Triple Differential Driver for Wideband Video AD8146/AD8147/AD8148 –IN B +IN B VS– VOCMA VOCMB 23 22 21 20 19 OPD 1 AD8146 VS– 2 –IN A 3 18 VOCMC 17 VS+ 16 –IN C 15 +IN C +IN A 4 VS– 5 B A 14 VS– C 12 +OUT C 06655-001 11 VS+ 10 –OUT B 9 +OUT B 8 VS+ 13 –OUT C 7 +OUT A –OUT A 6 –IN R 3 +IN R 4 VS– 5 –OUT R 6 –IN G +IN G VS– (SYNC) VSYNC HSYNC 21 20 19 AD8147/ AD8148 ×2 B A 7 8 9 C 10 11 12 18 SYNC LEVEL 17 VS+ (SYNC) 16 –IN B 15 +IN B 14 VS– 13 –OUT B 06655-002 2 22 +OUT B VS– 23 VS+ 1 24 –OUT G OPD VS+ Figure 1. +OUT G QXGA or 1080p video transmission KVM networking Video over unshielded twisted pair (UTP) Differential signal multiplexing 24 VS+ APPLICATIONS VS+ FUNCTIONAL BLOCK DIAGRAMS Triple high speed fully differential driver 700 MHz, −3 dB, 2 V p-p bandwidth (AD8146/AD8148) 600 MHz, −3 dB, 2 V p-p bandwidth (AD8147) 200 MHz, 0.1 dB, 2 V p-p bandwidth 3000 V/μs slew rate Fixed gain (AD8146/AD8147: G = 2, AD8148: G = 4) Differential or single-ended input to differential output Can be used as differential-to-differential receiver Drives one or two 100 Ω UTP cables Adjustable output common-mode voltage (AD8146) Internal common-mode feedback network Output balance error −50 dB @ 50 MHz On-chip, sync-on common-mode encoding (AD8147/AD8148) Output pull-down feature for line isolation Low power: 57 mA @ 5 V for 3 drivers (AD8146) Wide supply voltage range: +5 V to ±5 V Available in a small 4 mm × 4 mm LFCSP +OUT R FEATURES Figure 2. GENERAL DESCRIPTION The AD8146/AD8147/AD8148 are high speed triple, differential or single-ended input to differential output drivers. The AD8146 and AD8147 have a fixed gain of 2, and the AD8148 has a fixed gain of 4. They are all specifically designed for the highest resolution component video signals but can be used for any type of analog signals or high speed data transmission over either Category 5 UTP cable or differential printed circuit board (PCB) transmission lines. of 700 MHz and fast slew rates. They have an internal commonmode feedback feature that provides output amplitude and phase matching that is balanced to −60 dB at 50 MHz, suppressing even-order harmonics and minimizing radiated electromagnetic interference (EMI). These drivers can be used with the AD8145 triple differentialto-singled-ended receiver, and the AD8117 crosspoint switch to produce a video distribution system capable of supporting UXGA or 1080p signals. The common-mode voltage of each AD8146 output can be set to any level, allowing transmission of signals over the commonmode voltages. The AD8147 and AD8148 encode the vertical and horizontal sync signals on the common-mode voltages of the outputs. All outputs can be independently set to low voltage states to be used with series diodes for line isolation, allowing easy differential multiplexing over the same twisted pair cable. Manufactured on the Analog Devices, Inc. second generation XFCB bipolar process, the drivers have large signal bandwidths The AD8146/AD8147/AD8148 are available in a 24-lead LFCSP and operate over a temperature range of −40°C to +85°C. Rev. 0 Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.461.3113 ©2007 Analog Devices, Inc. All rights reserved. AD8146/AD8147/AD8148 TABLE OF CONTENTS Features .............................................................................................. 1 Applications....................................................................................... 1 Input Common-Mode Voltage Range in Single-Supply Applications ................................................................................ 15 Functional Block Diagrams............................................................. 1 Output Common-Mode Control ............................................. 15 General Description ......................................................................... 1 Sync-On Common-Mode ......................................................... 15 Revision History ............................................................................... 2 Applications..................................................................................... 16 Specifications..................................................................................... 3 Driving RGB Video Signals Over Category-5 UTP Cable.... 16 Absolute Maximum Ratings............................................................ 7 Video Sync-On Common-Mode.............................................. 16 Thermal Resistance ...................................................................... 7 Driving Two UTP Cables With One Driver ........................... 18 ESD Caution.................................................................................. 7 Using the AD8146 as a Receiver............................................... 18 Pin Configuration and Function Descriptions............................. 8 Output Pull-Down (OPD) ........................................................ 19 Typical Performance Characteristics ........................................... 10 Layout and Power Supply Decoupling Considerations......... 19 Theory of Operation ...................................................................... 14 Driving a Capacitive Load......................................................... 19 Definition of Terms.................................................................... 14 Adding Pre-Emphasis to the AD8148 ..................................... 20 Analyzing an Application Circuit............................................. 14 Exposed Paddle (EP).................................................................. 21 Closed-Loop Gain ...................................................................... 14 Outline Dimensions ....................................................................... 22 Calculating the Input Impedance............................................. 15 Ordering Guide .......................................................................... 