AD815

REV. B

–10–

Choice of Feedback and Gain Resistors

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

G =

+1

562

–1

499

499

+2

499

499

+5

499

125

+10

1 k

110

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

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.

(OPTIONAL)

+OUT

–OUT

+IN

–IN

Figure 41. Signal Ground Connected in “Star”

Configuration

1V

20ns

SIDE B

SIDE A

G = –1

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,

∆V

∆I

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.

× g

of the input stage. Using this amplifier as a follower with gain,

Figure 40, basic analysis yields the following result:

V

O

V

IN

= G ×

T

()

T

where:

G

R

R

F

G

=+

1

≈ 25 Ω

Figure 40. Current Feedback Amplifier Operation

Recognizing that G

× R

the first order that bandwidth for this amplifier is independent

of gain (G).

Considering that additional poles contribute excess phase at

high frequencies, there is a minimum feedback resistance below

which peaking or oscillation may result. This fact is used to

parasitic capacitance at the inverting input terminal will also add

can be difficult.

Achieving and maintaining gain flatness of better than 0.1 dB at

frequencies above 10 MHz requires careful consideration of

several issues.