AD815AVR

a FEATURES Flexible Configuration Differential Input and Output Driver or Two Single-Ended Drivers High Output Power Power Package 26 dBm Differential Line Drive for ADSL Application 40 V p-p Differential Output Voltage, RL = 50 500 mA Minimum Output Drive/Amp, RL = 5 Thermally Enhanced SOIC 400 mA Minimum Output Drive/Amp, RL = 10 Low Distortion –66 dB @ 1 MHz THD, R L = 200 , VOUT = 40 V p-p 0.05% and 0.45 Differential Gain and Phase, RL = 25 (6 Back-Terminated Video Loads) High Speed 120 MHz Bandwidth (–3 dB) 900 V/ s Differential Slew Rate 70 ns Settling Time to 0.1% Thermal Shutdown APPLICATIONS ADSL, HDSL and VDSL Line Interface Driver Coil or Transformer Driver CRT Convergence and Astigmatism Adjustment Video Distribution Amp Twisted Pair Cable Driver PRODUCT DESCRIPTION High Output Current Differential Driver AD815 FUNCTIONAL BLOCK DIAGRAM 15-Lead Through-Hole SIP (Y) and Surface-Mount DDPAK(VR) 15 14 13 TAB IS +VS 12 11 10 9 8 7 6 5 4 3 2 1 NC = NO CONNECT REFER TO PAGE 3 FOR 24-LEAD SOIC PACKAGE NC NC NC NC +IN2 –IN2 OUT2 +VS –VS OUT1 –IN1 +IN1 NC NC NC AD815 coupling transformer with a greater than 1:1 turns ratio. The low harmonic distortion of –66 dB @ 1 MHz into 200 Ω combined with the wide bandwidth and high current drive make the differential driver ideal for communication applications such as subscriber line interfaces for ADSL, HDSL and VDSL. The AD815 differential slew rate of 900 V/µs and high load drive are suitable for fast dynamic control of coils or transformers, and the video performance of 0.05% and 0.45° differential gain and phase into a load of 25 Ω enable up to 12 back-terminated loads to be driven. Three package styles are available, and all work over the industrial temperature range (–40°C to +85 °C). Maximum output power is achieved with the power package available for through-hole mounting (Y) and surface-mounting (VR). The 24-lead SOIC (RB) is capable of driving 26 dBm for full rate ADSL with proper heat sinking. +15V 100 The AD815 consists of two high speed amplifiers capable of supplying a minimum of 500 mA. They are typically configured as a differential driver enabling an output signal of 40 V p-p on ± 15 V supplies. This can be increased further with the use of a –40 TOTAL HARMONIC DISTORTION – dBc –50 –60 –70 –80 –90 –100 –110 100 RL = 50 (DIFFERENTIAL) RL = 200 (DIFFERENTIAL) VS = 15V G = +10 VOUT = 40V p-p 1/2 AD815 AMP1 499 R1 = 15 VIN = 4Vp-p 110 G = +10 499 VD = 40Vp-p RL 120 VOUT = 40Vp-p 100 1k 10k 100k FREQUENCY – Hz 1M 10M AMP2 1/2 AD815 –15V R2 = 15 1:2 TRANSFORMER Total Harmonic Distortion vs. Frequency Subscriber Line Differential Driver REV. B Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781/329-4700 World Wide Web Site: http://www.analog.com Fax: 781/326-8703 © Analog Devices, Inc., 1999 AD815–SPECIFICATIONS (@ T = +25 C, V = A S 15 V dc, RFB = 1 k and RLOAD = 100 VS ± 15 ±5 ± 15 ±5 ± 15 ± 15 ± 15 ± 5, ± 15 ± 5, ± 15 ± 5, ± 15 ± 15 ± 15 ±5 ± 15 Min 100 90 unless otherwise noted) AD815A Typ Max 120 110 40 10 900 70 –66 1.85 1.8 19 0.05 0.45 5 10 20 0.5 0.5 10 10 2 10 8 15 30 2 4 5 90 150 5 5 75 100 Units MHz MHz MHz MHz V/µs ns dBc nV/ √Hz pA/ √Hz pA/ √Hz % Degrees mV mV mV µV/°C mV