22 REVISION HISTORY 5/07—Revision 0: Initial Version Rev. 0 | Page 2 of 24 AD8146/AD8147/AD8148 SPECIFICATIONS VS = ±5V, VOCM = 0 V (AD8146); SYNC LEVEL = 0 V (AD8147/AD8148); T = 25°C; RL, dm = 200 Ω, unless otherwise noted. TMIN to TMAX = −40°C to +85°C. Table 1. Parameter DIFFERENTIAL INPUT AC Dynamic Performance −3 dB Small Signal Bandwidth −3 dB Large Signal Bandwidth Bandwidth for 0.1 dB Flatness Slew Rate Isolation Between Amplifiers DIFFERENTIAL INPUT DC Input Common-Mode Voltage Range Input Resistance Input Capacitance DC CMRR DIFFERENTIAL OUTPUT Differential Signal Gain Output Voltage Swing Output Offset Voltage Output Offset Drift Output Balance Error Output Voltage Noise (RTO) Output Short-Circuit Current VOCM DYNAMIC PERFORMANCE (AD8146 ONLY) −3 dB Bandwidth Slew Rate DC Gain VOCM INPUT CHARACTERISTICS (AD8146 ONLY) Input Voltage Range Input Resistance Input Offset Voltage DC CMRR SYNC DYNAMIC PERFORMANCE (AD8147/AD8148 ONLY) Slew Rate Conditions Min 700/600 MHz 200/235 MHz 3000 −86/−80 V/μs dB −5 to +5 1.0 1.13 2 −53/−49/−55 V kΩ kΩ pF dB 1.95 2.00 V/V −3.42 −3/−2.25/−3.42 +3.5 +3.4/+3.4/+3.5 V/V V +19 mV μV/°C dB −41/−44 dB nV/√Hz mA −19 TMIN to TMAX ΔVOUT, cm/ΔVIN, dm, ΔVOUT, dm = 2 V p-p, f = 50 MHz, AD8146 and AD8147/AD8148 DC, AD8146 and AD8148/AD8147 f = 1 MHz, AD8146 and AD8147/AD8148 Short to GND, source/sink ±8 −52/−49 25/42 +87/−67 340 800 0.986 1.000 MHz V/μs V/V +20 −48 V kΩ mV dB ±3 12.5 −20 ΔVOUT, dm/ΔVOCM, ΔVOCM = ±1 V VOUT, cm = −1 V to +1 V; 25% to 75% Rev. 0 | Page 3 of 24 Unit MHz Differential Single-ended input Differential ΔVOUT, dm/ΔVIN, cm, ΔVIN, cm = ±1 V, AD8146/AD8147/AD8148 ΔVOCM = 100 mV p-p VOCM = −1 V to +1 V, 25% to 75% ΔVOCM = ±1 V Max 900/780 VO = 0.2 V p-p, AD8146 and AD8148/AD8147 VO = 2 V p-p, AD8146 and AD8148/AD8147 VO = 2 V p-p, AD8146 and AD8147/AD8148 VO = 2 V p-p, 25% to 75% f = 10 MHz, between amplifiers, AD8146 and AD8147/AD8148 ΔVOUT, dm/ΔVIN, dm; ΔVIN, dm = ±1 V, AD8146 and AD8147 ΔVOUT, dm/ΔVIN, dm; ΔVIN, dm = ±1 V, AD8148 Each single-ended output, AD8146/AD8147/AD8148 Typ 1000 V/μs AD8146/AD8147/AD8148 Parameter HSYNC AND VSYNC INPUTS (AD8147/AD8148 ONLY) Input Low Voltage Input High Voltage SYNC LEVEL INPUT (AD8147/AD8148 ONLY) Setting to 0.5 V Pulse Levels Gain to Red Common-Mode Output Gain to Green Common-Mode Output Gain to Blue Common-Mode Output POWER SUPPLY Operating Range Quiescent Current, Positive Supply Quiescent Current, Negative Supply PSRR OUTPUT PULL-DOWN OPD Input Low Voltage OPD Input High Voltage OPD Input Bias Current OPD Assert Time OPD Deassert Time Output Voltage When OPD Asserted Conditions Min Typ Max 1.5 to 1.7 1.5 to 1.7 V V 0.5 ΔVO, cm/ΔVSYNC LEVEL (AD8147/AD8148) ΔVO, cm/ΔVSYNC LEVEL (AD8147/AD8148) ΔVO, cm/ΔVSYNC LEVEL (AD8147/AD8148) AD8146 AD8147/AD8148 Disabled, AD8146/AD8147 and AD8148 AD8146 AD8147/AD8148 Disabled ΔVOUT, dm/ΔVS; ΔVS = ±1 V (AD8146/AD8147/AD8148) 0.93/0.965 1.91/1.935 −1.08/−1.035 1.08/1.04 2.11/2.05 −0.93/−0.965 +4.5 ±5.5 57 61.5/62.5 6/21.5 −57 −60.5/−62 −37 −66/−52/−55 1.1 2.1 520 1 10 Each output, OPD input @ VS+ Rev. 0 | Page 4 of 24 −4.2 Unit −3.8 V V/V V/V V/V V mA mA mA mA mA mA dB V V μA μs ns V AD8146/AD8147/AD8148 VS = +5 V or ±2.5 V; VOCM = midsupply (AD8146); SYNC LEVEL = 0 V (AD8147/AD8148); T = 25°C; RL, dm = 200 Ω, unless otherwise noted. TMIN to TMAX = −40°C to +85°C. Table 2. Parameter DIFFERENTIAL INPUT AC Dynamic Performance −3 dB Small Signal Bandwidth −3 dB Large Signal Bandwidth Bandwidth for 0.1 dB Flatness DIFFERENTIAL INPUT DC Input Common-Mode Voltage Range Input Resistance Input Capacitance DC CMRR DIFFERENTIAL OUTPUT Differential Signal Gain Output Voltage Swing Output Offset Voltage Output Offset Drift Output Balance Error Output Voltage Noise (RTO) Output Short-Circuit Current VOCM DYNAMIC PERFORMANCE (AD8146 ONLY) −3 dB Bandwidth Slew Rate DC Gain VOCM INPUT CHARACTERISTICS (AD8146 ONLY) Input Voltage Range Input Resistance Input Offset Voltage DC CMRR SYNC DYNAMIC PERFORMANCE (AD8147/AD8148 ONLY) Slew Rate HSYNC AND VSYNC INPUTS (AD8147/AD8148 ONLY) Input Low Voltage Input High Voltage Conditions Min Unit MHz 590/620 MHz 165/200 MHz 0 to 5 1.0 1.13 2 −53/−49/−55 V kΩ kΩ pF dB 1.95 2.013 V/V 3.87 −1.17/−1.23 4.00 +1.24/+1.26 V/V V +17 mV μV/°C dB −41/−44 dB nV/√Hz mA Differential Single-ended input Differential ΔVOUT, dm/ΔVIN, cm; ΔVIN, cm = ±1 V, AD8146/AD8147/AD8148 −17 TMIN to TMAX ΔVOUT, cm/ΔVIN, dm, ΔVOUT, dm = 2 V p-p, f = 50 MHz, AD8146 and AD8147/AD8148 DC, AD8146 and AD8148/AD8147 f = 1 MHz, AD8146, AD8147/AD8148 Short to GND, source/sink ΔVOCM = 100 mV p-p VOCM = −1 V to +1 V, 25% to 75% ΔVOCM = ±1 V, TMIN to TMAX Max 870/680 VO = 0.2 V p-p, AD8146/AD8147 and AD8148 VO = 2 V p-p, AD8147/AD8146 and AD8148 VO = 2 V p-p, AD8146 and AD8147/AD8148 ΔVOUT, dm/ΔVIN, dm; ΔVIN, dm = ±1 V, AD8146 and AD8147 ΔVOUT, dm/ΔVIN, dm; ΔVIN, dm = ±1 V, AD8148 Each single-ended output AD8146 and AD8147/AD8148 Typ ±8 −53/−49 25/42 +63/−48 310 800 0.99 1.00 ±1.2 12.5 MHz V/μs V/V ΔVO, dm/ΔVOCM; ΔVOCM = ±1 V −42 V kΩ mV dB VOUT, cm = −1 V to +1 V; 25% to 75% 800 V/μs 1.3 to 1.5 1.3 to 1.5 V V −20 Rev. 0 | Page 5 of 24 +20 AD8146/AD8147/AD8148 Parameter SYNC LEVEL INPUT (AD8147/AD8148 ONLY) Setting to 0.5 V Pulse Levels Gain to Red Common-Mode Output Gain to Green Common-Mode Output Gain to Blue Common-Mode Output POWER SUPPLY Operating Range Quiescent Current Positive Supply Quiescent Current Negative Supply PSRR OUTPUT PULL-DOWN OPD Input Low Voltage OPD Input High Voltage OPD Input Bias Current OPD Assert Time OPD Deassert Time Output Voltage When OPD Asserted Conditions Min ΔVO, cm/ΔVSYNC LEVEL, AD8147/AD8148 ΔVO, cm/ΔVSYNC LEVEL, AD8147/AD8148 ΔVO, cm/ΔVSYNC LEVEL, AD8147/AD8148 Typ Max Unit 0.88/0.925 1.83/1.85 −1.07/−1 1.07/1.00 2.05/2.00 −0.88/−0.925 V V/V V/V V/V +4.5 ±5.5 50 55.5/52 4/12 0.5 AD8146 AD8147/AD8148 Disabled, AD8146/AD8147 and AD8148 AD8146 AD8147/AD8148 Disabled, AD8146/AD8147/AD8148 ΔVOUT, dm/ΔVS; ΔVS = ±1 V, AD8146/AD8147/AD8148 −50 −55/−51 −14/−18.2/−15 −70/−54/−60 1.0 2.0 160 600 10 Each output, OPD input @ VS+ Rev. 0 | Page 6 of 24 −1.71 −1.6 V mA mA mA mA mA mA dB V V μA ns ns V AD8146/AD8147/AD8148 ABSOLUTE MAXIMUM RATINGS Table 3. Rating 11 V ±VS See Figure 3 ±VS −65°C to +125°C −40°C to +85°C 300°C 150°C Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only and functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. THERMAL RESISTANCE Airflow reduces θJA. In addition, more metal directly in contact with the package leads from metal traces, through holes, ground, and power planes reduces the θJA. The exposed paddle on the underside of the package must be soldered to a pad on the PCB surface that is thermally connected to a ground plane to achieve the specified θJA. Figure 3 shows the maximum safe power dissipation in the package vs. the ambient temperature for the 24-lead LFCSP (57°C/W) package on a JEDEC standard 4-layer board with the underside paddle soldered to a pad that is thermally connected to a ground plane. θJA values are approximations. 