AD828

REV. B

–10–

THEORY OF OPERATION

The AD828 is a low cost, dual video operational amplifier

designed to excel in high performance, high output current video

applications.

The AD828 (Figure 40) consists of a degenerated NPN differen-

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

The output buffer stage employs emitter followers in a class AB

amplifier that delivers the necessary current to the load while

maintaining low levels of distortion.

The AD828 will drive terminated cables and capacitive loads of

10 pF or less. As the closed-loop gain is increased, the AD828

will drive heavier cap loads without oscillating.

–IN

+IN

OUTPUT

Figure 40. AD828 Simplified Schematic

INPUT CONSIDERATIONS

cuits where the input to the AD828 will be subjected to transient

or continuous overload voltages exceeding the

±6 V maximum

differential limit. This resistor provides protection for the input

transistors by limiting their maximum base current.

For high performance circuits, it is recommended that a “bal-

ancing” resistor be used to reduce the offset errors caused by

bias current flowing through the input and feedback resistors.

terminal. The offset voltage error will then be reduced by more

than an order of magnitude.

APPLYING THE AD828

The AD828 is a breakthrough dual amp that delivers precision

and speed at low cost with low power consumption. The AD828

offers excellent static and dynamic matching characteristics,

combined with the ability to drive heavy resistive loads.

As with all high frequency circuits, care should be taken to main-

tain overall device performance as well as their matching. The

following items are presented as general design considerations.

Circuit Board Layout

Input and output runs should be laid out so as to physically

isolate them from remaining runs. In addition, the feedback

resistor of each amplifier should be placed away from the feedback

resistor of the other amplifier, since this greatly reduces interamp

coupling.

Choosing Feedback and Gain Resistors

In order to prevent the stray capacitance present at each

amplifier’s summing junction from limiting its performance, the

feedback resistors should be

≤ 1 kΩ. Since the summing junction

capacitance may cause peaking, a small capacitor (1 pF–5 pF)

may be paralleled with Rf to neutralize this effect. Finally, sock-

ets should be avoided, because of their tendency to increase

interlead capacitance.

Power Supply Bypassing

Proper power supply decoupling is critical to preserve the integ-

rity of high frequency signals. In carefully laid out designs,

decoupling capacitors should be placed in close proximity to the

supply pins, while their lead lengths should be kept to a mini-

mum. These measures greatly reduce undesired inductive effects

on the amplifier’s response.

Though two 0.1

µF capacitors will typically be effective in de-

coupling the supplies, several capacitors of different values can

be paralleled to cover a wider frequency range.

PARALLEL AMPS PROVIDE 100 mA TO LOAD

By taking advantage of the superior matching characteristics of

the AD828, enhanced performance can easily be achieved by

employing the circuit in Figure 41. Here, two identical cells are

paralleled to obtain even higher load driving capability than that

of a single amplifier (100 mA min guaranteed). R1 and R2 are

included to limit current flow between amplifier outputs that

would arise in the presence of any residual mismatch.

2

3

8

1k

R2

5

1/2

AD828

1/2

AD828

1 F

0.1 F

7

5

6

1

1 F

0.1 F

4

R1

5

1k

1k

1k

Figure 41. Parallel Amp Configuration