mV mV µV/°C µA µA µA µA µA µA MΩ MΩ MΩ Ω pF ±V ±V dB dB ±V ±V ±V ±V mA mA mA A Ω dB ± 18 30 40 40 55 V mA mA mA mA dB Model DYNAMIC PERFORMANCE Small Signal Bandwidth (–3 dB) Bandwidth (0.1 dB) Differential Slew Rate Settling Time to 0.1% NOISE/HARMONIC PERFORMANCE Total Harmonic Distortion Input Voltage Noise Input Current Noise (+IIN) Input Current Noise (–IIN) Differential Gain Error Differential Phase Error DC PERFORMANCE Input Offset Voltage Conditions G = +1 G = +1 G = +2 G = +2 VOUT = 20 V p-p, G = +2 10 V Step, G = +2 f = 1 MHz, RLOAD = 200 Ω, VOUT = 40 V p-p f = 10 kHz, G = +2 (Single Ended) f = 10 kHz, G = +2 f = 10 kHz, G = +2 NTSC, G = +2, RLOAD = 25 Ω NTSC, G = +2, RLOAD = 25 Ω 800 TMIN – TMAX Input Offset Voltage Drift Differential Offset Voltage TMIN – TMAX Differential Offset Voltage Drift –Input Bias Current TMIN – TMAX +Input Bias Current TMIN – TMAX Differential Input Bias Current TMIN – TMAX Open-Loop Transresistance TMIN – TMAX INPUT CHARACTERISTICS Differential Input Resistance Differential Input Capacitance Input Common-Mode Voltage Range Common-Mode Rejection Ratio Differential Common-Mode Rejection Ratio OUTPUT CHARACTERISTICS Voltage Swing TMIN – TMAX TMIN – TMAX Single Ended, RLOAD = 25 Ω Differential, RLOAD = 50 Ω TMIN – TMAX RLOAD = 5 Ω RLOAD = 10 Ω +Input –Input ± 15 ± 15 ± 15 ±5 ± 5, ± 15 ± 5, ± 15 ± 15 ±5 ± 15 ± 15 ± 15 ±5 ± 15 ± 15 ± 15 ± 15 ±5 ± 15 ±5 ± 15 ± 5, ± 15 ± 5, ± 15 ± 5, ± 15 ± 5, ± 15 ± 5, ± 15 1.0 0.5 ±5 ± 15 5.0 57 80 11.0 1.1 21 22.5 500 350 400 7 15 1.4 13.5 3.5 65 100 11.7 1.8 23 24.5 750 400 500 1.0 13 –65 Output Current1, 2 VR, Y RB-24 Short Circuit Current Output Resistance MATCHING CHARACTERISTICS Crosstalk POWER SUPPLY Operating Range3 Quiescent Current f = 1 MHz TMIN – TMAX TMIN – TMAX 23 30 Power Supply Rejection Ratio TMIN – TMAX –55 –66 NOTES 1Output current is limited in the 24-lead SOIC package to the maximum power dissipation. See absolute maximum ratings and derating curves. 2See Figure 12 for bandwidth, gain, output drive recommended operation range. 3Observe derating curves for maximum junction temperature. Specifications subject to change without notice. –2– REV. B AD815 Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . ± 18 V Total Internal Power Dissipation2 Plastic (Y and VR) . . 3.05 Watts (Observe Derating Curves) Small Outline (RB) . . 2.4 Watts (Observe Derating Curves) Input Voltage (Common Mode) . . . . . . . . . . . . . . . . . . . . ± VS Differential Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . ± 6 V Output Short Circuit Duration . . . . . . . . . . . . . . . . . . . . . . Observe Power Derating Curves Can Only Short to Ground Storage Temperature Range Y, VR and RB Package . . . . . . . . . . . . . . . –65° C to +125°C Operating Temperature Range AD815A . . . . . . . . . . . . . . . . . . . . . . . . . . . –40°C to +85°C Lead Temperature Range (Soldering, 10 sec) . . . . . . . +300°C NOTES 1 Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; functional operation of the device at these or any other conditions above those indicated in the operational section of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. 