3.5 MAXIMUM POWER DISSIPATION (W) θJA is specified for the worst-case conditions, that is, θJA is specified for the device soldered in a circuit board in still air. Table 4. Thermal Resistance with the Underside Pad Connected to the Plane Package Type/PCB Type 24-Lead LFCSP/4-Layer θJA 57 Unit °C/W Maximum Power Dissipation The maximum safe power dissipation in the AD8146/ AD8147/AD8148 package is limited by the associated rise in junction temperature (TJ) on the die. At approximately 150°C, which is the glass transition temperature, the plastic changes its properties. Even temporarily exceeding this temperature limit can change the stresses that the package exerts on the die, permanently shifting the parametric performance of the AD8146/AD8147/AD8148. Exceeding a junction temperature of 175°C for an extended time can result in changes in the silicon devices, potentially causing failure. 3.0 2.5 2.0 1.5 1.0 0.5 0 –40 06655-021 Parameter Supply Voltage All VOCM Power Dissipation Input Common-Mode Voltage Storage Temperature Range Operating Temperature Range Lead Temperature (Soldering, 10 sec) Junction Temperature and common-mode currents flowing to the loads, as well as currents flowing through the internal differential and commonmode feedback loops. The internal resistor tap used in the common-mode feedback loop places a 4 kΩ differential load on the output. Differential feedback, network resistor values are given in the Theory of Operation section and Applications section. RMS output voltages should be considered when dealing with ac signals. –20 0 20 40 60 80 AMBIENT TEMPERATURE (°C) Figure 3. Maximum Power Dissipation vs. Temperature for a 4-Layer Board ESD CAUTION The power dissipated in the package (PD) is the sum of the quiescent power dissipation and the power dissipated in the package due to the load drive for all outputs. The quiescent power is the voltage between the supply pins (VS) times the quiescent current (IS). The load current consists of differential Rev. 0 | Page 7 of 24 AD8146/AD8147/AD8148 24 23 22 21 20 19 VS+ –IN B +IN B VS– VOCMA VOCMB PIN CONFIGURATION AND FUNCTION DESCRIPTIONS PIN 1 INDICATOR AD8146 TOP VIEW (Not to Scale) 18 17 16 15 14 13 VOCMC VS+ –IN C +IN C VS– –OUT C 06655-004 1 2 3 4 5 6 +OUT A 7 VS+ 8 +OUT B 9 –OUT B 10 VS+ 11 +OUT C 12 OPD VS– –IN A +IN A VS– –OUT A Figure 4. AD8146 Pin Configuration Table 5. AD8146 Pin Function Descriptions Pin No. 1 2, 5, 14, 21 3 4 6 7 8, 11, 17, 24 9 10 12 13 15 16 18 19 20 22 23 Mnemonic OPD VS− −IN A +IN A −OUT A +OUT A VS+ +OUT B −OUT B +OUT C −OUT C +IN C −IN C VOCMC VOCMB VOCMA +IN B −IN B Description Output Pull-Down. Negative Power Supply Voltage. Inverting Input, Amplifier A. Noninverting Input, Amplifier A. Negative Output, Amplifier A. Positive Output, Amplifier A. Positive Power Supply Voltage. Positive Output, Amplifier B. Negative Output, Amplifier B. Positive Output, Amplifier C. Negative Output, Amplifier C. Noninverting Input, Amplifier C. Inverting Input, Amplifier C. The voltage applied to this pin controls output common-mode voltage, Amplifier C. The voltage applied to this pin controls output common-mode voltage, Amplifier B. The voltage applied to this pin controls output common-mode voltage, Amplifier A. Noninverting Input, Amplifier B. Inverting Input, Amplifier B. Rev. 0 | Page 8 of 24 24 23 22 21 20 19 VS+ –IN G +IN G VS– (SYNC) VSYNC HSYNC AD8146/AD8147/AD8148 PIN 1 INDICATOR AD8147/ AD8148 TOP VIEW (Not to Scale) 18 17 16 15 14 13 SYNC LEVEL VS+ (SYNC) –IN B +IN B VS– –OUT B 06655-005 1 2 3 4 5 6 +OUT R 7 VS+ 8 +OUT G 9 –OUT G 10 VS+ 11 +OUT B 12 OPD VS– –IN R +IN R VS– –OUT R Figure 5. AD8147/AD8148 Pin Configuration Table 6. AD8147/AD8148 Pin Function Descriptions Pin No. 1 2, 5, 14 3 4 6 7 8, 11, 24 9 10 12 13 15 16 17 18 Mnemonic OPD VS− −IN R +IN R −OUT R +OUT R VS+ +OUT G −OUT G +OUT B −OUT B +IN B −IN B VS+ (SYNC) SYNC LEVEL 19 20 21 22 23 Exposed Paddle HSYNC VSYNC VS− (SYNC) +IN G −IN G GND Description Output Pull-Down. Negative Power Supply Voltage. Inverting Input, Red Amplifier. Noninverting Input, Red Amplifier. Negative Output, Red Amplifier. Positive Output, Red Amplifier. Positive Power Supply Voltage. Positive Output, Green Amplifier. Negative Output, Green Amplifier. Positive Output, Blue Amplifier. Negative Output, Blue Amplifier. Noninverting Input, Blue Amplifier. Inverting Input, Blue Amplifier. Positive Power Supply Voltage for Sync. The voltage applied to this pin controls the amplitude of the sync pulses that are applied to the common-mode voltages. Horizontal Sync Pulse Input. Vertical Sync Pulse Input. Negative Power Supply Voltage for Sync. Noninverting Input, Green Amplifier. Inverting Input, Green Amplifier. Signal Ground Reference. Rev. 0 | Page 9 of 24 AD8146/AD8147/AD8148 TYPICAL PERFORMANCE CHARACTERISTICS VS = ±5V; VOCM = 0 V (AD8146); SYNC LEVEL = 0 V (AD8147/AD8148); T = 25°C; RL, dm = 200 Ω; CL, dm = 0 pF, unless otherwise noted. TMIN to TMAX = −40°C to +85°C. 15 VOUT, dm = 2V p-p 7 13 6 12 GAIN (dB) 5 AD8146 (±2.5V) AD8146 (±5.0V) AD8147 (±2.5V) AD8147 (±5.0V) 4 3 7 0 6 1000 FREQUENCY (MHz) Figure 6. AD8146/AD8147 Large Signal Frequency Response for Various Supplies 5 10 Figure 9. AD8148 Large Signal Frequency Response for Various Supplies 15 VOUT, dm = 0.2V p-p VOUT, dm = 0.2V p-p 14 13 6 12 5 11 GAIN (dB) 7 AD8146 (±2.5V) AD8146 (±5.0V) AD8147 (±2.5V) AD8147 (±5.0V) 4 3 10 9 ±2.5V 2 8 1 7 0 6 1000 100 FREQUENCY (MHz) 5 10 06655-011 –1 10 ±5.0V Figure 7. AD8146/AD8147 Small Signal Frequency Response for Various Supplies 1000 100 FREQUENCY (MHz) 06655-014 8 Figure 10. AD8148 Small Signal