variables set (and including the parasitic input capacitance
for the OPA687 added to CD), the feedback capacitor value
(CF) may be set to control the frequency response.
FIGURE 3. Wideband, High Sensitivity, OC-3
Transimpedance Amplifier.
To achieve a maximally flat 2nd-order Butterworth frequency response, the feedback pole should be set to:
1/(2πRFCF) = √(GBP/(4πRFCD))
Adding the common-mode and differential-mode input capacitance (1.2 + 2.5)pF to the 1pF diode source capacitance
of Figure 3 (CD), and targeting a 12kΩ transimpedance gain
using the 3600MHz GBP for the OPA687, will require a
feedback pole set to 71MHz to get a maximum bandwidth
design. This will require a total feedback capacitance of
0.16pF.
Using this maximum bandwidth, maximally flat frequency
response target will give an approximate –3dB bandwidth
set by:
f
–3dB
= √(GBP/2πRFCD)Hz
The example of Figure 3 will give approximately 100MHz
flat bandwidth using the 0.16pF feedback compensation
capacitor. This bandwidth will easily support an OC-3 receiver with exceptional sensitivity.
If the total output noise is bandlimited to a frequency less
than the feedback pole frequency, a very simple expression
for the equivalent input noise current can be derived as:
Where:
iEQ= Equivalent input noise current if the output noise is
bandlimited to f < 1/(2πRFCD)
iN= Input current noise for the op amp inverting input
eN= Input voltage noise for the op amp
CD= Total Inverting Node Capacitance
f = Bandlimiting frequency in Hz (usually a post filter
prior to further signal processing)
Evaluating this expression up to the feedback pole frequency
at 71MHz for the circuit of Figure 3, gives an equivalent
input noise current of 3.0pA/√Hz. This is somewhat higher
than the 2.5pA/√Hz for just the op amp itself. This total
equivalent input current noise is being slightly increased by
the last term in the equivalent input noise expression. It is
essential in this case to use a low voltage noise op amp. For
example, if a slightly higher input noise voltage, but otherwise identical, op amp were used instead of the OPA687 in
this application (say 2.0nV/√Hz), the total input-referred
current noise would increase to 4.0pA/√Hz. Low input
voltage noise is required for the best sensitivity in these
wideband transimpedance applications. This is often unspecified for dedicated transimpedance amplifiers with a
total output noise for a specified source capacitance given
instead. It is the relatively high input voltage noise for those
components that cause higher than expected output noise if
the source capacitance is higher than expected.
LOW GAIN COMPENSATION FOR IMPROVED SFDR
A new external compensation technique may be used at low
signal gains to retain the full slew rate and noise benefits of
the OPA687, while maintaining the increased loop gain and
the associated improvement in distortion offered by the
decompensated architecture. This technique shapes the loop
gain for good stability while giving an easily controlled
second-order low pass frequency response. This technique
was used for the circuit on the front page of the data sheet
in a differential configuration to achieve extremely high
SFDR through high frequencies. That circuit is set up for a
differential gain of 8.5V/V from a differential input signal to
the output. Using the transformer shown will improve the
noise figure and translate from a single to a differential
ii
kT
R
e
R
eCf
EQ N
F
N
F
ND
=++
+
()
2
2
2
4
23π
R
F
12kΩ
12kΩ
0.1µF100pF
Supply Decoupling
Not Shown
λ
OPA687
+5V
–5V
–V
B
C
F
0.16pF
1pF
Photodiode
Page 10
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®
OPA687
signal. If the source is differential already, it may be connected through blocking capacitors into the gain setting
resistors. To set the compensation capacitors for this circuit
(CS and CF), consider the 1/2 circuit of Figure 4 where the
50Ω source is reflected through the 1:2 transformer and then
cut in 1/2 and grounded to give a total impedance to AC
ground (for the circuit on the front page of this data sheet)
equal to the 200Ω.