2 Specification is for device in free air with 0 ft/min air flow: 15-Lead Through-Hole and Surface Mount: θJA = 41 °C/W; 24-Lead Surface Mount: θJA = 52°C/W. ABSOLUTE MAXIMUM RATINGS 1 MAXIMUM POWER DISSIPATION The maximum power that can be safely dissipated by the AD815 is limited by the associated rise in junction temperature. The maximum safe junction temperature for the plastic encapsulated parts is determined by the glass transition temperature of the plastic, about 150° C. Exceeding this limit temporarily may cause a shift in parametric performance due to a change in the stresses exerted on the die by the package. Exceeding a junction temperature of 175° C for an extended period can result in device failure. The AD815 has thermal shutdown protection, which guarantees that the maximum junction temperature of the die remains below a safe level, even when the output is shorted to ground. Shorting the output to either power supply will result in device failure. To ensure proper operation, it is important to observe the derating curves and refer to the section on power considerations. It must also be noted that in high (noninverting) gain configurations (with low values of gain resistor), a high level of input overdrive can result in a large input error current, which may result in a significant power dissipation in the input stage. This power must be included when computing the junction temperature rise due to total internal power. 14 MAXIMUM POWER DISSIPATION – Watts 13 12 11 10 9 8 7 6 5 4 3 2 1 TJ = 150 C PIN CONFIGURATION 24-Lead Thermally-Enhanced SOIC (RB-24) NC 1 NC 2 NC 3 NC 4 5 THERMAL HEAT TABS +VS* 6 24 NC 23 NC 22 NC 21 NC θJA = 16 C/W SOLDERED DOWN TO COPPER HEAT SINK (STILL AIR = 0FT/MIN) AD815 AVR, AY θJA = 41 C/W (STILL AIR = 0FT/MIN) NO HEAT SINK AD815 AVR, AY AD815 20 19 THERMAL HEAT TABS +VS* TOP VIEW 7 (Not to Scale) 18 8 17 +IN1 9 –IN1 10 OUT1 11 –VS 12 16 +IN2 15 –IN2 14 OUT2 13 +VS θJA = 52 C/W (STILL AIR = 0 FT/MIN) NO HEAT SINK AD815ARB-24 70 80 90 0 –50 –40 –30 –20 –10 0 10 20 30 40 50 60 AMBIENT TEMPERATURE – C NC = NO CONNECT *HEAT TABS ARE CONNECTED TO THE POSITIVE SUPPLY. Plot of Maximum Power Dissipation vs. Temperature ORDERING GUIDE Model AD815ARB-24 AD815ARB-24-REEL AD815AVR AD815AY AD815AYS AD815-EB 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 Package Description 24-Lead Thermally Enhanced SOIC 24-Lead Thermally Enhanced SOIC 15-Lead Surface Mount DDPAK 15-Lead Through-Hole SIP with Staggered Leads and 90° Lead Form 15-Lead Through-Hole SIP with Staggered Leads and Straight Lead Form Evaluation Board Package Option RB-24 RB-24 VR-15 Y-15 YS-15 CAUTION ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate on the human body and test equipment and can discharge without detection. Although the AD815 features proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance degradation or loss of functionality. WARNING! ESD SENSITIVE DEVICE REV. B –3– AD815–Typical Performance Characteristics AD815 20 Volts 36 34 VS = SUPPLY CURRENT – mA 15 32 30 28 26 VS = 24 22 20 0 18 –40 5V 15V COMMON-MODE VOLTAGE RANGE – 10 5 0 5 10 SUPPLY VOLTAGE – 15 Volts 20 –20 0 20 40 60 JUNCTION TEMPERATURE – C 80 100 Figure 1. Input Common-Mode Voltage Range vs. Supply Voltage Figure 