a

High Output Current Differential Driver AD815

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 0.45ⴗ 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

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

TOTAL HARMONIC DISTORTION – dBc

–40 –50

VS = 615V G = +10 VOUT = 40V p-p

FUNCTIONAL BLOCK DIAGRAM 15-Lead Through-Hole SIP (Y) and Surface-Mount DDPAK(VR)

TAB IS +VS

15

NC

14

NC

13

NC

12 11

NC

10

–IN2

9 8

OUT2 +VS

AD815

+IN2

7

–VS

6

OUT1

5

–IN1

4 3

+IN1

2

NC

1

NC

NC

NC = NO CONNECT REFER TO PAGE 3 FOR 24-LEAD SOIC PACKAGE

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

–60

1/2 AD815

100V

–70 –80

AMP1 RL = 50V (DIFFERENTIAL)

499V RL = 200V (DIFFERENTIAL)

–90

VIN = 4Vp-p

110V

G = +10

RL 120V

VD = 40Vp-p

VOUT = 40Vp-p

499V

–100 –110 100

R1 = 15V

100V 1k

10k 100k FREQUENCY – Hz

1M

1/2 AD815

10M

Total Harmonic Distortion vs. Frequency

R2 = 15V

AMP2

1:2 TRANSFORMER

–15V

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 = +25ⴗC, V = ⴞ15 V dc, R A

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

S

FB

= 1 k⍀ and RLOAD = 100 ⍀ unless otherwise noted)

Conditions

VS

Min

G = +1 G = +1 G = +2 G = +2 VOUT = 20 V p-p, G = +2 10 V Step, G = +2

± 15 ±5 ± 15 ±5 ± 15 ± 15

100 90

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

AD815A Typ Max 120 110 40 10 900 70

MHz MHz MHz MHz V/µs ns

± 15 ± 5, ± 15 ± 5, ± 15 ± 5, ± 15 ± 15 ± 15

–66 1.85 1.8 19 0.05 0.45

dBc nV/√Hz pA/√Hz pA/√Hz % Degrees

±5 ± 15

5 10

±5 ± 15

20 0.5 0.5

± 5, ± 15

10 10

± 5, ± 15

2

± 5, ± 15

10

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

± 5, ± 15

Open-Loop Transresistance TMIN – TMAX INPUT CHARACTERISTICS Differential Input Resistance

Output Current1, 2 VR, Y RB-24 Short Circuit Current Output Resistance MATCHING CHARACTERISTICS Crosstalk POWER SUPPLY Operating Range3 Quiescent Current

2 4 5 90 150 5 5 75 100

5.0

mV mV mV µV/°C mV mV mV µV/°C µA µA µA µA µA µA MΩ MΩ

± 15 ± 15 ±5 ± 5, ± 15 ± 5, ± 15

57 80

± 15 ±5 ± 15 ± 15

11.0 1.1 21 22.5

11.7 1.8 23 24.5

±V ±V ±V ±V

RLOAD = 10 Ω

± 15 ±5 ± 15 ± 15 ± 15

500 350 400

750 400 500 1.0 13

mA mA mA A Ω

f = 1 MHz

± 15

–65

dB

±5 ± 15 ±5 ± 15 ± 5, ± 15

23 30

TMIN – TMAX TMIN – TMAX Single Ended, RLOAD = 25 Ω Differential, RLOAD = 50 Ω TMIN – TMAX RLOAD = 5 Ω

TMIN – TMAX TMIN – TMAX

Power Supply Rejection Ratio

8 15 30

7 15 1.4 13.5 3.5 65 100

Differential Input Capacitance Input Common-Mode Voltage Range Common-Mode Rejection Ratio Differential Common-Mode Rejection Ratio OUTPUT CHARACTERISTICS Voltage Swing

1.0 0.5

± 15

+Input –Input

Units

TMIN – TMAX

–55

–66

± 18 30 40 40 55

MΩ Ω pF ±V ±V dB dB

V mA mA mA mA dB

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 ABSOLUTE MAXIMUM RATINGS 1

MAXIMUM POWER DISSIPATION

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

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.

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.

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.

PIN CONFIGURATION 24-Lead Thermally-Enhanced SOIC (RB-24) 24 NC

NC 2

23 NC

NC 3

22 NC

NC 4

21 NC

5 THERMAL HEAT TABS +VS*

AD815

6

20 19

TOP VIEW 7 (Not to Scale) 18 8

MAXIMUM POWER DISSIPATION – Watts

NC 1

14

THERMAL HEAT TABS +VS*

17

TJ = 1508C

13

θJA = 168C/W SOLDERED DOWN TO COPPER HEAT SINK (STILL AIR = 0FT/MIN) AD815 AVR, AY

12 11 10 9 8 7 6 5 4 3

θJA = 418C/W (STILL AIR = 0FT/MIN) NO HEAT SINK AD815 AVR, AY

θJA = 528C/W

+IN1 9

16 +IN2

–IN1 10

15 –IN2

1

OUT1 11

14 OUT2

0 –50 –40 –30 –20 –10 0 10 20 30 40 50 60 AMBIENT TEMPERATURE – 8C

–VS 12

13 +VS

NC = NO CONNECT

2

(STILL AIR = 0 FT/MIN) NO HEAT SINK

AD815ARB-24 70 80

90

Plot of Maximum Power Dissipation vs. Temperature

*HEAT TABS ARE CONNECTED TO THE POSITIVE SUPPLY.