Frequency Response for Various Supplies 12.5 6.5 VOUT, dm = 2V p-p 6.4 VOUT, dm = 2V p-p 12.4 12.3 6.2 12.2 6.1 12.1 GAIN (dB) 6.3 6.0 5.9 AD8146 (±2.5V) AD8146 (±5.0V) AD8147 (±2.5V) AD8147 (±5.0V) 5.8 5.7 ±2.5V 12.0 11.9 ±5.0V 11.8 11.7 11.6 1 10 100 FREQUENCY (MHz) 1000 06655-012 5.6 Figure 8. AD8146/AD8147 Large Signal 0.1 dB Flatness for Various Supplies Rev. 0 | Page 10 of 24 11.5 1 10 100 FREQUENCY (MHz) 1000 06655-015 GAIN (dB) 1000 100 FREQUENCY (MHz) 9 GAIN (dB) 9 8 100 ±5.0V 10 1 –1 10 ±2.5V 11 2 5.5 VOUT, dm = 2V p-p 14 06655-010 GAIN (dB) 8 06655-013 9 Figure 11. AD8148 Large Signal 0.1 dB Flatness for Various Supplies AD8146/AD8147/AD8148 1.5 1.5 VOUT, dm = 2V p-p VS = ±2.5V VOUT, dm = 2V p-p VS = ±2.5V 1.0 1.0 VS = ±5.0V 0 –0.5 –0.5 –1.0 –1.0 2 4 6 8 10 12 14 16 18 20 TIME (ns) –1.5 06655-016 0 Figure 12. AD8146/AD8147 Large Signal Transient Response for Various Supplies 6 8 10 12 14 16 18 20 Figure 15. AD8148 Large Signal Transient Response for Various Supplies 150 VOUT, dm = 0.2V p-p VS = ±2.5V 100 100 0 –50 0 –50 –100 2 4 6 8 10 12 14 16 18 TIME (ns) 20 –150 06655-017 0 Figure 13. AD8146/AD8147 Small Signal Transient Response for Various Supplies 2 4 6 8 10 12 14 16 18 TIME (ns) 20 Figure 16. AD8148 Small Signal Transient Response for Various Supplies –20 –20 ΔVOUT, cm/ΔVOUT, dm ΔVOUT, dm = 2V p-p ΔVOUT, dm/ΔVIN, cm ΔVIN, cm = 2V p-p AD8146 –35 –40 –45 –50 AD8147 –55 AD8148 06655-024 –60 –65 1 10 100 –30 AD8148 –40 AD8147 –50 –60 AD8146 –70 –80 1000 06655-027 –30 COMMON-MODE REJECTION (dB) –25 –70 0 06655-020 –100 VS = ±5.0V 50 VOLTAGE (mV) VS = ±5.0V 50 VOLTAGE (mV) 4 VOUT, dm = 0.2V p-p VS = ±2.5V OUTPUT BALANCE ERROR (dB) 2 TIME (ns) 150 –150 0 06655-019 0 –1.5 VS = ±5.0V 0.5 VOLTAGE (V) VOLTAGE (V) 0.5 1 10 100 FREQUENCY (MHz) FREQUENCY (MHz) Figure 14. Output Balance vs. Frequency Figure 17. CMRR vs. Frequency Rev. 0 | Page 11 of 24 1000 AD8146/AD8147/AD8148 –20 –20 ΔVOUT, dm/ΔVS+ ΔVS = 2V p-p –40 –50 AD8148 –60 AD8147 –80 AD8146 –100 0.1 1 10 –60 –70 AD8146 –80 –90 –110 0.1 1000 100 1 10 FREQUENCY (MHz) 1000 100 FREQUENCY (MHz) Figure 18. Positive Power Supply Rejection vs. Frequency 1000 AD8148 AD8147 –50 –100 06655-028 –90 –40 06655-051 –70 ΔVOUT, dm/ΔVS– ΔVS = 2V p-p –30 POWER SUPPLY REJECTION (dB) POWER SUPPLY REJECTION (dB) –30 Figure 21. Negative Power Supply Rejection vs. Frequency –20 VS = ±5V ΔVOUT, dmB/ΔVIN, dmA ΔVIN, dmA = 1V p-p –30 –40 AD8148 ISOLATION (dB) NOISE (nV/ Hz) –50 AD8146 100 AD8147 –60 –70 AD8146 –80 –90 AD8148 10 0.01 06655-029 AD8147 0.1 1 10 100 1000 10000 06655-052 –100 –110 –120 0.1 100000 1 10 FREQUENCY (kHz) Figure 19. Output-Referred Voltage Noise vs. Frequency Figure 22. Amplifier-to-Amplifier Isolation vs. Frequency 10 10 INPUT × 2 (VS = ±5.0V) OUTPUT (VS = ±5.0V) 6 VOLTAGE (V) –2 2 0 –4 –6 –6 –8 100 200 300 400 500 600 700 800 900 OUTPUT (VS = ±2.5V) –2 –4 06655-030 VOLTAGE (V) OUTPUT (VS = ±2.5V) 0 INPUT × 4 (VS = ±2.5V) 4 2 0 OUTPUT (VS = ±5.0V) 6 INPUT × 2 (VS = ±2.5V) 4 INPUT × 4 (VS = ±5.0V) 8 06655-033 8 –10 1000 100 FREQUENCY (MHz) –8 –10 1000 0 100 200 300 400 500 600 700 800 TIME (ns) TIME (ns) Figure 20. AD8146/AD8147 Output Overdrive Recovery Figure 23. AD8148 Output Overdrive Recovery Rev. 0 | Page 12 of 24 900 1000 AD8146/AD8147/AD8148 59 62 IS+ (±5.0V) 60 SUPPLY CURRENT (mA) 55 53 RL, dm = OPEN CIRCUIT 51 IS+ (±2.5V) 49 45 –60 –40 –20 0 20 40 60 80 100 RL, dm = OPEN CIRCUIT 56 54 52 IS+ (±2.5V) 50 06655-054 47 IS+ (±5.0V) 58 48 –60 120 06655-056 SUPPLY CURRENT (mA) 57 –40 –20 TEMPERATURE (°C) 40 60 80 100 120 Figure 26. AD8147/AD8148 Supply Current vs. Temperature –35 1.5 ΔVOUT, dm/ΔVOCM ΔVOCM = 2V p-p ±VS = 5.0V –40 ±VS = 2.5V 1.0 0.5 AD8146 –50 0 VOUT, cm = 2V p-p –55 –0.5 –60 –1.0 1 10 100 –1.5 1000 FREQUENCY (MHz) Figure 25. VOCM Common-Mode Rejection Ratio 06655-037 VOLTAGE (V) –45 06655-061 VOCM CMRR (dB) 20 TEMPERATURE (°C) Figure 24. AD8146 Supply Current vs. Temperature –65 0 0 5 10 15 20 25 30 35 40 TIME (ns) Figure 27. AD8146 Large Signal VOCM Transient Response for Various Supplies Rev. 0 | Page 13 of 24 AD8146/AD8147/AD8148 THEORY OF OPERATION Each differential driver differs from a conventional op amp in that it has two outputs whose voltages move in opposite directions. Like an op amp, it relies on high open-loop gain and negative feedback to force these outputs to the desired voltages. The drivers make it easy to perform single-ended-to-differential conversion, common-mode level shifting, and amplification of differential signals. Common-Mode Voltage Previous differential drivers, both discrete and integrated designs, were based on using two independent amplifiers and two independent feedback loops, one to control each of the outputs. When these circuits are driven from a single-ended source, the resulting outputs are typically not well balanced. Achieving a balanced output has typically required exceptional matching of the amplifiers and feedback networks. Output balance is a measure of how well the differential output signals are matched in amplitude and how close they are to exactly 180° apart in phase. Balance is most easily determined by placing a well-matched resistor divider between the differential output voltage nodes and comparing the magnitude of the signal at the divider’s midpoint with the magnitude of the differential signal. By this definition, output balance error is the magnitude of the change in output common-mode voltage divided by the magnitude of the change in output differential mode voltage in response to a differential input signal. DC common-mode level shifting has also been difficult with previous differential drivers. Level shifting has required the use of a third amplifier and feedback loop to control the output common-mode level. Sometimes, the third amplifier was also used to attempt to correct an inherently unbalanced circuit. Excellent performance over a wide frequency range has proven difficult with this approach. Each of the drivers uses two feedback loops to separately control the differential and common-mode output voltages. The differential feedback, set by the internal resistors, controls only the differential output voltage. The internal commonmode feedback loop controls