LMH6738

LMH6738 Very Wideband, Low Distortion Triple Op Amp July 2004 LMH6738 Very Wideband, Low Distortion Triple Op Amp General Description The LMH6738 is a very wideband, DC coupled monolithic operational amplifier designed specifically for ultra high resolution video systems as well as wide dynamic range systems requiring exceptional signal fidelity. Benefiting from National’s current feedback architecture, the LMH6738 offers a gain range of ± 1 to ± 10 while providing stable, operation without external compensation, even at unity gain. At a gain of +2 the LMH6738 supports ultra high resolution video systems with a 400 MHz 2 VPP –3 dB Bandwidth. With 12-bit distortion levels through 30 MHz (RL = 100Ω), 2.3 nV/ Hz input referred noise, the LMH6738 is the ideal driver or buffer for high speed flash A/D and D/A converters. Wide dynamic range systems such as radar and communication receivers requiring a wideband amplifier offering exceptional signal purity will find the LMH6738’s low input referred noise and low harmonic distortion make it an attractive solution. Features n n n n n n n 750 MHz −3 dB small signal bandwidth (AV = +1) −85 dBc 3rd harmonic distortion (20 MHz) 2.3 nV/ Hz input noise voltage 3300 V/µs slew rate 33 mA supply current (11.3 mA per op amp) 90 mA linear output current 0.02/0.01 Diff. Gain / Diff. Phase (RL = 150Ω) Applications n n n n n n n n n RGB video driver High resolution projectors Flash A/D driver D/A transimpedance buffer Wide dynamic range IF amp Radar/communication receivers DDS post-amps Wideband inverting summer Line driver Connection Diagram 16-Pin SSOP 20097510 Top View VIP10™ is a trademark of National Semiconductor Corporation. © 2004 National Semiconductor Corporation DS200975 www.national.com LMH6738 Absolute Maximum Ratings (Note 1) If Military/Aerospace specified devices are required, please contact the National Semiconductor Sales Office/ Distributors for availability and specifications. Supply Voltage (V+ - V– ) IOUT Common Mode Input Voltage Maximum Junction Temperature Storage Temperature Range Soldering Information Infrared or Convection (20 sec.) Wave Soldering (10 sec.) 235˚C 260˚C 13.2V (Note 3) ESD Tolerance (Note 4) Human Body Model Machine Model Storage Temperature Range 2000V 200V −65˚C to +150˚C ± VCC +150˚C −65˚C to +150˚C Operating Ratings (Note 1) Thermal Resistance Package 16-Pin SSOP Operating Temperature Range Supply Voltage (V+ - V– ) (θJC) 36˚C/W −40˚C 8V to (θJA) 120˚C/W +85˚C 12V Electrical Characteristics (Note 2) AV = +2, VCC = ± 5V, RL = 100Ω, RF = 549Ω; unless otherwise specified. Symbol UGBW SSBW LSBW 0.1 dB Bandwidth GFPL GFR1 GFR2 TRS TRL SR ts te td Distortion HD2L HD2 HD2H HD3L HD3 HD3H Equivalent Input Noise VN ICN NCN DG DP VIO IBN IBI Non-Inverting Voltage Inverting Current Non-Inverting Current Differential Gain Differential Phase Input Offset Voltage (Note 6) Input Bias Current (Note 6) Input Bias Current (Note 6) Non-Inverting Inverting −15 −20 3rd Harmonic Distortion 2nd Harmonic Distortion 2 VPP, 5 MHz 2 VPP, 20 MHz 2 VPP, 50 MHz 2 VPP, 5 MHz 2 VPP, 20 MHz 2 VPP, 50 MHz −80 −71 −55 −90 −85 −65 2.3 12 3 .02 .01 0.5 −7 −2 nV/ pA/ pA/ % ˚ dBc dBc Peaking Rolloff Rolloff Rise and Fall Time (10% to 90%) Slew Rate Settling Time to 0.1% Enable Time Disable Time Parameter -3 dB Bandwidth -3 dB Bandwidth Conditions Unity Gain, VOUT = 200 mVPP VOUT = 200 mVPP VOUT = 2 VPP VOUT = 2 VPP DC to 75 MHz DC to 150 MHz, VOUT = 2 VPP @ 300 MHz, VOUT = 2 VPP Min Typ 750 480 400 150 0 0.1 1.0 0.9 1.7 3300 10 7.3 4.5 Max Units MHz MHz MHz dB dB dB Frequency Domain Performance Time Domain Response 2V Step 5V Step 5V Step 2V Step From Disable = rising edge. From Disable = falling edge. ns V/µs ns ns ns > 1 MHz > 1 MHz > 1 MHz 4.43 MHz, RL = 150Ω 4.43 MHz, RL = 150Ω Video Performance Static, DC Performance ± 2.5 ± 4.5 0 +5 mV µA µA ± 25 ± 35 www.national.com 2 LMH6738 Electrical Characteristics (Note 2) Symbol PSRR CMRR XTLK Parameter Power Supply Rejection Ratio (Note 6) Common Mode Rejection Ratio (Note 6) Crosstalk (Continued) AV = +2, VCC = ± 5V, RL = 100Ω, RF = 549Ω; unless otherwise specified. Conditions Min 50 48.5 46 44 Input Referred, f=10MHz, Drive channels A,C measure channel B All three amps Enabled, No Load RL = ∞ RL = ∞ Typ 53 50 −80 Max Units dB dB dB ICC Supply Current (Note 6) Supply Current