AD811

REV. D

–9–

APPLICATIONS

General Design Considerations

The AD811 is a current feedback amplifier optimized for use in

high performance video and data acquisition applications. Since

it uses a current feedback architecture, its closed-loop –3 dB

bandwidth is dependent on the magnitude of the feedback resis-

tor. The desired closed-loop gain and bandwidth are obtained

contains recommended resistor values for a variety of useful

closed-loop gains and supply voltages.

Table I. –3 dB Bandwidth vs. Closed-Loop Gain and

Resistance Values

15 V

Closed-Loop

–3 dB BW

Gain

(MHz)

+1

750

Ω

140

+2

649

Ω

649

Ω

120

+10

511

Ω

56.2

Ω

100

–1

590

Ω

590

Ω

115

–10

511

Ω

51.1

Ω

95

5 V

Closed-Loop

–3 dB BW

Gain

(MHz)

+1

619

Ω

80

+2

562

Ω

562

Ω

80

+10

442

Ω

48.7

Ω

65

–1

562

Ω

562

Ω

75

–10

442

Ω

44.2

Ω

65

10 V

Closed-Loop

–3 dB BW

Gain

(MHz)

+1

649

Ω

105

+2

590

Ω

590

Ω

105

+10

499

Ω

49.9

Ω

80

–1

590

Ω

590

Ω

105

–10

499

Ω

49.9

Ω

80

Figures 11 and 12 illustrate the relationship between the feed-

back resistor and the frequency and time domain response char-

acteristics for a closed-loop gain of +2. (The response at other

gains will be similar.)

The 3 dB bandwidth is somewhat dependent on the power supply

voltage. As the supply voltage is decreased for example, the

magnitude of internal junction capacitances is increased, causing

a reduction in closed-loop bandwidth. To compensate for this,

smaller values of feedback resistor are used at lower supply

voltages.

Achieving the Flattest Gain Response at High Frequency

Achieving and maintaining gain flatness of better than 0.1 dB at

frequencies above 10 MHz requires careful consideration of

several issues.

Choice of Feedback and Gain Resistors

Because of the above-mentioned relationship between the 3 dB

bandwidth and the feedback resistor, the fine scale gain flatness

will, to some extent, vary with feedback resistor tolerance. It is,

therefore, recommended that resistors with a 1% tolerance be

used if it is desired to maintain flatness over a wide range of

production lots. In addition, resistors of different construction

have different associated parasitic capacitance and inductance.

Metal-film resistors were used for the bulk of the characteriza-

tion for this data sheet. It is possible that values other than those

indicated will be optimal for other resistor types.

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 (3/16" is plenty) should be left around

the signal lines to minimize coupling. Additionally, signal lines

connecting the feedback and gain resistors should be short

enough so that their associated inductance does not cause

high frequency gain errors. Line lengths less than 1/4" are

recommended.

Quality of Coaxial Cable

Optimum flatness when driving a coax cable is possible only

when the driven cable is terminated at each end with a resistor

matching its characteristic impedance. If the coax was ideal,

then the resulting flatness would not be affected by the length of

the cable. While outstanding results can be achieved using inex-

pensive cables, it should be noted that some variation in flatness

due to varying cable lengths may be experienced.

Power Supply Bypassing

Adequate power supply bypassing can be critical when optimiz-

ing 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. Although the recommended

0.1

µF power supply bypass capacitors will be sufficient in many

applications, more elaborate bypassing (such as using two paral-

leled capacitors) may be required in some cases.