ADA4855-3

Rev. 0 | Page 14 of 20

THEORY OF OPERATION

The ADA4855 is a voltage feedback op amp that employs a new

input stage that achieves a high slew rate while maintaining a

wide common-mode input range. The input common-mode

range of the ADA4855 extends from 200 mV below the negative

rail to 1 V below the positive rail. This feature makes the ADA4855

ideal for single-supply applications. In addition, this new input

stage does not sacrifice noise performance for slew rate. At

6.8 nV/√Hz, the ADA4855 is one of the lowest noise rail-to-

rail output video amplifiers in the market.

Besides a novel input stage, the ADA4855 employs the Analog

Devices, Inc., patented rail-to-rail output stage. This output

stage makes efficient use of the power supplies, allowing the op

amp to drive up to three video loads to within 350 mV of the

positive power rail. In addition, this output stage provides the

amplifier with very fast overdrive characteristics, which is an

important property in video applications.

The ADA4855 comes in a 16-lead LFCSP that has an exposed

thermal pad for lower operating temperature. This pad is internally

connected to the negative rail. To avoid printed circuit board (PCB)

layout problems, the ADA4855 features a new pinout flow that

is optimized for video applications. As shown in Figure 4, the

noninverting input and output pins of each amplifier are

adjacent to each other for ease of layout.

The ADA4855 is fabricated in Analog Devices dielectrically

isolated eXtra Fast Complementary Bipolar 3 (XFCB3) process,

which results in the outstanding speed and dynamic range

displayed by the amplifier.

OUT

+IN

–IN

C

R

Figure 46. High Level Design Schematic