Considering only the noise gain (this is the same as the noninverting signal gain) for the circuit of Figure 4, the low
frequency noise gain, (NG1) will be set by the resistor ratios
while the high frequency noise gain (NG2) will be set by the
capacitor ratios. The capacitor values set both the transition
frequencies and the high frequency noise gain. If the high
frequency noise gain, determined by NG2 = 1 + CS/CF, is set
to a value greater than the recommended minimum stable
gain for the op amp, and the noise gain pole, set by 1/RFCF,
is placed correctly, a very well-controlled, second-order low
pass frequency response will result.
To choose the values for both CS and CF, two parameters and
only three equations need to be solved. The first parameter
is the target high frequency noise gain NG2, which should be
greater than the minimum stable gain for the OPA687. Here,
a target NG2 of 24 will be used. The second parameter is the
desired low frequency signal gain, which also sets the low
frequency noise gain NG1. To simplify this discussion, we
will target a maximally flat second-order low pass Butterworth
frequency response (Q = 0.707). The signal gain of –4.25
shown in Figure 4 will set the low frequency noise gain to
NG1 = 1 + RF/RG (= 5.25 in this example). Then, using only
these two gains and the GBP for the OPA687 (3600MHz),
the key frequency in the compensation can be determined as:
ZO=
GBP
NG
1
2
1–
NG
1
NG
2
–1–2
NG
1
NG
2
CF=
1
2π•R
FZO
NG
2
CS= NG2–1
()
C
F
f
–3dB
≅ ZOGBP
Physically, this Z0 (4.1MHz for the values shown above) is
set by 1/(2π • RF(CF + CS)) and is the frequency at which the
rising portion of the noise gain would intersect unity gain if
projected back to 0dB gain. The actual zero in the noise gain
occurs at NG1 • Z0 and the pole in the noise gain occurs at
NG2 • Z0. Since GBP is expressed in Hz, multiply Z0 by 2π
and use this to get CF by solving:
Finally, since CS and CF set the high frequency noise gain,
determine CS by [Using NG2 = 24]:
The resulting closed-loop bandwidth will be approximately
equal to:
For the values shown in Figure 4, the f
–3dB
will be approximately 121MHz. This is less than that predicted by simply
dividing the GBP product by NG1. The compensation
network controls the bandwidth to a lower value while
providing the full slew rate at the output and an exceptional distortion performance due to increased loop gain at
frequencies below NG1 • Z0. The capacitor values shown
in Figure 4 are calculated for NG1 = 5.25 and NG2 = 24
with no adjustment for parasitics. The full circuit on the
front page of this datasheet shows the capacitors adjusted
for parasitics.
The front page of this data sheet shows the measured 2-tone,
3rd-order distortion for just the amplifier portion of the
circuit.
The upper curve is for a total 2-tone envelope of 4Vp-p,
requiring two tones, each at 2Vp-p across the OPA687
outputs. The lower curve is for a 2Vp-p envelope requiring
each tone to be 1Vp-p. The basic measurement dynamic
range for the two close-in spurious tones is approximately
85dBc. The 4Vp-p test does not show measurable 3rd-order
spurious until 25MHz ,while the 2Vp-p is unmeasurable up
to 40MHz center frequency. Two-tone, 2nd-order
intermodulation distortion was unmeasurable for the circuit
on the front page of this data sheet.
(= 1.90pF)
(= 43.8pF)
(= 121MHz)
FIGURE 4. Broadband Low Inverting Gain External Com-
pensation.
R
F
850Ω
C
S
44pF
OPA687
+5V
–5V
V
O
V
I
C
F
1.9pF
R
G
200Ω
Page 11
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OPA687
LOW NOISE FIGURE, HIGH DYNAMIC
RANGE AMPLIFIER
The low input noise voltage of the OPA687 and its very high
2-tone intercept can be used to good advantage as a fixedgain IF amplifier. While input noise figures in the 10dB
range (for a matched 50Ω input) are easily achieved with
just the OPA687, Figure 5 shows a technique to reduce the
noise figure even further while providing a broadband, high
gain IF amplifier stage using two stages of the OPA687.