4. Total Supply Current vs. Temperature 40 SINGLE-ENDED OUTPUT VOLTAGE – V p-p 80 DIFFERENTIAL OUTPUT VOLTAGE – V p-p 33 TOTAL SUPPLY CURRENT – mA TA = +25 C 30 30 NO LOAD 60 27 20 RL = 50 (DIFFERENTIAL) RL = 25 (SINGLE-ENDED) 40 24 10 20 21 0 0 5 10 SUPPLY VOLTAGE – 15 Volts 0 20 18 0 2 4 6 8 10 12 SUPPLY VOLTAGE – Volts 14 16 Figure 2. Output Voltage Swing vs. Supply Voltage Figure 5. Total Supply Current vs. Supply Voltage 30 SINGLE-ENDED OUTPUT VOLTAGE – Volts p-p VS = 25 15V 60 DIFFERENTIAL OUTPUT VOLTAGE – Volts p-p 10 0 INPUT BIAS CURRENT – A –10 –20 –30 –40 –50 –60 SIDE A, B VS = 15V, +I B 5V 50 20 40 VS = 5V 15 30 SIDE B –IB SIDE A SIDE B SIDE A –I B 15V 100 10 VS = 5 5V 20 10 –70 VS = –80 –40 –20 0 20 40 60 JUNCTION TEMPERATURE – C 80 0 10 100 LOAD RESISTANCE – (Differential – 1k ) (Single-Ended – 0 10k /2) Figure 3. Output Voltage Swing vs. Load Resistance Figure 6. Input Bias Current vs. Temperature –4– REV. B AD815 0 –2 INPUT OFFSET VOLTAGE – mV –4 VS = –6 –8 –10 VS = –12 –14 –40 15V –40 1k –60 –2.0 –1.6 –1.2 0 –0.8 –0.4 0.4 0.8 LOAD CURRENT – Amps 5V 80 TA = 25 C 60 40 RTI OFFSET – mV 20 0 –20 VIN f = 0.1Hz 100 49.9 1k VS = 5V VS = 10V VS = 15V 1/2 AD815 VOUT RL= 5 –20 0 20 40 60 JUNCTION TEMPERATURE – C 80 100 1.2 1.6 2.0 Figure 7. Input Offset Voltage vs. Temperature Figure 10. Thermal Nonlinearity vs. Output Current Drive 750 VS = SHORT CIRCUIT CURRENT – mA 700 SOURCE 650 15V CLOSED-LOOP OUTPUT RESISTANCE – 100 10 VS = 5V 600 SINK 550 1 VS = 15V 0.1 500 0.01 450 –60 –40 –20 0 20 40 60 80 100 JUNCTION TEMPERATURE – C 120 140 30k 100k 300k 1M 3M 10M FREQUENCY – Hz 30M 100M 300M Figure 8. Short Circuit Current vs. Temperature Figure 11. Closed-Loop Output Resistance vs. Frequency 15 TA = 25 C RL = 25 VS = RTI OFFSET – mV 5 5V VS = 10V VS = 15V DIFFERENTIAL OUTPUT VOLTAGE – V p-p 40 TA = 25 C VS = ±15V RL = 100 30 RL = 50 20 RL = 25 10 RL = 1 0 –16 –12 –8 –4 0 4 VOUT – Volts 8 12 16 20 0 2 4 10 6 8 FREQUENCY – MHz 12 14 10 0 VIN f = 0.1Hz 100 49.9 –5 1/2 AD815 VOUT RL= 25 –10 1k –15 –20 1k Figure 9. Gain Nonlinearity vs. Output Voltage Figure 12. Large Signal Frequency Response REV. B –5– AD815 100 100 120 110 VOLTAGE NOISE – nV/ √ Hz TRANSIMPEDANCE – dB INVERTING INPUT CURRENT NOISE CURRENT NOISE – pA/ √ Hz 100 90 80 70 60 50 40 INPUT VOLTAGE NOISE 100 1k FREQUENCY – Hz 10k 30 1 100k 100 1k 10k 100k 1M FREQUENCY – Hz 10M PHASE 100 500 PHASE – Degrees 0 –50 –100 –150 –200 –250 100M TRANSIMPEDANCE 10 10 NONINVERTING INPUT CURRENT NOISE 1 10 Figure 13. Input Current and Voltage Noise vs. Frequency Figure 16. Open-Loop Transimpedance vs. Frequency 90 TOTAL HARMONIC DISTORTION – dBc 80 VS = 70 60 50 40 30 20 10 10k 562 VIN 562 562 SIDE B SIDE A 562 VOUT 15V –40 –50 –60 –70 –80 –90 –100 –110 100 RL = 50 (DIFFERENTIAL) RL = 200 (DIFFERENTIAL) VS = 15V G = +10 VOUT = 40V p-p COMMON-MODE REJECTION – dB 1/2 AD815 100k 1M FREQUENCY – Hz 10M 100M 1k 10k 100k FREQUENCY – Hz 1M 10M Figure 14. Common-Mode Rejection vs. Frequency Figure 17. Total Harmonic Distortion vs. Frequency 0 OUTPUT SWING FROM ±V TO 0 – Volts –10 –20 –30 PSRR – dB –40 –PSRR –50 –60 –70 –80 –90 –100 0.01 0.1 1 10 FREQUENCY – MHz 100 300 +PSRR VS = 15V G = +2 RL = 100 10 8 1% 6 4 2 0 –2 –4 –6 1% –8 0 20 60 40 70 SETTLING TIME – ns 80 100 0.1% GAIN = +2 VS = 15V 0.1% –10 Figure 15. Power Supply Rejection vs. Frequency Figure 18. Output Swing and Error vs. Settling Time –6– REV. B AD815 700 G = +10 SINGLE-ENDED SLEW RATE – V/ s (PER AMPLIFIER) DIFFERENTIAL SLEW RATE – V/ s 600 500 G = +2 400 300 200 100 0 0 5 10 15 OUTPUT STEP SIZE – V p-p 20 25 800 600 400 200 0 1200 1000 OPEN-LOOP TRANSRESISTANCE – M 4 SIDE B 3 SIDE A 2 –TZ 1 SIDE B +TZ SIDE A 1400 5 0 –40 –20 0 20 40 60 JUNCTION TEMPERATURE – C 80 100 Figure 19. Slew Rate vs. Output Step Size Figure 22. Open-Loop Transresistance vs. Temperature –85 VS = SIDE B –80 SIDE A –75 OUTPUT SWING – Volts +PSRR 15V 15 VS = 14 15V RL = 150 +VOUT | –VOUT | +VOUT 12 | –VOUT | 11 –PSRR RL = 25 PSRR – dB 13 –70 SIDE A –65 SIDE B –60 –40 –20 0 20 40 60 JUNCTION TEMPERATURE – C 80 100 10 –40 –20 0 20 40 60 JUNCTION TEMPERATURE – C 80 100 Figure 20. PSRR vs. Temperature Figure 23. Single-Ended Output Swing vs. Temperature –74 –73 27 26 OUTPUT SWING – Volts –72 CMRR – dB –71 –70 –69 –CMRR –68 –67 –66 –40 +CMRR 23 VS = 15V RL = 50 25 –VOUT +VOUT 24 –20 0 20 40 60 JUNCTION TEMPERATURE – C 80 100 22 –40 –20 0 20 40 60 JUNCTION TEMPERATURE – C 80 100 Figure 21. CMRR vs. Temperature Figure 24. Differential Output Swing vs. Temperature REV. B –7– AD815 DIFF PHASE – Degrees 6 BACK TERMINATED LOADS (25 ) 0.04 0.03 0.02 0.01 0.00 –0.01 –0.02 –0.03 –0.04 0.010 0.005 0.000 –0.005 –0.010 –0.015 –0.020 –0.025 –0.030 0.5 0.4 0.3 0.2 0.1 0.0 –0.1 –0.2 –0.3 0.12 0.10 0.08 0.06 0.04 0.02 0.00 –0.02 –0.04 DIFF GAIN – % 1 0 5V 0.1 NORMALIZED FLATNESS – dB 0 –0.1 –0.2 –0.3 –0.4 –0.5 –0.6 –0.7 0.1 VIN 100 49.9 499 499 100 VOUT 5V B B A A 15V –1 –2 –3 –4 –5 –6 –7 –8 –9 300 NORMALIZED FREQUENCY RESPONSE – dB 15V PHASE G = +2 RF = 1k NTSC GAIN 1 2 3 4 5 6 7 8 9 10 11 DIFF GAIN – % PHASE GAIN G = +2 RF = 1k NTSC PHASE 3 4 5 6 7 8 9 10 11 GAIN DIFF PHASE – Degrees 2 BACK TERMINATED LOADS (75 ) 1 2 1 10 FREQUENCY – MHz 100 Figure 25. Differential Gain and Differential Phase (per Amplifier) Figure 28. Bandwidth vs. Frequency, G = +2 –10 NORMALIZED OUTPUT VOLTAGE – dB –20 –30 CROSSTALK – dB –40 –50 –60 –70 SIDE A –80 –90 –100 –110 0.03 G = +2 RF = 499 VS = 15V, 5V VIN = 400mVrms RL = 100 SIDE B 1 VS = 0 –1 –2 –3 VIN –4 49.9 –5 –6 –7 0.1 124 499 100 100 VOUT SIDE B 15V SIDE A 0.1 1 10 FREQUENCY – MHz 100 300 1 10 FREQUENCY – MHz 100 300 Figure 26. Output-to-Output Crosstalk vs. Frequency Figure 29. –3 dB Bandwidth vs. Frequency, G = +5 2 1 0 OUTPUT VOLTAGE – dB –1 –2 –3 –4 –5 –6 –7 –9 0.1 49.9 562 100 10 0% VS = 15V VIN = 0 dBm SIDE A SIDE B 100 90 VIN 100 VOUT 5V 1s 1 10 FREQUENCY – MHz 100 300 Figure 27. –3 dB Bandwidth vs. Frequency, G = +1 Figure 30. 40 V p-p Differential Sine Wave, RL = 50 Ω, f = 100 kHz –8– REV. B AD815 562 +15V 10 F 0.1 F 8 RF +15V 10 F 0.1 F RS 8 1/2 AD815 100 VIN PULSE GENERATOR TR/TF = 250ps 50 –15V 7 1/2 AD815 0.1 F RL = 100 VIN PULSE GENERATOR TR/TF = 250ps 100 7 0.1 F 10 F –15V RL = 100 10 F 50 Figure 31. Test Circuit, Gain = +1 SIDE A Figure 35. Test Circuit, Gain = 1 + R F /RS G = +1 RF = 698 RL = 100 SIDE A G = +5 RF = 562 RL = 100 RS = 140 SIDE B SIDE B 100mV 20ns 5V 100ns Figure 32. 