only the common-mode output voltage. This architecture makes it easy to transmit signals over the common-mode voltage channels by simply applying the signal voltages to the VOCM inputs. The output common-mode voltage is forced, by internal common-mode feedback, to equal the voltage applied to the VOCM input, without affecting the differential output voltage. The driver architecture results in outputs that are highly balanced over a wide frequency range without requiring external components or adjustments. The common-mode feedback loop forces the signal component of the output common-mode voltage to be zeroed. The result is nearly perfectly balanced differential outputs of identical amplitude that are exactly 180° apart in phase. Common-mode voltage refers to the average of two node voltages with respect to a common reference. The output common-mode voltage is defined as VOUT, cm = (VOP + VON)/2 Output Balance Output Balance Error = ΔVOUT , cm ΔVOUT , dm ANALYZING AN APPLICATION CIRCUIT The drivers use high open-loop gain and negative feedback to force their differential and common-mode output voltages to minimize the differential and common-mode input error voltages. The differential input error voltage is defined as the voltage between the differential inputs labeled VAP and VAN in Figure 28. For most purposes, this voltage can be assumed to be zero. Similarly, the difference between the actual output commonmode voltage and the voltage applied to VOCM can also be assumed to be zero. Starting from these two assumptions, any application circuit can be analyzed. CLOSED-LOOP GAIN The differential mode gain of the circuit in Figure 28 can be described by VOUT, dm VIN, dm Differential Voltage Differential voltage refers to the difference between two node voltages that are balanced with respect to each other. For example, in Figure 28 the output differential voltage (or equivalently output differential mode voltage) is defined as RF RG where: RF is 1.0 kΩ and RG is 500 Ω nominally for the AD8146 and AD8147. RF is 2.0 kΩ and RG is 500 Ω nominally for the AD8148. VOUT, dm = (VOP − VON) RF + VIP VIN, dm VOCM – VIN RG VAP RL, dm RG VAN RF VON VOUT, dm VOP 06655-006 DEFINITION OF TERMS = Figure 28. Internal Architecture and Signal Name Definitions Rev. 0 | Page 14 of 24 AD8146/AD8147/AD8148 CALCULATING THE INPUT IMPEDANCE OUTPUT COMMON-MODE CONTROL The effective input impedance of a circuit such as that in Figure 28 at VIP and VIN depends on whether the amplifier is being driven by a single-ended or differential signal source. For balanced differential input signals, the differential input impedance, RIN, dm, between the inputs VIP and VIN for all devices is The AD8146 allows the user to control each of the three common-mode output levels independently through the three VOCM input pins. The VOCM pins pass a signal to the commonmode output level of each of their respective amplifiers with 330 MHz of small signal bandwidth and an internally fixed gain of 1. In this way, additional control and communication signals can be embedded on the common-mode levels as users see fit. RIN, dm = 2 × RG In the case of a single-ended input signal (for example, if VIN is grounded and the input signal is applied to VIP), the input impedance becomes R IN, dm ⎞ ⎛ ⎟ ⎜ R G ⎟ =⎜ RF ⎟ ⎜ ⎜ 1 − 2 × (R + R ) ⎟ F ⎠ G ⎝ SYNC-ON COMMON-MODE The single-ended input impedance of the AD8146 and the AD8147 is therefore 750 Ω, and the single-ended input impedance of the AD8148 is 833 Ω. The input impedance of the circuit is effectively higher than it would be for a conventional op amp connected as an inverter because a fraction of the differential output voltage appears at the inputs as a common-mode signal, partially bootstrapping the voltage across the input resistor RG. INPUT COMMON-MODE VOLTAGE RANGE IN SINGLE-SUPPLY APPLICATIONS The driver inputs are designed to facilitate level-shifting of ground-referenced input signals on a single power supply. For a single-ended input, this implies, for example, that the voltage at VIN in Figure 28 would be 0 V when the negative power supply voltage of the amplifier is also set to 0 V. It is important to ensure that the common-mode voltage at the amplifier inputs, VAP and VAN, stays within its specified range. Because voltages VAP and VAN are driven to be essentially equal by negative feedback, the input common-mode voltage of the amplifier can be expressed as a single term, VACM. VACM can be calculated as VACM = VOCM + 2VICM 3 where VICM is the common-mode voltage of the input signal, that is, VICM = (VIP + VIN)/2. With no external circuitry, the level at the VOCM input of each amplifier defaults to approximately midsupply. An internal resistive divider with an impedance of approximately 12.5 kΩ sets this level. To limit common-mode noise in dc commonmode applications, external bypass capacitors should be connected from each of the VOCM input pins to ground. The AD8147 and AD8148 are specifically targeted at driving RGB video signals over UTP cable using a sync-on commonmode technique. The common-mode outputs of each of the R, G, and B differential outputs are set using circuitry contained within the device. This circuitry embeds the horizontal and vertical sync pulses on the three common-mode outputs in a way that also results in low radiated energy. For a more detailed description of the sync scheme, see the Applications section. The sync-on common-mode circuit generates a current based on the SYNC LEVEL input pin (Pin 18). With the SYNC LEVEL input tied to GND, the common-mode output of all drivers is set at (VS+ + VS−)/2. Using a resistor divider, a voltage can be applied between GND and SYNC LEVEL that determines the maximum deviation of the common-mode outputs from their midsupply level. If, for instance, SYNC LEVEL = 0.5 V and the supply voltage is 5 V, the common-mode outputs fall within an envelope of 2.5 V ± 0.5 V. The state of each VOUT, cm output based on the HSYNC and VSYNC inputs is determined by the equations defined in the Applications section. In most cases, the sync-on common-mode circuit can be used by directly applying the HSYNC and VSYNC signals to their respective AD8147 or