This circuit uses two stages of forward gain with an overall
feedback loop to set the input impedance match. The input
transformer provides both a noiseless voltage gain and a
signal inversion to retain an overall non-inverting signal
path from PI to PO—since the 2nd amplifier stage is inverting to provide the correct feedback polarity through the
6.19kΩ resistor. To achieve a 50Ω input match at the
primary of the 1:2 transformer, the secondary must see a
200Ω load impedance. At higher frequencies, the match is
provided by the 200Ω resistor in series with 10pF. At lower
signal frequencies (f < 80MHz), the input match is set by the
feedback through the 6.19kΩ resistor. The low noise figure
(5dB) for this circuit is achieved by using the transformer,
the low voltage noise OPA687, and the input match set by
feedback. The first stage amplifier provides a gain of +15.
The very high SFDR is provided by operating the output
stage a low signal gain of –2 and using the inverting
compensation to hold it stable. Depending on the load that
is driven, this circuit can give a 2-tone SFDR that exceeds
90dB through 30MHz. Besides offering a very high dynamic
range, this circuit improves on standard IF amplifiers by
offering a precisely controlled gain and a very flexible
output load driving capability.
DESIGN-IN TOOLS
DEMONSTRATION BOARDS
Two PC boards are available to assist in the initial evaluation of circuit performance using the OPA687 in its two
package styles. Both of these are available free as an
unpopulated PC board delivered with descriptive documentation. The summary information for these boards is shown
in the table below.
BOARD LITERATURE
PART REQUEST
PRODUCT PACKAGE NUMBER NUMBER
OPA687U 8-Pin SO-8 DEM-OPA68xU MKT-351
OPA687N 6-Lead SOT23-6 DEM-OPA6xxN MKT-348
Contact the Burr-Brown applications support line to request
any of these boards.
MACROMODELS AND APPLICATIONS SUPPORT
Computer simulation of circuit performance using SPICE is
often useful when analyzing the performance of analog
circuits and systems. This is particularly true for video and
RF amplifier circuits where parasitic capacitance and inductance can have a major effect on circuit performance. A
SPICE model for the OPA687 is available through either the
Burr-Brown Internet web page (http://www.burr-brown.com)
or as one model on a disk from the Burr-Brown Applications department (1-800-548-6132). The Applications department is also available for design assistance at this
number. These models do a good job of predicting smallsignal AC and transient performance under a wide variety of
operating conditions. They do not do as well in predicting
the harmonic distortion characteristics. These models do not
attempt to distinguish between the package types in their
small-signal AC performance.
FIGURE 5. Very High Dynamic Range High Gain Amplifier.
OPA687
10pF
P
I
1.6pF
6.19kΩ
750Ω
1.5kΩ
200Ω
30.1Ω
420Ω
5dB
Noise
Figure
1:2
50Ω Source
P
O
OPA687
46pF
Input match
set by this
feedback path
Overall Gain = 35.6dB
P
O
P
I
Page 12
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OPA687
OPERATING SUGGESTIONS
SETTING RESISTOR VALUES TO MINIMIZE NOISE
The OPA687 provides a very low input noise voltage while
requiring a low 18.5mA of quiescent current. To take full
advantage of this low input noise, careful attention to the
other possible noise contributors is required. Figure 6 shows
the op amp noise analysis model with all the noise terms
included. In this model, all the noise terms are taken to be
noise voltage or current density terms in either nV/√Hz or
pA/√Hz.
E E I R kTR NG I R kTR NG
ONIBNS S BIF F
=+
()
+
()
+
()
+
2
2
2
2
44
EN= E
NI
2
+ IBNR
S
()
2
+4kTRS+
I
BIRF
NG
2
+
4kTR
F
NG
4kT
R
G
R
G
R
F
R
S
OPA687
I
BI
E
O
I
BN
4kT = 1.6E –20J
at 290°K
E
RS
E
NI
√4kTR
S
√4kTR
F
FIGURE 6. Op Amp Noise Analysis Model.
The total output spot-noise voltage can be computed as the
square root of the squared contributing terms to the output
noise voltage. This computation is adding all the contributing noise powers at the output by superposition, then taking
the square root to get back to a spot-noise voltage. Equation
1 shows the general form for this output noise voltage using
the terms shown in Figure 6.