ORDERING GUIDE Model

Temperature Range

Package Description

Package Option

AD815ARB-24 AD815ARB-24-REEL AD815AVR AD815AY AD815AYS AD815-EB

–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

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

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.

REV. B

–3–

WARNING! ESD SENSITIVE DEVICE

AD815–Typical Performance Characteristics AD815 36 34

10

5

0

5

10 15 SUPPLY VOLTAGE – 6Volts

VS = 65V

24

30

60 NO LOAD

20

RL = 50V (DIFFERENTIAL) RL = 25V (SINGLE-ENDED)

40

20

10

0

5

10 15 SUPPLY VOLTAGE – 6Volts

40

30

15

VS = 65V

20

10

27

24

21

2

10

4

6 8 10 12 SUPPLY VOLTAGE – 6Volts

16

14

SIDE A, B

0 INPUT BIAS CURRENT – mA

20

TA = +258C

Figure 5. Total Supply Current vs. Supply Voltage

DIFFERENTIAL OUTPUT VOLTAGE – Volts p-p

50

25

100

30

0

60 VS = 615V

80

18

0 20

Figure 2. Output Voltage Swing vs. Supply Voltage

30

0 20 40 60 JUNCTION TEMPERATURE – 8C

33

TOTAL SUPPLY CURRENT – mA

80

–20

Figure 4. Total Supply Current vs. Temperature

DIFFERENTIAL OUTPUT VOLTAGE – V p-p

SINGLE-ENDED OUTPUT VOLTAGE – V p-p

26

18 –40

20

40

0

SINGLE-ENDED OUTPUT VOLTAGE – Volts p-p

28

22

Figure 1. Input Common-Mode Voltage Range vs. Supply Voltage

5

30

20 0

10

VS = 615V

32

15

SUPPLY CURRENT – mA

COMMON-MODE VOLTAGE RANGE – 6Volts

20

+I B

VS = 615V, 65V

–10 –20 VS = 65V

–30

SIDE B –I B

–40

SIDE A –50 SIDE B

–60 SIDE A –70

–I B

VS = 615V

0 0 10 100 1k 10k LOAD RESISTANCE – (Differential – V) (Single-Ended – V/2)

