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OPA2684

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Texas Instruments semiconductor products and disclaimers thereto appears at the end of this data sheet.

SBOS239D – APRIL 2002 – REVISED JULY 2008

Low-Power, Dual Current-Feedback

OPERATIONAL AMPLIFIER

FEATURES

● MINIMAL BANDWIDTH CHANGE VERSUS GAIN

● 170MHz BANDWIDTH AT G = +2

● > 120MHz BANDWIDTH TO GAIN > +10

● LOW DISTORTION: < –82dBc at 5MHz

● HIGH OUTPUT CURRENT: 120mA

● SINGLE +5V TO +12V SUPPLY OPERATION

● DUAL ±2.5 TO ±6.0V SUPPLY OPERATION

● LOW SUPPLY CURRENT: 3.4mA Total

V+

V–

ERR

(S)

ERR

Low-Power Amplifier

1 of 2 Channels

DESCRIPTION

The OPA2684 provides a new level of performance in low-

power, wideband, current-feedback (CFB) amplifiers. This

CFB

PLUS

amplifier is among the first to use an internally

closed-loop input buffer stage that enhances performance

significantly over earlier low-power CFB amplifiers. While

retaining the benefits of very low power operation, this new

architecture provides many of the benefits of a more ideal

CFB amplifier. The closed-loop input stage buffer gives a

very low and linearized impedance path at the inverting input

to sense the feedback error current. This improved inverting

input impedance retains exceptional bandwidth to much

higher gains and improves harmonic distortion over earlier

solutions limited by inverting input linearity. Beyond simple

high-gain applications, the OPA2684 CFB

PLUS

amplifier per-

mits the gain setting element to be set with considerable

APPLICATIONS

● SHORT-LOOP ADSL CO DRIVER

● LOW-POWER BROADCAST VIDEO DRIVERS

● DIFFERENTIAL EQUALIZING FILTERS

● DIFFERENTIAL SAW FILTER POST AMPLIFIER

● MULTICHANNEL SUMMING AMPLIFIERS

● PROFESSIONAL CAMERAS

● ADC INPUT DRIVERS

freedom from amplifier bandwidth interaction. This allows

frequency response peaking elements to be added, multiple

input inverting summing circuits to have greater bandwidth,

and low-power line drivers to meet the demanding require-

ments of studio cameras and broadcast video.

The output capability of the OPA2684 also sets a new mark

in performance for low-power, current-feedback amplifiers.

Delivering a full ±4Vp-p swing on ±5V supplies, the OPA2684

also has the output current to support > ±3Vp-p into 50Ω

loads. This minimal output headroom requirement is comple-

mented by a similar 1.2V input stage headroom giving

exceptional capability for single +5V operation.

The OPA2684’s low 3.4mA supply current is precisely trimmed

at +25°C. This trim, along with low shift over temperature and

supply voltage, gives a very robust design over a wide range

of operating conditions.

–3

–6

–9

–12

–15

–18

–21

–24

Normalized Gain (3dB/div)

= 800Ω

G = 100

G = 50

G = 20

G = 10

G = 2

G = 1

MHz

100 200

BW (MHz) vs GAIN

G = 5

US Patent #6,724,260

PRODUCTION DATA information is current as of publication date.

Products conform to specifications per the terms of Texas Instruments

standard warranty. Production processing does not necessarily include

testing of all parameters.

All trademarks are the property of their respective owners.

Summary of content (34 pages)