Equation 1
Dividing this expression by the noise gain (NG = 1 + RF/RG)
will give the equivalent input-referred, spot-noise voltage at
the non-inverting input as shown in Equation 2.
Equation 2
Putting high resistor values into Equation 2 can quickly
dominate the total equivalent input-referred noise. A source
impedance on the non-inverting input of 56Ω will add a
Johnson voltage noise term equal to just that for the amplifier itself. Holding the gain and source resistors low (as was
used in the Typical Performance Curves) will minimize the
resistor noise contribution in Equation 2. Evaluating Equa-
tion 2 for the circuit of Figure 1 will give a total equivalent
input noise of 1.4nV/√Hz. This is slightly increased from the
0.95nV/√Hz for the op amp itself due to the contribution of
the resistor and bias current noise terms.
FREQUENCY RESPONSE CONTROL
Voltage feedback op amps exhibit decreasing closed-loop
bandwidth as the signal gain is increased. In theory, this
relationship is described by the Gain Bandwidth Product
(GBP) shown in the specifications. Ideally, dividing GBP by
the non-inverting signal gain (also called the Noise Gain, or
NG) will predict the closed-loop bandwidth. In practice, this
only holds true when the phase margin approaches 90°, as
it does in high gain configurations. At low gains (increased
feedback factors), most high-speed amplifiers will exhibit a
more complex response with lower phase margin. The
OPA687 is compensated to give a maximally flat 2nd-order
Butterworth closed-loop response at a non-inverting gain of
+20 (Figure 1). This results in a typical gain of +20 bandwidth of 290MHz, far exceeding that predicted by dividing
the 3600MHz GBP by 20. Increasing the gain will cause the
phase margin to approach 90° and the bandwidth to more
closely approach the predicted value of (GBP/NG). At a
gain of +50, the OPA687 will very nearly match the 72MHz
bandwidth predicted using the simple formula and the typical GBP of 3600MHz.
Inverting operation offers some interesting opportunities to
increase the available gain bandwidth product. When the
source impedance is matched by the gain resistor (Figure 2),
the signal gain is (1 + RF/RG) while the noise gain for
bandwidth purposes is (1 + RF/2RG). This cuts the noise gain
almost in half, increasing the minimum stable gain for
inverting operation under these condition to –20 and the
equivalent gain bandwidth product to 7.2GHz.
DRIVING CAPACITIVE LOADS
One of the most demanding, and yet very common, load
conditions for an op amp is capacitive loading. Often, the
capacitive load is the input of an A/D converter, including
additional external capacitance which may be recommended
to improve A/D linearity. A high-speed, high open-loop
gain amplifier like the OPA687 can be very susceptible to
decreased stability and closed-loop response peaking when
a capacitive load is placed directly on the output pin. When
the amplifier’s open-loop output resistance is considered,
this capacitive load introduces an additional pole in the
signal path that can decrease the phase margin. Several
external solutions to this problem have been suggested.
When the primary considerations are frequency response
flatness, pulse response fidelity and/or distortion, the simplest and most effective solution is to isolate the capacitive
load from the feedback loop by inserting a series isolation
resistor between the amplifier output and the capacitive
load. This does not eliminate the pole from the loop response, but rather shifts it and adds a zero at a higher
frequency. The additional zero acts to cancel the phase lag
from the capacitive load pole, thus increasing the phase
margin and improving stability.
Page 13
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OPA687
The Typical Performance Curves show the recommended
RS vs Capacitive Load and the resulting frequency response
at the load. Parasitic capacitive loads greater than 2pF can
begin to degrade the performance of the OPA687. Long PC
board traces, unmatched cables, and connections to multiple
devices can easily cause this value to be exceeded. Always
consider this effect carefully, and add the recommended
series resistor as close as possible to the OPA687 output pin
(see Board Layout Guidelines).
The criterion for setting this RS resistor is a maximum
bandwidth, flat frequency response at the load. For the
OPA687 operating in a gain of +20, the frequency response
at the output pin is very flat to begin with, allowing relatively small values of RS to be used for low capacitive loads.