–80 –40

Figure 3. Output Voltage Swing vs. Load Resistance

–20

0 20 40 60 JUNCTION TEMPERATURE – 8C

80

100

Figure 6. Input Bias Current vs. Temperature

–4–

REV. B

AD815 0

80

–2

60

–4

40 RTI OFFSET – mV

INPUT OFFSET VOLTAGE – mV

TA = 258C

VS = 65V –6 –8

VS = 615V VS = 610V

20 VS = 65V

0

VIN 1/2 f = 0.1Hz 100V AD815 VOUT

–20

–10

49.9V

VS = 615V –12

–40

–14 –40

–60 –2.0 –1.6 –1.2

1kV

0 20 40 60 JUNCTION TEMPERATURE – 8C

–20

80

100

Figure 7. Input Offset Voltage vs. Temperature

RL= 5V

1kV

0 –0.8 –0.4 0.4 0.8 LOAD CURRENT – Amps

1.2

1.6

2.0

Figure 10. Thermal Nonlinearity vs. Output Current Drive

750 CLOSED-LOOP OUTPUT RESISTANCE – V

SHORT CIRCUIT CURRENT – mA

VS = 615V 700 SOURCE 650

600 SINK 550

500

450 –60

–40

–20

0 20 40 60 80 100 JUNCTION TEMPERATURE – 8C

120

100

10

VS = 65V VS = 615V

1

0.1

0.01

140

30k

Figure 8. Short Circuit Current vs. Temperature

100k

300k

1M 3M 10M FREQUENCY – Hz

30M

100M

300M

Figure 11. Closed-Loop Output Resistance vs. Frequency

RTI OFFSET – mV

10

DIFFERENTIAL OUTPUT VOLTAGE – V p-p

15 VS = 610V

TA = 258C RL = 25V

VS = 65V

VS = 615V

5

0 VIN 1/2 f = 0.1Hz 100V AD815 VOUT

–5

49.9V –10

1kV

1kV –15 –20

RL= 25V

RL = 100V 30 RL = 50V 20 RL = 25V 10 RL = 1V 0

–16

–12

–8

–4 0 4 VOUT – Volts

8

12

16

0

20

Figure 9. Gain Nonlinearity vs. Output Voltage

REV. B

TA = 258C VS = ±15V

40

2

4

10 6 8 FREQUENCY – MHz

12

14

Figure 12. Large Signal Frequency Response

–5–

AD815 100

100 120

TRANSIMPEDANCE

10

NONINVERTING INPUT CURRENT NOISE

INPUT VOLTAGE NOISE

1 10

100

1k FREQUENCY – Hz

100

500

80

0 –50

60

–100

50

–150

40

–200

30

–250

100

Figure 13. Input Current and Voltage Noise vs. Frequency

80

TOTAL HARMONIC DISTORTION – dBc

COMMON-MODE REJECTION – dB

10k 100k 1M FREQUENCY – Hz

10M

100M

–40

VS = 615V

70 SIDE B

60

SIDE A 50

562V 562V

VOUT

VIN

30

562V

1/2 AD815

562V 20 10 10k

100k

1M FREQUENCY – Hz

–50 –60 –70 –80

1k

10k 100k FREQUENCY – Hz

1M

10M

Figure 17. Total Harmonic Distortion vs. Frequency

10 VS = 615V G = +2 RL = 100V

–30 –40 –PSRR –50 +PSRR

–60 –70 –80 –90

–100 0.01

RL = 200V (DIFFERENTIAL)

–100

OUTPUT SWING FROM ±V TO 0 – Volts

–20

RL = 50V (DIFFERENTIAL)

–90

0 –10

VS = 615V G = +10 VOUT = 40V p-p

–110 100

100M

10M

Figure 14. Common-Mode Rejection vs. Frequency

PSRR – dB

1k

Figure 16. Open-Loop Transimpedance vs. Frequency

90

40

PHASE

70

1 100k

10k

100

90

PHASE – Degrees

10

TRANSIMPEDANCE – dB

INVERTING INPUT CURRENT NOISE

CURRENT NOISE – pA/ √ Hz

VOLTAGE NOISE – nV/ √ Hz

110

8 1%

0.1%

6 GAIN = +2 VS = 615V

4 2 0 –2 –4 –6 1%

0.1%

–8

–10 0.1

1 10 FREQUENCY – MHz

100

300

0

Figure 15. Power Supply Rejection vs. Frequency

20

60 40 70 SETTLING TIME – ns

80

100

Figure 18. Output Swing and Error vs. Settling Time

–6–

REV. B

AD815 700

1400

5

1200

500

1000 G = +2

400

800

300

600

200

400

100

200

0

OPEN-LOOP TRANSRESISTANCE – MV

600

DIFFERENTIAL SLEW RATE – V/ms

SINGLE-ENDED SLEW RATE – V/ms (PER AMPLIFIER)