PAGE 1
OPA2684 OPA 268 4 SBOS239D – APRIL 2002 – REVISED JULY 2008 www.ti.com Low-Power, Dual Current-Feedback OPERATIONAL AMPLIFIER FEATURES APPLICATIONS ● ● ● ● ● ● ● ● ● ● ● ● ● ● ● MINIMAL BANDWIDTH CHANGE VERSUS GAIN 170MHz BANDWIDTH AT G = +2 > 120MHz BANDWIDTH TO GAIN > +10 LOW DISTORTION: < –82dBc at 5MHz HIGH OUTPUT CURRENT: 120mA SINGLE +5V TO +12V SUPPLY OPERATION DUAL ±2.5 TO ±6.0V SUPPLY OPERATION LOW SUPPLY CURRENT: 3.
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ELECTROSTATIC DISCHARGE SENSITIVITY ABSOLUTE MAXIMUM RATINGS(1) Power Supply ............................................................................... ±6.5VDC Internal Power Dissipation ................................. See Thermal Information Differential Input Voltage .................................................................. ±1.2V Input Voltage Range ............................................................................ ±VS Storage Temperature Range: ID, IDBV ........................
PAGE 3
ELECTRICAL CHARACTERISTICS: VS = ±5V Boldface limits are tested at +25°C. RF = 800Ω, RL = 100Ω, and G = +2, (see Figure 1 for AC performance only), unless otherwise noted. OPA2684ID, IDCN TYP PARAMETER AC PERFORMANCE (see Figure 1) Small-Signal Bandwidth (VO = 0.5Vp-p) Bandwidth for 0.
PAGE 4
ELECTRICAL CHARACTERISTICS: VS = +5V Boldface limits are tested at +25°C. RF = 1kΩ, RL = 100Ω, and G = +2, (see Figure 3 for AC performance only), unless otherwise noted. OPA2684ID, IDCN TYP PARAMETER AC PERFORMANCE (see Figure 3) Small-Signal Bandwidth (VO = 0.5Vp-p) Bandwidth for 0.
PAGE 5
TYPICAL CHARACTERISTICS: VS = ±5V At TA = +25°C, G = +2, RF = 800Ω, and RL = 100Ω, unless otherwise noted. NONINVERTING SMALL-SIGNAL FREQUENCY RESPONSE VO = 0.5Vp-p RF = 800Ω 3 G=1 G=2 0 –3 –6 G=5 G = 10 –9 G = 20 –12 G = 50 –15 See Figure 1 VO = 0.5Vp-p RF = 800Ω 0 –3 –6 G = 100 –18 See Figure 2 –12 1 10 100 200 1 10 Frequency (MHz) G = +2 RL = 100Ω VO = 0.5Vp-p G = –1 RL = 100Ω VO = 0.
PAGE 6
TYPICAL CHARACTERISTICS: VS = ±5V (Cont.) At TA = +25°C, G = +2, RF = 800Ω, and RL = 100Ω, unless otherwise noted. HARMONIC DISTORTION vs LOAD RESISTANCE HARMONIC DISTORTION vs FREQUENCY –50 –50 VO = 2Vp-p f = 5MHz G = +2 –60 VO = 2Vp-p RL = 100Ω Harmonic Distortion (dBc) Harmonic Distortion (dBc) –55 –65 2nd-Harmonic –70 –75 3rd-Harmonic –80 –60 2nd-Harmonic –70 3rd-Harmonic –80 –85 See Figure 1 –90 See Figure 1 –90 100 0.
PAGE 7
TYPICAL CHARACTERISTICS: VS = ±5V (Cont.) At TA = +25°C, G = +2, RF = 800Ω, and RL = 100Ω, unless otherwise noted. 2-TONE, 3RD-ORDER INTERMODULATION DISTORTION INPUT VOLTAGE AND CURRENT NOISE DENSITY –50 100 20MHz 3rd-Order Spurious Level (dBc) Noninverting Current Noise 9.4pA/√Hz 10 Voltage Noise 3.
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TYPICAL CHARACTERISTICS: VS = ±5V (Cont.) At TA = +25°C, G = +2, RF = 800Ω, and RL = 100Ω, unless otherwise noted. COMPOSITE VIDEO DIFFERENTIAL GAIN/PHASE OUTPUT CURRENT AND VOLTAGE LIMITATIONS 3 0.07 2 0.06 dG 0.05 1W Power Limit =1 00Ω 0.08 VO (V) 50 Ω 1 0 –1 0.04 –2 0.03 dP 0.02 –3 0.
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TYPICAL CHARACTERISTICS: VS = ±5V (Cont.) At TA = +25°C, G = +2, RF = 800Ω, and RL = 100Ω, unless otherwise noted. 8.0 8.0 3.2 6.4 6.4 6.4 2.4 4.8 4.8 4.8 1.6 3.2 1.6 0 –0.8 0 Output Voltage Right-Scale –1.6 See Figure 1 –1.6 –2.4 –3.2 –3.2 –4.8 Input Voltage Left-Scale –4.0 Input Voltage (1.6V/div) 0.8 Output Voltage (1.6V/div) Input Voltage (0.8V/div) 8.0 3.2 3.2 Output Voltage Right-Scale 1.6 1.6 0 0 –1.6 –1.6 –3.2 –3.2 –4.8 –6.4 –6.4 –8.0 –8.
PAGE 10
TYPICAL CHARACTERISTICS: VS = ±5V (Cont.) At TA = +25°C, G = +2, RF = 800Ω, RL = 100Ω, unless otherwise noted.
PAGE 11