As the signal gain is increased, the unloaded phase margin
will also increase. Driving capacitive loads at higher gains
will require lower RS values than shown for a gain of +20.
DISTORTION PERFORMANCE
The OPA687 is capable of delivering an exceptionally low
distortion signal at high frequencies over a wide range of
gains. The distortion plots in the Typical Performance Curves
show the typical distortion under a wide variety of conditions. Most of these plots are limited to 110dB dynamic
range. The OPA687’s distortion driving a 500Ω load does
not rise above –90dBc until either the signal level exceeds
3.0V and/or the fundamental frequency exceeds 5MHz.
Generally, until the fundamental signal reaches very high
frequencies or powers, the 2nd harmonic will dominate the
distortion with negligible 3rd harmonic component. Focusing then on the 2nd harmonic, increasing the load impedance
improves distortion directly. Remember that the total load
includes the feedback network, in the non-inverting configuration this is sum of RF + RG, while in the inverting
configuration this is just RF (Figure 2). Increasing output
voltage swing increases harmonic distortion directly. A 6dB
increase in output swing will generally increase the 2nd
harmonic 12dB and the 3rd harmonic 18dB. Increasing the
signal gain will also increase the 2nd harmonic distortion.
Again, a 6dB increase in gain will increase the 2nd and 3rd
harmonic by about 6dB even with a constant output power
and frequency. And finally, the distortion increases as the
fundamental frequency increases due to the roll-off in the
loop gain with frequency. Conversely, the distortion will
improve going to lower frequencies down to the dominant
open-loop pole at approximately 200kHz.
In most applications, the 2nd harmonic will set the limit to
dynamic range. Even order non-linearities arise from slight
imbalances between the positive and negative halves of an
output sinusoid. These imbalanced non-linearities arise from
such mechanisms as voltage dependent base-collector capacitances and imbalanced source impedances looking out
of the two amplifier power pins. Once a circuit and board
layout has been determined, these imbalances can typically
be nulled out by adjusting the DC operating point for the
signal. An example DC tune is shown in Figure 7. This
circuit has a DC-coupled inverting signal path to the output
pin that provides gain for a small DC offsetting signal
brought into the non-inverting input pin. The output is ACcoupled to block off this DC operating point from interacting with the next stage.
FIGURE 7. DC Adjustment for 2nd Harmonic Distortion.
For a 1Vp-p output swing in the 10MHz to 20MHz region,
an output DC voltage in the ±1.5V range will null the 2nd
harmonic distortion. Tests into a 200Ω converter input load
have shown > 20dB decrease in the 2nd harmonic using this
technique. Once the required voltage is found for a particular
board, circuit, and signal requirement, that voltage is very
repeatable from part to part and may be set permanently on
the non-inverting input. Minimal degradation from this improved 2nd harmonic distortion over temperature will be
observed. An alternative means to eliminate the 2nd harmonic distortion is to operate two OPA687s differentially as
shown on the front page of the data sheet. Both single-tone
and 2-tone even order harmonic distortions for this differential configuration are essentially unmeasureable through
30MHz for a good layout.