AD8148 inputs. The logic thresholds of the HSYNC and VSYNC inputs are set to nominally 1.4 V with respect to GND, and the exposed paddles of the AD8147 and AD8148 are used as the GND references for the incoming sync pulses. When ±2.5 V supplies are used, however, external protection is required to limit the positive excursion to less than 2.5 V. For more details, see the Applications section. The input paths from the HSYNC and VSYNC inputs to the switches in the current mode level-shifting circuit are well matched to eliminate false switching transients, maximizing commonmode balance and minimizing radiated energy. Rev. 0 | Page 15 of 24 AD8146/AD8147/AD8148 APPLICATIONS DRIVING RGB VIDEO SIGNALS OVER CATEGORY-5 UTP CABLE The foremost application of the drivers is the transmission of RGB video signals over UTP cable in KVM networks. The excellent balance of the differential outputs ensures low radiated energy from each of the twisted pairs. Single-ended video signals are easily converted to differential signals for transmission over the cable, and the internally fixed gain of 2 or 4 automatically compensates for the losses incurred by the source and load terminations. The common topologies used in KVM networks, such as daisy-chained, star, and point-to-point, are supported by the drivers. Figure 29 shows the AD8146 in a triple singleended-to-differential application when driven from a 75 Ω source, which is typical of how RGB video is driven over an UTP cable. +5V 0.1µF ON ALL VS+ PINS VS+ 1kΩ 75Ω AD8146 500Ω 82.5Ω VIDEO SOURCE A +2.5V 500Ω 39.2Ω 49.9Ω VOCM A 49.9Ω – OUT A + 1kΩ 1kΩ 49.9Ω 500Ω 75Ω 82.5Ω VIDEO SOURCE B +2.5V 500Ω 39.2Ω VOCM B 49.9Ω – OUT B + 1kΩ 1kΩ 500Ω 82.5Ω VIDEO SOURCE C OUTPUT PULLDOWN +2.5V 500Ω 39.2Ω 49.9Ω VOCM C 49.9Ω In computer video applications, the horizontal and vertical sync signals are often separate from the video information signals. For example, in typical computer monitor applications, the red, green, and blue (RGB) color signals are transmitted over separate cables, as are the vertical and horizontal sync signals. When transmitting these types of video signals over long distances on UTP cable, it is desirable to reduce the required number of physical channels. One way to do this is to encode the vertical and horizontal sync signals as weighted sums and differences of the output common-mode signals. The RGB color signals are each transmitted differentially over separate physical channels. The fact that the differential and common-mode signals are orthogonal allows the RGB color and sync signals to be separated at the channel’s receiver. Cat-5 cable contains four balanced twisted-pair physical channels that can support both differential and common-mode signals. Transmitting typical computer monitor video over this cable can be accomplished by using three of the twisted pairs for the RGB and sync signals and one wire of the fourth pair as a return path for the Schottky diode bias currents. Each color is transmitted differentially, one on each of the three pairs, and the encoded sync signals are transmitted among the common-mode signals of each of the three pairs. To minimize EMI from the sync signals, the common-mode signals on each of the three pairs produced by the sync encoding scheme induce electric and magnetic fields that for the most part cancel each other. A conceptual block diagram of the sync encoding scheme is presented in Figure 30. Because the AD8147/AD8148 have the sync encoding scheme implemented internally, the user simply applies the horizontal and vertical sync signals to the appropriate inputs. (See the Specifications tables for the high and low levels of the horizontal and vertical sync pulse voltages). – OUT C + 1kΩ OPD VS– 06655-007 75Ω VIDEO SYNC-ON COMMON-MODE Figure 29. AD8146 in Single-Ended-to-Differential Application Rev. 0 | Page 16 of 24 AD8146/AD8147/AD8148 3.1 1kΩ AD8147/AD8148 3.0 500Ω 500Ω –IN R 2.9 –OUT R 2.7 +OUT R 1kΩ VSYNC G 2.8 VOCM R VOLTS +IN R 2.6 2.5 R 2.4 2.3 HSYNC 2.1 500Ω +IN G ×2 –IN G 2.2 1kΩ SYNC LEVEL 500Ω B 2.0 –OUT G 5.0 VOCM G +OUT G 4.5 1kΩ 4.0 1kΩ 3.5 –IN B 500Ω 500Ω VOLTS 3.0 +IN B –OUT B VOCM B 2.5 HSYNC 2.0 +OUT B VSYNC 1.5 1kΩ 1.0 06655-009 OPD 0.5 VOCM WEIGHTING EQUATIONS: 0 0.98 06655-008 RED VOCM = K(VSYNC – HSYNC ) + VMIDSUPPLY 2 GREEN VOCM = K(–2V SYNC ) + VMIDSUPPLY 2 BLUE VOCM = K(VSYNC + HSYNC ) + VMIDSUPPLY 2 Figure 30. AD8147/AD8148 Sync-On Common-Mode Encoding Scheme 0.99 1.00 1.01 1.02 1.03 TIME (µs) 1.04 1.05 1.06 1.07 Figure 31. AD8147 Sync-On Common-Mode Signals in Single 5 V Application The transmitted common-mode sync signal magnitudes are scaled by applying a dc voltage to the SYNC LEVEL input, referenced to GND. The difference between the voltage applied to the SYNC LEVEL input and GND sets the peak deviation of the encoded sync signals about the midsupply, common-mode voltage. For example, with the SYNC LEVEL input set at 500 mV, the deviation of the encoded sync pulses about the nominal midsupply, common-mode voltage is typically ±500 mV. The equations in Figure 30 describe how the VSYNC and HSYNC signals are encoded on each color’s midsupply common-mode signal. In these equations, the weights of the VSYNC and HSYNC signals are ±1 (+1 for high and −1 for low), and the constant K is equal to the peak deviation of the encoded sync signals. Figure 31 shows how the sync signals appear on each commonmode voltage in a single 5 V supply application when the voltage applied to the SYNC LEVEL input is 500 mV, which is the typical setting for most applications. Rev. 0 | Page 17 of 24 AD8146/AD8147/AD8148 Sync pulse amplitudes applied to the AD8147 and AD8148 must be less than or equal to the positive supply voltage. In low positive supply applications, such as those that use ±2.5 V supplies, external limiting may be required because many logic families produce amplitudes up to 5 V. Figure 32 illustrates how to use a monolithic triple diode to limit a sync pulse with 5 V amplitude to an amplitude of approximately 2 V. Driver bandwidth is affected to a small degree when driving the 100 Ω load presented by the two cables, as compared with driving a typical 200 Ω load. Figure 34 illustrates the AD8146/ AD8147/AD8148 bandwidths when driving a 100 Ω load. 15 12 AD8148 INCOMING SYNC PULSE +5V 2 3 6 5 0V 06655-036 HN2D02FUTW1T1 GAIN (dB) 1 0V 9 LIMITED SYNC PULSE +2V 301Ω 4 6 AD8147 3 AD8146 Figure 32. Limiting Sync Pulse Amplitude in Low Positive Supply Applications 0 DRIVING TWO UTP CABLES WITH ONE DRIVER Some applications require driving two UTP cables with a single driver. Each individual driver of the AD8146/AD8147/AD8148 is capable of driving two doubly terminated cables, which places a differential load of 100 Ω across the outputs of the driver. Figure 33 illustrates how to drive two cables. 