500 mV Step Response, G = +1 Figure 36. 20 V Step Response, G = +5 SIDE A G = +1 RF = 562 RL = 100 562 +15V 10 F 0.1 F 562 VIN PULSE GENERATOR TR/TF = 250ps 55 100 7 8 SIDE B 1/2 AD815 0.1 F 10 F –15V RL = 100 1V 20ns Figure 33. 4 V Step Response, G = +1 Figure 37. Test Circuit, Gain = –1 SIDE A G = +1 RF = 562 RL = 100 SIDE A G = –1 RF = 562 RL = 100 SIDE B SIDE B 2V 50ns 100mV 20ns Figure 34. 10 V Step Response, G = +1 Figure 38. 500 mV Step Response, G = –1 REV. B –9– AD815 Choice of Feedback and Gain Resistors SIDE A G = –1 RF = 562 RL = 100 SIDE B The fine scale gain flatness will, to some extent, vary with feedback resistance. It therefore is recommended that once optimum resistor values have been determined, 1% tolerance values should be used if it is desired to maintain flatness over a wide range of production lots. Table I shows optimum values for several useful configurations. These should be used as starting point in any application. Table I. Resistor Values 1V 20ns RF ( ) G= +1 –1 +2 +5 +10 562 499 499 499 1k RG ( ) 499 499 125 110 Figure 39. 4 V Step Response, G = –1 THEORY OF OPERATION The AD815 is a dual current feedback amplifier with high (500 mA) output current capability. Being a current feedback amplifier, the AD815’s open-loop behavior is expressed as transimpedance, ∆ VO/ ∆ I–IN , or TZ . The open-loop transimpedance behaves just as the open-loop voltage gain of a voltage feedback amplifier, that is, it has a large dc value and decreases at roughly 6 dB/octave in frequency. Since RIN is proportional to 1/gM, the equivalent voltage gain is just TZ × gM , where the gM in question is the transconductance of the input stage. Using this amplifier as a follower with gain, Figure 40, basic analysis yields the following result: T Z (S ) VO =G× VIN T Z (S ) + G × RIN + RF where: RF RG R IN = 1/gM ≈ 25 Ω G = 1+ RF RG RIN RN VIN PRINTED CIRCUIT BOARD LAYOUT CONSIDERATIONS As to be expected for a wideband amplifier, PC board parasitics can affect the overall closed-loop performance. Of concern are stray capacitances at the output and the inverting input nodes. If a ground plane is to be used on the same side of the board as the signal traces, a space (5 mm min) should be left around the signal lines to minimize coupling. POWER SUPPLY BYPASSING VOUT Adequate power supply bypassing can be critical when optimizing the performance of a high frequency circuit. Inductance in the power supply leads can form resonant circuits that produce peaking in the amplifier’s response. In addition, if large current transients must be delivered to the load, then bypass capacitors (typically greater than 1 µF) will be required to provide the best settling time and lowest distortion. A parallel combination of 10.0 µF and 0.1 µF is recommended. Under some low frequency applications, a bypass capacitance of greater than 10 µF may be necessary. Due to the large load currents delivered by the AD815, special consideration must be given to careful bypassing. The ground returns on both supply bypass capacitors as well as signal common must be “star” connected as shown in Figure 41. +VS +IN Figure 40. Current Feedback Amplifier Operation RF RG (OPTIONAL) RF +OUT Recognizing that G × RIN