The OPA687 has an extremely low 3rd-order harmonic
distortion. This also gives a high 2-tone, 3rd-order
intermodulation intercept as shown in the Typical Performance Curves. This intercept curve is defined at the 50Ω
load when driven through a 50Ω matching resistor to allow
direct comparisons to RF MMIC devices. This network
attenuates the voltage swing from the output pin to the load
by 6dB. If the OPA687 drives directly into the input of a
high impedance device, such as an ADC, this 6dB attenuation is not taken. Under these conditions, the intercept will
increase by a minimum 6dBm. The intercept is used to
R
F
Supply Decoupling
Not Shown
5kΩ
5kΩ
R
G
V
I
20Ω
10kΩ
0.1µF
–5V
+5V
OPA687
+V
S
–V
S
V
O
Page 14
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OPA687
predict the intermodulation spurious for two closely-spaced
frequencies. If the two test frequencies, f1 and f2, are
specified in terms of average and delta frequency, fO = (f1 +
f2)/2 and ∆f = |f2 – f1| /2, the two, 3-order, close-in spurious
tones will appear at fO ±3 • ∆f. The difference between two
equal test-tone power levels and these intermodulation spurious power levels is given by (dBc = 2 • (IM3 – PO)) where
IM3 is the intercept taken from the Typical Performance
Curve and PO is the power level in dBm at the 50Ω load for
one of the two, closely-spaced test frequencies. For instance,
at 20MHz, the OPA687—at a gain of +20, has an intercept
of 43dBm at a matched 50Ω load. If the full envelope of the
two frequencies needs to be 2Vp-p, this requires each tone
to be 4dBm. The 3rd-order intermodulation spurious tones
will then be 2 • (43 – 4)=78dBc below the test-tone power
level (–74dBm). If this same 2Vp-p, 2-tone envelope were
delivered directly into the input of an ADC without the
matching loss or the loading of the 50Ω network, the
intercept would increase to at least 49dBm. With the same
signal and gain conditions, but now driving directly into a
light load, the spurious tones will then be at least 2 • (49 –
4) = 90dBc below the 4dBm test-tone power levels centered
on 20MHz. Tests have shown that, in reality, they are much
lower due to the lighter loading presented by most ADCs.
DC ACCURACY AND OFFSET CONTROL
The OPA687 can provide excellent DC signal accuracy due
to its high open-loop gain, high common-mode rejection,
high power supply rejection, and low input offset voltage
and bias current offset errors. To take full advantage of its
low ±1.0mV input offset voltage, careful attention to input
bias current cancellation is also required. The low noise
input stage for the OPA687 has a relatively high input bias
current (20µA typ into the pins) but with a very close match
between the two input currents—typically ±200nA input
offset current. The total output offset voltage may be considerably reduced by matching the source impedances looking
out of the two inputs. For example, one way to add bias
current cancellation to the circuit of Figure 1 would be to
insert a 12.1Ω series resistor into the non-inverting input
from the 50Ω terminating resistor. When the 50Ω source
resistor is DC-coupled, this will increase the source impedance for the non-inverting input bias current to 37.1Ω. Since
this is now equal to the impedance looking out of the
inverting input (RF || RG) for Figure 1, the circuit will cancel
the gains for the bias currents to the output leaving only the
offset current times the feedback resistor as a residual DC
error term at the output. Using the 750Ω feedback resistor,
this output error will now be less than ±1.8µA • 750Ω =
±1.4mV over the full temperature range for the circuit of
Figure 1 with a 12.1Ω resistor added as described. The
output DC offset will then be dominated by the input offset
voltage multiplied by the signal gain. For the circuit of
Figure 1, this will give a worst-case output DC offset of
±1.6mV • 20 = ±32mV over the full temperature range.
A fine-scale output offset null, or DC operating point adjustment is sometimes required. Numerous techniques are available for introducing a DC offset control into an op amp
circuit. Most of these techniques eventually reduce to setting
up a DC current through the feedback resistor. One key
consideration to selecting a technique is to insure that it has
a minimal impact on the desired signal path frequency
response. If the signal path is intended to be non-inverting,
the offset control is best applied as an inverting summing
signal to avoid interaction with the signal source. If the
signal path is intended to be inverting, applying the offset
control to the non-inverting input can be considered. For a
DC-coupled inverting input signal, this DC offset signal will
set up a DC current back into the source that must be
considered. An offset adjustment placed on the inverting op
amp input can also change the noise gain and frequency
response flatness. Figure 8 shows one example of an offset
adjustment for a DC-coupled signal path that will have
minimum impact on the signal frequency response. In this
case, the input is brought into an inverting gain resistor with
the DC adjustment and additional current summed into the
inverting node. The resistor values setting this offset adjustment are much larger than the signal path resistors. This will
insure that this adjustment has minimal impact on the loop
gain and hence, the frequency response.
FIGURE 8. DC-Coupled, Inverting Gain of –40, with Offset
Adjustment.