TYPICAL CHARACTERISTICS: VS = +5V At TA = +25°C, VS = 5V, G = +2, RF = 1.0kΩ, and RL = 100Ω, unless otherwise noted. NONINVERTING SMALL-SIGNAL FREQUENCY RESPONSE RF = 1kΩ 3 Normalized Gain (3dB/div) INVERTING SMALL-SIGNAL FREQUENCY RESPONSE 3 G = 50 RF = 1.
PAGE 12
TYPICAL CHARACTERISTICS: VS = +5V (Cont.) At TA = +25°C, VS = 5V, G = +2, RF = 1.0kΩ, and RL = 100Ω, unless otherwise noted. HARMONIC DISTORTION vs LOAD RESISTANCE HARMONIC DISTORTION vs FREQUENCY –50 –50 VO = 2Vp-p f = 5MHz Harmonic Distortion (dBc) Harmonic Distortion (dBc) –55 VO = 2Vp-p RL = 100Ω –60 3rd-Harmonic –65 –70 –75 –80 2nd-Harmonic –60 2nd-Harmonic –70 3rd-Harmonic –80 –85 See Figure 3 –90 100 0.
PAGE 13
TYPICAL CHARACTERISTICS: VS = +5V (Cont.) At TA = +25°C, VS = 5V, G = +2, RF = 1.0Ω, and RL = 100Ω, unless otherwise noted. DIFFERENTIAL PERFORMANCE TEST CIRCUIT DIFFERENTIAL SMALL-SIGNAL FREQUENCY RESPONSE 6 +5V VO = 200mVp-p RL = 100Ω 3 0.01µF VI 0.01µF 1/2 OPA2684 RG Normalized Gain (dB) GD = +2.5V 1kΩ RG 1kΩ RL 1kΩ RG VO G=1 G=2 0 –3 –6 –9 G=5 –12 –15 –18 G = 20 –21 1/2 OPA2684 G = 10 –24 1 +2.
PAGE 14
APPLICATIONS INFORMATION LOW-POWER, CURRENT-FEEDBACK OPERATION The dual channel OPA2684 gives a new level of performance in low-power, current-feedback op amps. Using a new input stage buffer architecture, the OPA2684 CFBPLUS amplifier holds nearly constant AC performance over a wide gain range. This closed-loop internal buffer gives a very low and linearized impedance at the inverting node, isolating the amplifier’s AC performance from gain element variations.
PAGE 15
into this midpoint voltage bias. The input voltage can swing to within 1.25V of either supply pin, giving a 2.5Vp-p input signal range centered between the supply pins. The input impedance of Figure 3 is set to give a 50Ω input match. If the source does not require a 50Ω match, remove this and drive directly into the blocking capacitor. The source will then see the 5kΩ load of the biasing network.
PAGE 16
This approach provides for a source termination impedance that is independent of the signal gain. For instance, simple differential filters may be included in the signal path right up to the noninverting inputs without interacting with the gain setting. The differential signal gain for the circuit of Figure 5 is: AD = 1 + 2 • RF /RG Since the OPA2684 is a CFBPLUS amplifier, its bandwidth is principally controlled with the feedback resistor value, Figure 5 shows the recommended value of 800Ω.
PAGE 17
Figure 9 designs the filter for a differential gain of 5 using the OPA2684. The resistor values have been adjusted slightly to account for the amplifier bandwidth effects. SINGLE TO DIFFERENTIAL CONVERSION 24 While this circuit is bipolar, using ±5V supplies, it can easily be adapted to single-supply operation. This is typically done by providing a supply midpoint reference at the noninverting inputs then adding DC blocking caps at each input and in series with the amplifier gain resistor, RG.
PAGE 18
SINGLE-SUPPLY, HIGH GAIN DIFFERENTIAL ADC DRIVER Where a very low power differential I/O interface to a moderate performance ADC is required, the circuit of Figure 11 may be considered. The circuit builds on the inverting differential I/O configuration of Figure 6 by adding the input transformer and the output low-pass filter.
PAGE 19
DESIGN-IN TOOLS +12V DEMONSTRATION FIXTURES 2kΩ Two printed circuit boards (PCBs) are available to assist in the initial evaluation of circuit performance using the OPA2684 in its two package styles. Both of these are offered free of charge as unpopulated PCBs, delivered with a user’s guide. The summary information for these fixtures is shown in Table I. +6V 1/2 OPA2684 RF 800Ω RO 18.2Ω RP 1.07kΩ RG 931Ω 2Vp-p max 2kΩ 1:1 12.67Vp-p → 135Ω V2 max RP 1.07kΩ RF 800Ω PRODUCT RO 18.