Disabled V+ Supply Current Disabled V− 32 1.9 1.1 1000 .8 35 40 2.2 1.3 mA mA mA kΩ pF Ω Ω Miscellaneous Performance RIN+ CIN+ RIN− RO VO Non-Inverting Input Resistance Non-Inverting Input Capacitance Inverting Input Impedance Output Impedance Output Voltage Range (Note 6) Output impedance of input buffer. DC RL = 100Ω RL = ∞ CMIR IO ISC IIH IIL VDMAX VDMIM Common Mode Input Range (Note 6) Linear Output Current (Notes 3, 6) Short Circuit Current (Note 5) Disable Pin Bias Current High Disable Pin Bias Current Low Voltage for Disable Voltage for Enable CMRR > 40 dB VIN = 0V, VOUT < ± 30 mV VIN = 2V Output Shorted to Ground Disable Pin = V+ Disable Pin = 0V Disable Pin ≤ VDMAX Disable Pin ≥ VDMIN 2.0 30 0.05 ± 3.25 ± 3.1 ± 3.65 ± 3.5 ± 1.9 ± 1.7 80 60 ± 3.5 ± 3.8 ± 2.0 90 160 10 −350 0.8 V V mA mA µA µA V V Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is intended to be functional, but specific performance is not guaranteed. For guaranteed specifications, see the Electrical Characteristics tables. Note 2: Electrical Table values apply only for factory testing conditions at the temperature indicated. Factory testing conditions result in very limited self-heating of the device such that TJ = TA. No guarantee of parametric performance is indicated in the electrical tables under conditions of internal self heating where TJ > TA. See Applications Section for information on temperature de-rating of this device." Min/Max ratings are based on product characterization and simulation. Individual parameters are tested as noted. Note 3: The maximum output current (IOUT) is determined by device power dissipation limitations. See the Power Dissipation section of the Application Section for more details. Note 4: Human body model: 1.5 kΩ in series with 100 pF. Machine model: 0Ω in series with 200 pF. Note 5: Short circuit current should be limited in duration to no more than 10 seconds. See the Power Dissipation section of the Application Section for more details. Note 6: Parameter 100% production tested at 25˚ C. Ordering Information Package 16-pin SSOP Part Number LMH6738MQ LMH6738MQX Package Marking LH6738MQ Transport Media 95 Units/Rail 2.5k Units Tape and Reel NSC Drawing MQA16 3 www.national.com LMH6738 Typical Performance Characteristics wise specified). Large Signal Frequency Response AV = +2, VCC = ± 5V, RL = 100Ω, RF = 549Ω; unless otherLarge Signal Frequency Response 20097520 20097528 Small Signal Frequency Response Frequency Response vs. VOUT 20097513 20097501 Frequency Response vs. Supply Voltage Pulse Response 20097516 20097522 www.national.com 4 LMH6738 Typical Performance Characteristics AV = +2, VCC = ±5V, RL = 100Ω, RF = 549Ω; unless otherwise specified). (Continued) Frequency Response vs. Capacitive Load Series Output Resistance vs. Capacitive Load 20097514 20097519 Open Loop Gain and Phase Distortion vs. Frequency 20097526 20097525 Distortion vs. Output Voltage Distortion vs. Supply Voltage 20097517 20097518 5 www.national.com LMH6738 Typical Performance Characteristics AV = +2, VCC = ±5V, RL = 100Ω, RF = 549Ω; unless otherwise specified). (Continued) CMRR vs. Frequency PSRR vs. Frequency 20097511 20097521 Crosstalk vs. Frequency Closed Loop Output Impedance |Z| 20097529 20097504 Disable Timing DC Errors vs. Temperature 20097524 20097512 www.national.com 6 LMH6738 Typical Performance Characteristics AV = +2, VCC = ±5V, RL = 100Ω, RF = 549Ω; unless otherwise specified). (Continued) Input Noise vs. Frequency 20097527 7 www.national.com LMH6738 Application Section FEEDBACK RESISTOR SELECTION One of the key benefits of a current feedback operational amplifier is the ability to maintain optimum frequency response independent of gain by using appropriate values for the feedback resistor (RF). The Electrical Characteristics and Typical Performance plots specify an RF of 550Ω, a gain of +2 V/V and ± 5V power supplies (unless otherwise specified). Generally, lowering RF from it’s recommended value will peak the frequency response and extend the bandwidth while increasing the value of RF will cause the frequency response to roll off faster. Reducing the value of RF too far below it’s recommended value will cause overshoot, ringing and, eventually, oscillation. 