100Ω UTP 100Ω 49.9Ω VOCM 100Ω UTP Figure 33. Driving Two UTP Cables With One Driver 100Ω 06655-034 49.9Ω 49.9Ω 1 10 100 1000 FREQUENCY (MHz) Figure 34. Large Signal Frequency Response Driving 100 Ω Loads USING THE AD8146 AS A RECEIVER While the AD8146 excels as a differential driver, it can also be used as a differential-to-differential receiver applied as an input buffer that protects a more sophisticated device, such as a differential crosspoint switch. See Figure 35 for an illustration of this type of application. 49.9Ω AD8146/AD8147/AD8148 –3 06655-044 RL, dm = 100Ω VOUT = 2V p-p Because the AD8146 VOCM input pins are uncommitted, any incoming common-mode signal, such as encoded sync pulses, can be reproduced at the AD8146 outputs by stripping it from the received signal and applying it directly to the VOCM pin. The two series 54.9 Ω resistors form a differential termination resistor of 109.8 Ω, which when loaded with the 1 kΩ differential input resistance of the AD8146, provides an overall termination of approximately 100 Ω. The received common-mode voltages are available at the center taps between the two resistors. Rev. 0 | Page 18 of 24 AD8146/AD8147/AD8148 VS+ = +2.5V AD8146 54.9Ω RED 100Ω CHANNEL UTP VPOS = +2.5V CROSSPOINT SWITCH 1kΩ 10Ω 500Ω 500Ω VOCM 10Ω 54.9Ω INPUT I, NEGATIVE PHASE INPUT I, POSITIVE PHASE 1kΩ 1kΩ 54.9Ω GREEN 100Ω CHANNEL UTP 10Ω 500Ω 500Ω VOCM 10Ω 54.9Ω INPUT J, NEGATIVE PHASE INPUT J, POSITIVE PHASE 1kΩ 1kΩ 54.9Ω BLUE 100Ω CHANNEL UTP 10Ω 500Ω 500Ω VOCM 10Ω 54.9Ω INPUT K, NEGATIVE PHASE INPUT K, POSITIVE PHASE VS– = –2.5V VNEG = –2.5V 06655-035 1kΩ Figure 35. Using the AD8146 as a Differential Receiver Terminations are not required between the AD8146 and the switch if the interconnection lengths are kept short (less than two inches). The 10 Ω series resistors buffer the input capacitance of the switch (typically 2 pF) and produce a lowpass rolloff that is down by only 0.025 dB at 600 MHz. OUTPUT PULL-DOWN (OPD) The output pull-down feature, when used in conjunction with series Schottky diodes, offers a convenient means to multiplex a number of driver outputs together to form a video network. The OPD pin is a binary input that controls the state of the outputs. Its binary input level is referenced to GND (see the Specifications section for the logic levels). When the OPD input is driven to its low state, the output is enabled and operates in normal fashion. In this state, the VOCM input can be used to provide a positive bias on the series diodes, allowing the drivers to transmit signals over the network. When the OPD input is driven to its high state, the outputs of the drivers are forced to a low voltage, irrespective of the level on the VOCM input, reverse-biasing the series diodes and thus presenting high impedance to the network. This feature allows a three-state output to be realized that maintains its high impedance state even when the drivers are not powered. LAYOUT AND POWER SUPPLY DECOUPLING CONSIDERATIONS Standard high speed PCB layout practices should be adhered to when designing with the drivers. A solid ground plane is required and good wideband power supply decoupling networks should be placed as close as possible to the supply pins. Small surface-mount ceramic capacitors are recommended for these networks, and tantalum capacitors are recommended for bulk supply decoupling. Source termination resistors on the differential outputs must be placed as close as possible to the output pins to minimize load capacitance due to the PCB traces. DRIVING A CAPACITIVE LOAD A purely capacitive load can react with the output impedance of any amplifier to produce an undesirable phase shift, which reduces phase margin and results in high frequency ringing in the pulse response. The best way to minimize this effect is to place a small resistor in series with each of the outputs of the amplifier to buffer the load capacitance. Most applications include 49.9 Ω source termination resistors, which effectively buffer any stray load capacitance. It is recommended that the output pull-down feature only be used in conjunction with series diodes in such a way as to ensure that the diodes are reverse-biased when the output pulldown feature is asserted, because some loading conditions can prevent the output voltage from being pulled all the way down. Rev. 0 | Page 19 of 24 AD8146/AD8147/AD8148 Under no circumstances should capacitance be intentionally added to an output to introduce frequency domain peaking. Figure 36 and Figure 37 illustrate how adding just 5 pF of excessive load capacitance influences time and frequency domain responses. ADDING PRE-EMPHASIS TO THE AD8148 UTP cables exhibit loss characteristics that are low pass in nature and are exponential functions of the square root of the frequency. Over wideband video bandwidths, the losses are predominantly due to the skin effect, which causes the resistance of the cable to increase with frequency. Even though the loss characteristics are nonlinear, suitable linear networks can be designed to approximately compensate for the losses. 