AD816

REV. B

–11–

Table I. Driver Resistor Values

G = +1

604

∞

–1

499

499

+2

499

499

+5

499

125

+10

1k

110

DRIVER DC ERRORS AND NOISE

There are three major noise and offset terms to consider in a

current feedback amplifier. For offset errors refer to the equa-

tion below. For noise error the terms are root-sum-squared to

give a net output error. In the circuit below (Figure 43), they

and subsequently multiplied by the noise gain always appear at

less than 4 nV/

√Hz. At low gains, however, the inverting input

layout and device matching contribute to better offset and drift.

The typical performance curves in conjunction with the equations

below can be used to predict the performance of the AD816 in

any application.









± I

1

+









± I

VIO

AD816

DRIVERS

Figure 43. Driver Output Offset Voltage

THEORY OF OPERATION (RECEIVER)

Each AD816 receiver is a wide band high performance opera-

tional amplifier. It also provides a constant slew rate, bandwidth

and settling time over its entire specified temperature range.

The AD816 receiver consists of a degenerated NPN differential

pair driving matched PNPs in a folded-cascode gain stage. The

output buffer stage employs emitter followers in a class AB

amplifier which deliver the necessary current to the load while

maintaining low levels of distortion.

A protection resistor in series with the noninverting input is

required in circuits where the input to the receiver could be

subject to transients on continuous overload voltages exceeding

the

±6 V maximum differential limit. The resistor provides

protection for the input transistors, by limiting their maximum

base current.

THEORY OF OPERATION (DRIVER)

The AD816 driver is a dual current feedback amplifier with high

(500 mA) output current capability. Being a current feedback

amplifier, the AD816 driver’s open-loop behavior is expressed

as transimpedance,

∆V

impedance behaves just as the open-loop voltage gain of a volt-

age feedback amplifier, that is, it has a large dc value and de-

creases at roughly 6 dB/octave in frequency.

× g

of the input stage. Figure 42 shows the driver connected as a

follower with gain. Basic analysis yields the following results:

V

O

V

IN

= G ×

T

()

T

where:

R

F

R

G

≈ 25 Ω

Figure 42. 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

phase in the feedback loop so that picking an optimum value for

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

The fine scale gain flatness will, to some extent, vary with

feedback resistance. It is therefore 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 gain configurations. These should be used as a

starting point in any application.