20097505 FIGURE 1. Recommended Non-Inverting Gain Circuit 20097503 FIGURE 3. Recommended RF vs. Gain See Figure 3, Recommended RF. vs Gain for selecting a feedback resistor value for gains of ± 1 to ± 10. Since each application is slightly different it is worth some experimentation to find the optimal RF for a given circuit. In general a value of RF that produces ~.1 dB of peaking is the best compromise between stability and maximal bandwidth. Note that it is not possible to use a current feedback amplifier with the output shorted directly to the inverting input. The buffer configuration of the LMH6738 requires a 750Ω feedback resistor for stable operation. The LMH6738 was optimized for high speed operation. As shown in Figure 3 the suggested value for RF decreases for higher gains. Due to the impedance of the input buffer there is a practical limit for how small RFcan go, based on the lowest practical value of RG. This limitation applies to both inverting and non inverting configurations. For the LMH6738 the input resistance of the inverting input is approximately 30Ω and 20Ω is a practical (but not hard and fast) lower limit for RG. The LMH6738 begins to operate in a gain bandwidth limited fashion in the region where RG is nearly equal to the input buffer impedance. Note that the amplifier will operate with RG values well below 20Ω, however results may be substantially different than predicted from ideal models. In particular the voltage potential between the Inverting and Non Inverting inputs cannot be expected to remain small. Inverting gain applications that require impedance matched inputs may limit gain flexibility somewhat (especially if maximum bandwidth is required). The impedance seen by the source is RG || RT (RT is optional). The value of RG is RF 8 20097506 FIGURE 2. Recommended Inverting Gain Circuit GENERAL INFORMATION The LMH6738 is a high speed current feedback amplifier, optimized for very high speed and low distortion. The LMH6738 has no internal ground reference so single or split supply configurations are both equally useful. EVALUATION BOARDS National Semiconductor provides the following evaluation boards as a guide for high frequency layout and as an aid in device testing and characterization. Many of the datasheet plots were measured with these boards. Device LMH6738MQA Package SSOP Evaluation Board Part Number LMH730275 A bare evaluation board is shipped when a sample request is placed with National Semiconductor. www.national.com LMH6738 Application Section (Continued) /Gain. Thus for an inverting gain of −7 V/V and an optimal value for RF the input impedance is equal to 50Ω. Using a termination resistor this can be brought down to match a 25Ω source, however, a 150Ω source cannot be matched. To match a 150Ω source would require using a 1050Ω feedback resistor and would result in reduced bandwidth. For more information see Application Note OA-13 which describes the relationship between RF and closed-loop frequency response for current feedback operational amplifiers. The value for the inverting input impedance for the LMH6738 is approximately 30Ω. The LMH6738 is designed for optimum performance at gains of +1 to +10 V/V and −1 to −9 V/V. Higher gain configurations are still useful, however, the bandwidth will fall as gain is increased, much like a typical voltage feedback amplifier. ACTIVE FILTER When using any current feedback Operational Amplifier as an active filter it is necessary to be careful using reactive components in the feedback loop. Reducing the feedback impedance, especially at higher frequencies, will almost certainly cause stability problems. Likewise capacitance on the inverting input should be avoided. See Application Notes OA-7 and OA-26 for more information on Active Filter applications for Current Feedback Op Amps. When using the LMH6738 as a low pass filter the value of RF can be substantially reduced from the value recommended in the RF vs. Gain charts. The benefit of reducing RF is increased gain at higher frequencies, which improves attenuation in the stop band. Stability problems are avoided because in the stop band additional device bandwidth is used to cancel the input signal rather than amplify it. The benefit of this change depends on the particulars of the circuit design. With a high pass filter configuration reducing RF will likely result in device instability and is not recommended. 