REV. C

–10–

AD828

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 consists of a degenerated NPN differential pair

driving matched PNPs in a folded-cascade gain stage (Figure 4).

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 4. Simplified Schematic

INPUT CONSIDERATIONS

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

continuous overload voltages exceeding the

±6 V maximum dif-

ferential limit. This resistor provides protection for the input

transistors by limiting their maximum base current.

For high performance circuits, the “balancing” resistor should be

used to reduce the offset errors caused by bias current flowing

through the input and feedback resistors. The balancing resistor

a matched impedance at each input 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 feed-

back resistor of the other amplifier, since this greatly reduces

interamp coupling.

Choosing Feedback and Gain Resistors

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 capaci-

tance may cause peaking, a small capacitor (1 pF to 5 pF) may

should be avoided, because of their tendency to increase interlead

capacitance.

Power Supply Bypassing

Proper power supply decoupling is critical to preserve the

integrity 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

minimum. These measures greatly reduce undesired inductive

effects on the amplifier’s response.

Though two 0.1

µF capacitors will typically be effective in

decoupling 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 employ-

ing the circuit in Figure 5. 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 5. Parallel Amp Configuration