G = +10

0 0

5

10 15 OUTPUT STEP SIZE – V p-p

20

4 SIDE B

Figure 19. Slew Rate vs. Output Step Size

+TZ

SIDE A 2 –TZ SIDE B

1

0 –40

25

SIDE A

3

–20

0 20 40 60 JUNCTION TEMPERATURE – 8C

100

Figure 22. Open-Loop Transresistance vs. Temperature

–85

15 VS = 615V

VS = 615V SIDE B –80

14 OUTPUT SWING – Volts

+PSRR

SIDE A

PSRR – dB

80

–75

–70 SIDE A SIDE B

–65

RL = 150V +VOUT | –VOUT |

13

+VOUT RL = 25V

12 | –VOUT | 11

–PSRR –60 –40

–20

0 20 40 60 JUNCTION TEMPERATURE – 8C

80

10 –40

100

Figure 20. PSRR vs. Temperature

–20

0 20 40 60 JUNCTION TEMPERATURE – 8C

80

100

Figure 23. Single-Ended Output Swing vs. Temperature

27

–74 –73

26 OUTPUT SWING – Volts

CMRR – dB

–72 –71 –70 –69 –CMRR

VS = 615V RL = 50V

25 –VOUT +VOUT 24

–68

–66 –40

23

+CMRR

–67

–20

0 20 40 60 JUNCTION TEMPERATURE – 8C

80

22 –40

100

0 20 40 60 JUNCTION TEMPERATURE – 8C

80

100

Figure 24. Differential Output Swing vs. Temperature

Figure 21. CMRR vs. Temperature

REV. B

–20

–7–

AD815

2

3

4

5

6

7

8

9

2 BACK TERMINATED LOADS (75V)

0.010 0.005 0.000 –0.005 –0.010 –0.015 –0.020 –0.025 –0.030

PHASE GAIN

GAIN

1

2

PHASE 3

4

5

6

7

8

9

0.12 0.10 0.08 G = +2 0.06 RF = 1kV 0.04 NTSC 0.02 0.00 –0.02 –0.04 10 11

–3 –4

B VIN

–0.3

–5

100V

–0.4

VOUT

–6

65V

49.9V

–0.5

499V

499V

100V

–7 –8

–0.6

1

10 FREQUENCY – MHz

100

–9 300

Figure 28. Bandwidth vs. Frequency, G = +2

1 VS = 615V

G = +2 RF = 499V VS = 615V, 65V VIN = 400mVrms RL = 100V SIDE B

–50 –60 –70

SIDE A –80 –90

–100 –110 0.03

A

A

–0.2

NORMALIZED OUTPUT VOLTAGE – dB

CROSSTALK – dB

–40

–2 B

–0.1

–10

–30

–1

0

–0.7 0.1

Figure 25. Differential Gain and Differential Phase (per Amplifier)

–20

615V

0.1

0

NORMALIZED FREQUENCY RESPONSE – dB

1

65V

NORMALIZED FLATNESS – dB

GAIN

1

615V

DIFF PHASE – Degrees

PHASE

0.5 0.4 0.3 G = +2 RF = 1kV 0.2 0.1 NTSC 0.0 –0.1 –0.2 –0.3 10 11

DIFF PHASE – Degrees

DIFF GAIN – %

DIFF GAIN – %

6 BACK TERMINATED LOADS (25V) 0.04 0.03 0.02 0.01 0.00 –0.01 –0.02 –0.03 –0.04

0.1

1 10 FREQUENCY – MHz

100

0

SIDE A SIDE B

–1 –2 –3 VIN

100V 49.9V

–5

124V

Figure 26. Output-to-Output Crosstalk vs. Frequency

499V

100V

–6 –7 0.1

300

VOUT

–4

1

10 FREQUENCY – MHz

100

300

Figure 29. –3 dB Bandwidth vs. Frequency, G = +5

2 1

VS = 615V VIN = 0 dBm

SIDE B

OUTPUT VOLTAGE – dB

0

100 90

SIDE A

–1 –2 –3

VIN

100V

VOUT

–4 –5

49.9V 562V

10

100V

0%

–6

5V

–7 –9 0.1

1

10 FREQUENCY – MHz

100

1ms

300

Figure 30. 40 V p-p Differential Sine Wave, RL = 50 Ω, f = 100 kHz

Figure 27. –3 dB Bandwidth vs. Frequency, G = +1

–8–

REV. B

AD815 RF

562V 10mF

+15V

0.1mF

0.1mF RS

8

8

1/2 AD815 100V VIN

7

PULSE GENERATOR

50V

1/2 AD815 0.1mF

50V

Figure 35. Test Circuit, Gain = 1 + R F /RS

G = +1 RF = 698V RL = 100V

G = +5 RF = 562V RL = 100V RS = 140V

SIDE A

SIDE B

SIDE B

5V

20ns

100ns

Figure 36. 20 V Step Response, G = +5

Figure 32. 500 mV Step Response, G = +1

SIDE A

RL = 100V

10mF –15V

TR/TF = 250ps

Figure 31. Test Circuit, Gain = +1

100mV

0.1mF 7

PULSE GENERATOR

–15V

SIDE A

100V

VIN

RL = 100V

10mF

TR/TF = 250ps

10mF

+15V

G = +1 RF = 562V RL = 100V

562V 10mF

+15V SIDE B

0.1mF 562V

8

VIN PULSE GENERATOR

55V

1/2 AD815 100V

0.1mF 7

TR/TF = 250ps

RL = 100V

10mF –15V

1V

20ns

Figure 33. 4 V Step Response, G = +1

Figure 37. Test Circuit, Gain = –1

SIDE A

SIDE A

G = +1 RF = 562V RL = 100V

SIDE B

SIDE B

2V

100mV

50ns

Figure 34. 10 V Step Response, G = +1

REV. B

G = –1 RF = 562V RL = 100V

20ns

Figure 38. 500 mV Step Response, G = –1

–9–

AD815 Choice of Feedback and Gain Resistors SIDE A

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.

G = –1 RF = 562V RL = 100V

SIDE B

Table I. Resistor Values 1V

20ns

G= 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.

562 499 499 499 1k

⬁ 499 499 125 110

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.

T Z (S ) VO =G× VIN T Z (S ) + G × RIN + RF

POWER SUPPLY BYPASSING

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.

where: RF RG R IN = 1/gM ≈ 25 Ω G = 1+

RF RG RIN

RG (⍀)

PRINTED CIRCUIT BOARD LAYOUT CONSIDERATIONS

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:

RN

+1 –1 +2 +5 +10

RF (⍀)

VOUT

+VS

VIN +IN

Figure 40. Current Feedback Amplifier Operation

RF

Recognizing that G × RIN