20097508 FIGURE 5. Decoupling Capacitive Loads DRIVING CAPACITIVE LOADS Capacitive output loading applications will benefit from the use of a series output resistor ROUT. Figure 5 shows the use of a series output resistor, ROUT, to stabilize the amplifier output under capacitive loading. Capacitive loads of 5 to 120 pF are the most critical, causing ringing, frequency response peaking and possible oscillation. The charts “Suggested ROUT vs. Cap Load” give a recommended value for selecting a series output resistor for mitigating capacitive loads. The values suggested in the charts are selected for .5 dB or less of peaking in the frequency response. This gives a good compromise between settling time and bandwidth. For applications where maximum frequency response is needed and some peaking is tolerable, the value of ROUT can be reduced slightly from the recommended values. An alternative approach is to place Rout inside the feedback loop as shown in Figure 6. This will preserve gain accuracy, but will still limit maximum output voltage swing. 20097509 FIGURE 6. Series Output Resistor Inside Feedback Loop INVERTING INPUT PARASITIC CAPACITANCE Parasitic capacitance is any capacitance in a circuit that was not intentionally added. It comes about from electrical interaction between conductors. Parasitic capacitance can be reduced but never entirely eliminated. Most parasitic capacitances that cause problems are related to board layout or lack of termination on transmission lines. Please see the section on Layout Considerations for hints on reducing problems due to parasitic capacitances on board traces. Transmission lines should be terminated in their characteristic impedance at both ends. High speed amplifiers are sensitive to capacitance between the inverting input and ground or power supplies. This shows up as gain peaking at high frequency. The capacitor raises device gain at high frequencies by making RG appear smaller. Capacitive output loading will exaggerate this effect. In general, avoid introducing unnecessary parasitic capacitance at both the inverting input and the output. 9 www.national.com 20097507 FIGURE 4. Typical Video Application LMH6738 Application Section (Continued) One possible remedy for this effect is to slightly increase the value of the feedback (and gain set) resistor. This will tend to offset the high frequency gain peaking while leaving other parameters relatively unchanged. If the device has a capacitive load as well as inverting input capacitance using a series output resistor as described in the section on “Driving Capacitive Loads” will help. LAYOUT CONSIDERATIONS Whenever questions about layout arise, use the evaluation board as a guide. The LMH730275 is the evaluation board supplied with samples of the LMH6738. To reduce parasitic capacitances ground and power planes should be removed near the input and output pins. Components in the feedback loop should be placed as close to the device as possible. For long signal paths controlled impedance lines should be used, along with impedance matching elements at both ends. Bypass capacitors should be placed as close to the device as possible. Bypass capacitors from each rail to ground are applied in pairs. The larger electrolytic bypass capacitors can be located farther from the device, the smaller ceramic capacitors should be placed as close to the device as possible. The LMH6738 has multiple power and ground pins for enhanced supply bypassing. Every pin should ideally have a separate bypass capacitor. Sharing bypass capacitors may slightly degrade second order harmonic performance, especially if the supply traces are thin and /or long. In Figure 1 and Figure 2 CSS is optional, but is recommended for best second harmonic distortion. Another option to using CSS is to use pairs of .01 µF and .1 µF ceramic capacitors for each supply bypass. VIDEO PERFORMANCE The LMH6738 has been designed to provide excellent performance with production quality video signals in a wide variety of formats such as HDTV and High Resolution VGA. NTSC and PAL performance is nearly flawless. Best performance will be obtained with back terminated loads. The back termination reduces reflections from the transmission line and effectively masks transmission line and other parasitic capacitances from the amplifier output stage. Figure 4 shows a