R
F
2kΩ
±250mV Output Adjustment
= – = –40
Supply Decoupling
Not Shown
5kΩ
5kΩ
95.3Ω
0.1µF
R
G
50Ω
V
I
20kΩ
10kΩ
0.1µF
–5V
+5V
OPA687
+5V
–5V
V
O
V
O
V
I
R
F
R
G
Page 15
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OPA687
DISABLE OPERATION
The OPA687 provides an optional disable feature that may
be used to reduce system power. If the DIS control pin is left
unconnected, the OPA687 will operate normally. To disable, the control pin must be asserted low. Figure 9 shows
a simplified internal circuit for the disable control feature.
Note that it is the power in the output stage and not in the
load that determines internal power dissipation.
As an absolute worst-case example, compute the maximum
TJ using an OPA687N (SOT23-6 package) in the circuit of
Figure 1 operating at the maximum specified ambient temperature of +85°C and driving a grounded 100Ω load.
PD = 10V (20.5mA) + 52/(4 • (100Ω || 789Ω)) = 275mW
Maximum TJ = +85°C + (0.28W • 150°C/W) = 127°C
All actual applications will operate at a lower junction
temperature than the 127°C computed above. Compute your
actual output stage power to get an accurate estimate of
maximum junction temperature, or use the results shown
here as an absolute maximum.
BOARD LAYOUT
Achieving optimum performance with a high frequency
amplifier like the OPA687 requires careful attention to
board layout parasitics and external component types. Recommendations that will optimize performance include:
a) Minimize parasitic capacitance to any AC ground for
all of the signal I/O pins. Parasitic capacitance on the
output and inverting input pins can cause instability: on the
non-inverting input, it can react with the source impedance
to cause unintentional bandlimiting. To reduce unwanted
capacitance, a window around the signal I/O pins should be
opened in all of the ground and power planes around those
pins. Otherwise, ground and power planes should be unbroken elsewhere on the board.
b) Minimize the distance (< 0.25") from the power
supply pins to high frequency 0.1µF decoupling capacitors. At the device pins, the ground and power plane layout
should not be in close proximity to the signal I/O pins. Avoid
narrow power and ground traces to minimize inductance
between the pins and the decoupling capacitors. The power
supply connections should always be decoupled with these
capacitors. Larger (2.2µF to 6.8µF) decoupling capacitors,
effective at lower frequency, should also be used on the
main supply pins. These may be placed somewhat farther
from the device and may be shared among several devices in
the same area of the PC board.
c) Careful selection and placement of external components will preserve the high frequency performance of
the OPA687. Resistors should be a very low reactance type.
Surface-mount resistors work best and allow a tighter overall layout. Metal-film and carbon composition, axially-leaded
resistors can also provide good high frequency performance.
Again, keep their leads and PC board trace length as short as
possible. Never use wirewound type resistors in a high
frequency application. Since the output pin and inverting
input pin are the most sensitive to parasitic capacitance,
always position the feedback and series output resistor, if
any, as close as possible to the output pin. Other network
components, such as non-inverting input termination resistors, should also be placed close to the package. Where
double-side component mounting is allowed, place the feed-
25kΩ 110kΩ
15kΩ
I
S
Control
–V
S
+V
S
V
DIS
Q1
FIGURE 9. Simplified Disabled Control Circuit.
In normal operation, base current to Q1 is provided through
the 110kΩ resistor while the emitter current through the
15kΩ resistor sets up a voltage drop that is inadequate to
turn on the two diodes in Q1’s emitter. As V
DIS
is pulled
low, additional current is pulled through the 15kΩ resistor,
eventually turning on these two diodes (≈ 100µA). At this
point, any further current pulled out of V
DIS
goes through
those diodes holding the emitter-based voltage of Q1 at
approximately zero volts. This shuts off the collector current
out of Q1, turning the amplifier off. The supply current in
the disable mode are only those required to operate the
circuit of Figure 9.
THERMAL ANALYSIS
The OPA687 will not require heatsinking or airflow in most
applications. Maximum desired junction temperature will
set the maximum allowed internal power dissipation as
described below. In no case should the maximum junction
temperature be allowed to exceed 175°C.