PAGE 20
The key elements of this current-feedback op amp model are: α ⇒ Buffer gain from the noninverting input to the inverting input RI ⇒ Buffer output impedance iERR ⇒ Feedback error current signal Z(s) ⇒ Frequency dependent open-loop transimpedance gain from iERR to VO The buffer gain is typically very close to 1.00 and is normally neglected from signal gain considerations. It will, however, set the CMRR for a single op amp differential amplifier configuration. For the buffer gain α < 1.
PAGE 21
the 1.6kΩ total feedback network impedance. This 94Ω load will require no more than 40mA output current to support the ±3.8V minimum output voltage swing specified for 100Ω loads. This is well under the specified minimum +120/–90mA specifications over the full temperature range. The specifications described above, though familiar in the industry, consider voltage and current limits separately. In many applications, it is the voltage • current, or V-I product, which is more relevant to circuit operation.
PAGE 22
lower 3rd-harmonic component. Focusing then on the 2ndharmonic, increasing the load impedance improves distortion directly. Remember that the total load includes the feedback network—in the noninverting configuration (see Figure 1) this is the sum of RF + RG, while in the inverting configuration it is just RF. Also, providing an additional supply decoupling capacitor (0.1µF) between the supply pins (for bipolar operation) improves the 2nd-order distortion slightly (3dB to 6dB).
PAGE 23
however, are somewhat higher and are unmatched. Whereas bias current cancellation techniques are very effective with most voltage-feedback op amps, they do not generally reduce the output DC offset for wideband current-feedback op amps. Since the two input bias currents are unrelated in both magnitude and polarity, matching the source impedance looking out of each input to reduce their error contribution to the output is ineffective.
PAGE 24
d) 24 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 CLOAD.
PAGE 25
Revision History DATE REVISION 7/08 D 5/06 C PAGE SECTION 2 Abs Max Ratings 3, 4 Electrical Characteristics, Power Supply 19 Design-In Tools DESCRIPTION Changed Storage Temperature Range from −40°C to +125C to −65°C to +125C. Added minimum supply voltage. Changed min and max quiescent current values from total to per channel. Board part numbers changed. NOTE: Page numbers for previous revisions may differ from page numbers in the current version. OPA2684 SBOS239D www.ti.
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PACKAGE OPTION ADDENDUM www.ti.
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PACKAGE OPTION ADDENDUM www.ti.com 31-Oct-2013 Important Information and Disclaimer:The information provided on this page represents TI's knowledge and belief as of the date that it is provided. TI bases its knowledge and belief on information provided by third parties, and makes no representation or warranty as to the accuracy of such information. Efforts are underway to better integrate information from third parties.
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PACKAGE MATERIALS INFORMATION www.ti.com 12-Aug-2013 TAPE AND REEL INFORMATION *All dimensions are nominal Device OPA2684IDCNT Package Package Pins Type Drawing SPQ SOT-23 250 DCN 8 Reel Reel A0 Diameter Width (mm) (mm) W1 (mm) 180.0 8.4 Pack Materials-Page 1 3.2 B0 (mm) K0 (mm) P1 (mm) 3.1 1.39 4.0 W Pin1 (mm) Quadrant 8.
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PACKAGE MATERIALS INFORMATION www.ti.com 12-Aug-2013 *All dimensions are nominal Device Package Type Package Drawing Pins SPQ Length (mm) Width (mm) Height (mm) OPA2684IDCNT SOT-23 DCN 8 250 210.0 185.0 35.
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