2.0 1.5 VS = ±5V RL, dm = 200Ω VOUT, dm = 2V p-p CL = 5pF VOLTAGE (V) 1.0 0.5 CL = 0pF 0 –0.5 –1.0 –2.0 0 2 4 6 8 10 12 14 16 18 20 TIME (ns) 06655-031 –1.5 Figure 36. Large Signal Transient Responses at Various Capacitive Loads 12 11 10 VS = ±5V RL, dm = 200Ω VOUT, dm = 2V p-p CL = 5pF GAIN (dB) 9 8 7 Because the AD8148 has an internally set, closed-loop gain of 4 (12 dB), it is possible to reduce the gain at low frequencies using external frequency selective components, then use these components to provide increasing gain with increasing frequency, back to a value close to 12 dB. These components, along with the AD8148, form the pre-emphasis network. When properly designed, the combined frequency response of the pre-emphasis network and cable is approximately flat with a gain of 2 (6 dB). Figure 38 illustrates how to construct a pre-emphasis network using the AD8148 that compensates for 30 meters of UTP cable. The network in the lower leg is required to match the transfer function of the two feedback loops. 6 CL = 0pF 5 Placing the compensation network at the transmitting end of the cable is referred to as pre-emphasis, because the higher frequencies are emphasized, or boosted, before they are sent, to compensate for the low-pass response of the cable. Because the higher frequencies experience more loss than the lower frequencies as they pass through the cable, the high and low frequencies arrive at approximately the same level and at the end of the cable when a properly designed pre-emphasis network is used at the transmitter. The ideal cascaded frequency response of the preemphasis network and the cable is therefore nominally flat. 4 2 10 100 FREQUENCY (MHz) 1000 06655-032 3 Figure 37. Large Signal Frequency Responses at Various Capacitive Loads While high frequency peaking is desirable in some cable equalization applications, it should be implemented using methods that do not compromise the stability of the driver and that do not depend on amplifier parasitic elements. The parasitic elements are affected by process variations and cannot be depended upon for circuit designs. The amplifier may break into oscillation when excess load capacitance is intentionally added. For more information on this topic, see the Adding PreEmphasis to the AD8148 section for a description on how to introduce a controlled amount of pre-emphasis for 30 meters of UTP using the AD8148. At dc, the capacitors are open circuits, and the network has a gain of approximately 6.5 dB. (The additional 0.5 dB is added to compensate for the cable flat loss that occurs at frequencies below where the skin effect begins to take effect.) Moving up in frequency, the 30 pF capacitor begins to take effect and introduces a zero into the frequency response, causing the gain to increase with frequency. Continuing to move up in frequency, the 30 pF capacitor becomes an effective short, and the 487 Ω resistor goes in parallel with the 442 Ω resistor, forming a pole in the response. Continuing to move up in frequency, the 15 pF capacitor takes effect, introducing another zero, and causes the gain to further increase with frequency until it becomes an effective short, and the gain starts to flatten out until the amplifier response begins to roll off. The gain does not reach 12 dB before the amplifier begins to roll off because the 12 dB value is a high frequency asymptote. The pole and zero locations cited in the previous discussion are qualitative, but the discussion describes the basic principles involved with the operation of the pre-emphasis network. Rev. 0 | Page 20 of 24 AD8146/AD8147/AD8148 Figure 39 illustrates the frequency response of the pre-emphasis network. Figure 40 illustrates the frequency response of the pre-emphasis circuit cascaded with the cable compared with that of the cable alone. It can be seen that the overall response is flat to within ±0.4 dB. The ±0.4 dB ripple in the response is due to the fact that the pre-emphasis network is linear, comprised of two realaxis pole/zero pairs, and the cable response is nonlinear. EXPOSED PADDLE (EP) The 24-lead LFCSP has an exposed paddle on the underside of its body. To achieve the specified thermal resistance, it must have a good thermal connection to one of the PCB planes. The exposed paddle must therefore be soldered to a pad on the top of the board that is connected to an inner plane with several thermal vias. The AD8147/AD8148 use the paddle as a ground reference; therefore, for these parts, the PCB plane used must be the ground plane. 18pF 442Ω 75Ω VIDEO SOURCE 2kΩ 30pF 487Ω 500Ω 49.9Ω + 100 FEET 100Ω UTP AD8148 82.5Ω 500Ω 442Ω 49.9Ω – 2kΩ 487Ω 30pF 18pF 06655-048 39.2Ω Figure 38. Pre-Emphasis Network Using the AD8148 for 30 Meters of UTP Cable 12 9 VS = ±5V 11 6 10 3 GAIN (dB) GAIN (dB) VS = ±5V 9 PRE-EMPHASIS NETWORK WITH CABLE 0 8 –3 7 –6 1 10 –9 0.1 100 FREQUENCY (MHz) 06655-050 6 0.1 06655-049 CABLE ALONE 1 10 100 FREQUENCY (MHz) Figure 40. AD8148 Pre-Emphasis Network Cascaded With 30 Meters of UTP Cable vs. UTP Cable Alone Figure 39. AD8148 Pre-Emphasis Network Frequency Response Rev. 0 | Page 21 of 24 AD8146/AD8147/AD8148 OUTLINE DIMENSIONS 0.60 MAX 4.00 BSC SQ PIN 1 INDICATOR 0.60 MAX TOP VIEW 0.50 BSC 3.75 BSC SQ 0.50 0.40 0.30 1.00 0.85 0.80 12° MAX SEATING PLANE 0.80 MAX 0.65 TYP 0.30 0.23 0.18 PIN 1 INDICATOR 24 1 19 18 2.25 2.10 SQ 1.95 EXPOSED PAD (BOTTOM VIEW) 13 12 7 6 0.25 MIN 2.50 REF 0.05 MAX 0.02 NOM 0.20 REF COPLANARITY 0.08 COMPLIANT TO JEDEC STANDARDS MO-220-VGGD-2 Figure 41. 24-Lead Lead Frame Chip Scale Package [LFCSP_VQ] 4 mm × 4 mm (CP-24-1) Dimensions shown in millimeters ORDERING GUIDE Model AD8146ACPZ-R2 1 AD8146ACPZ-R71 AD8146ACPZ-RL1 AD8147ACPZ-R21 AD8147ACPZ-R71 AD8147ACPZ-RL1 AD8148ACPZ-R21 AD8148ACPZ-R71 AD8148ACPZ-RL1 1 Temperature Range −40°C to +85°C −40°C to +85°C −40°C to +85°C −40°C to +85°C −40°C to +85°C −40°C to +85°C −40°C to +85°C −40°C to +85°C −40°C to +85°C Package Description 24-Lead Lead Frame Chip Scale Package [LFCSP_VQ] 24-Lead Lead Frame Chip Scale Package [LFCSP_VQ] 24-Lead Lead Frame Chip Scale Package [LFCSP_VQ] 24-Lead Lead Frame Chip Scale Package [LFCSP_VQ] 24-Lead Lead Frame Chip Scale Package [LFCSP_VQ] 24-Lead Lead Frame Chip Scale Package [LFCSP_VQ] 24-Lead Lead Frame Chip Scale Package [LFCSP_VQ] 24-Lead Lead Frame Chip Scale Package [LFCSP_VQ] 24-Lead Lead Frame Chip Scale Package [LFCSP_VQ] Z = RoHS Compliant Part. Rev. 0 | Page 22 of 24 Package Option CP-24-1 CP-24-1 CP-24-1 CP-24-1 CP-24-1 CP-24-1 CP-24-1 CP-24-1 CP-24-1 AD8146/AD8147/AD8148 NOTES Rev. 0 | Page 23 of 24 AD8146/AD8147/AD8148 NOTES ©2007 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D06655-0-5/07(0) Rev. 0 | Page 24 of 24