typical configuration for driving a 75Ω Cable. The amplifier is configured for a gain of two to make up for the 6 dB of loss in ROUT. POWER DISSIPATION The LMH6738 is optimized for maximum speed and performance in the small form factor of the standard SSOP-16 package. To achieve its high level of performance, the LMH6738 consumes an appreciable amount of quiescent current which cannot be neglected when considering the total package power dissipation limit. The quiescent current contributes to about 40˚ C rise in junction temperature when no additional heat sink is used (VS = ± 5V, all 3 channels on). Therefore, it is easy to see the need for proper precautions to be taken in order to make sure the junction temperature’s absolute maximum rating of 150˚C is not violated. To ensure maximum output drive and highest performance, thermal shutdown is not provided. Therefore, it is of utmost importance to make sure that the TJMAX is never exceeded due to the overall power dissipation (all 3 channels). With the LMH6738 used in a back-terminated 75Ω RGB analog video system (with 2 VPP output voltage), the total power dissipation is around 435 mW of which 340 mW is due to the quiescent device dissipation (output black level at 0V). With no additional heat sink used, that puts the junction temperature to about 140˚ C when operated at 85˚C ambient. To reduce the junction temperature many options are available. Forced air cooling is the easiest option. An external add-on heat-sink can be added to the SSOP-16 package, or alternatively, additional board metal (copper) area can be utilized as heat-sink. An effective way to reduce the junction temperature for the SSOP-16 package (and other plastic packages) is to use the copper board area to conduct heat. With no enhancement the major heat flow path in this package is from the die through the metal lead frame (inside the package) and onto the surrounding copper through the interconnecting leads. Since high frequency performance requires limited metal near the device pins the best way to use board copper to remove heat is through the bottom of the package. A gap filler with high thermal conductivity can be used to conduct heat from the bottom of the package to copper on the circuit board. Vias to a ground or power plane on the back side of the circuit board will provide additional heat dissipation. A combination of front side copper and vias to the back side can be combined as well. Follow these steps to determine the Maximum power dissipation for the LMH6738: 1. Calculate the quiescent (no-load) power: PAMP = ICC* (VS) VS = V+-V− 2. Calculate the RMS power dissipated in the output stage: PD (rms) = rms ((VS - VOUT)*IOUT) where VOUT and IOUT are the voltage and current across the external load and VS is the total supply current 3. Calculate the total RMS power: PT = PAMP+PD The maximum power that the LMH6738, package can dissipate at a given temperature can be derived with the following equation (See Figure 7): PMAX = (150o – TAMB)/ θJA, where TAMB = Ambient temperature (˚C) and θJA = Thermal resistance, from junction to ambient, for a given package (˚C/W). For the SSOP package θJA is 120˚C/W. 20097502 FIGURE 7. Maximum Power Dissipation www.national.com 10 LMH6738 Application Section ESD PROTECTION (Continued) The LMH6738 is protected against electrostatic discharge (ESD) on all pins. The LMH6738 will survive 2000V Human Body model and 200V Machine model events. Under closed loop operation the ESD diodes have no effect on circuit performance. There are occasions, however, when the ESD diodes will be evident. If the LMH6738 is driven by a large signal while the device is powered down the ESD diodes will conduct. The current that flows through the ESD diodes will either exit the chip through the supply pins or will flow through the device, hence it is possible to power up a chip with a large signal applied to the input pins. Shorting the power pins to each other will prevent the chip from being powered up through the input. 11 www.national.com LMH6738 Very Wideband, Low Distortion Triple Op Amp Physical Dimensions unless otherwise noted inches (millimeters) 16-Pin SSOP NS Package Number MQA16 LIFE SUPPORT POLICY NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury to the user. 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