Operating junction temperature (TJ) is given by TA + PD •
θ
JA
. The total internal power dissipation (PD) is the sum of
quiescent power (PDQ) and additional power dissipated in
the output stage (PDL) to deliver load power. Quiescent
power is simply the specified no-load supply current times
the total supply voltage across the part. PDL will depend on
the required output signal and load but would, for a grounded
resistive load, be at a maximum when the output is fixed at
a voltage equal to 1/2 either supply voltage (for equal bipolar
supplies). Under this condition PDL = V
S
2
/(4 • RL) where R
L
includes feedback network loading. This is the absolute
highest power that can be dissipated for a given RL. All
actual applications will dissipate less power in the output
stage.
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OPA687
back resistor directly under the package on the other side of
the board between the output and inverting input pins. Even
with a low parasitic capacitance shunting the external resistors, excessively high resistor values can create significant
time constants that can degrade performance. Good axial
metal-film or surface-mount resistors have approximately
0.2pF in shunt with the resistor. For resistor values > 2.0kΩ,
this parasitic capacitance can add a pole and/or a zero below
400MHz that can effect circuit operation. Keep resistor
values as low as possible, consistent with load driving
considerations. It has been suggested here that a good
starting point for design would be set the RG be set to 39.2Ω
for non-inverting applications. Doing this will automatically
keep the resistor noise terms low, and minimize the effect of
their parasitic capacitance.
d) Connections to other wideband devices on the board
may be made with short direct traces or through onboard transmission lines. For short connections, consider
the trace and the input to the next device as a lumped
capacitive load. Relatively wide traces (50mils to 100mils)
should be used, preferably with ground and power planes
opened up around them. Estimate the total capacitive load
and set RS from the plot of recommended RS vs Capacitive
Load. Low parasitic capacitive loads (< 4pF) may not need
an RS since the OPA687 is nominally compensated to
operate with a 2pF parasitic load. Higher parasitic cap. loads
without an RS are allowed as the signal gain increases
(increasing the unloaded phase margin). If a long trace is
required, and the 6dB signal loss intrinsic to a doublyterminated transmission line is acceptable, implement a
matched impedance transmission line using microstrip or
stripline techniques (consult an ECL design handbook for
microstrip and stripline layout techniques). A 50Ω environment is normally not necessary on board, and in fact, a
higher impedance environment will improve distortion as
shown in the distortion versus load plots. With a characteristic board trace impedance defined based on board material
and trace dimensions, a matching series resistor into the
trace from the output of the OPA687 is used as well as a
terminating shunt resistor at the input of the destination
device. Remember also that the terminating impedance will
be the parallel combination of the shunt resistor and the
input impedance of the destination device: this total effective impedance should be set to match the trace impedance.
If the 6dB attenuation of a doubly terminated transmission
line is unacceptable, a long trace can be series-terminated at
the source end only. Treat the trace as a capacitive load in
this case and set the series resistor value as shown in the plot
of RS vs Capacitive Load. This will not preserve signal
integrity as well as a doubly-terminated line. If the input
impedance of the destination device is low, there will be
some signal attenuation due to the voltage divider formed by
the series output into the terminating impedance.
e) Socketing a high speed part like the OPA687 is not
recommended. The additional lead length and pin-to-pin
capacitance introduced by the socket can create an extremely troublesome parasitic network which can make it
almost impossible to achieve a smooth, stable frequency
response. Best results are obtained by soldering the OPA687
onto the board.
INPUT AND ESD PROTECTION
The OPA687 is built using a very high speed complementary bipolar process. The internal junction breakdown voltages are relatively low for these very small geometry devices. These breakdowns are reflected in the Absolute Maximum Ratings table. All device pins are protected with
internal ESD protection diodes to the power supplies as
shown in Figure 10.
External
Pin
+V
CC
–V
CC
Internal
Circuitry
These diodes provide moderate protection to input overdrive
voltages above the supplies as well. The protection diodes
can typically support 30mA continuous current. Where higher
currents are possible (e.g., in systems with ±15V supply
parts driving into the OPA687), current-limiting series resistors should be added into the two inputs. Keep these resistor
values as low as possible since high values degrade both
noise performance and frequency response.
FIGURE 10. Internal ESD Protection.
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