ADA4856-3

Rev. 0 | Page 12 of 20

THEORY OF OPERATION

The ADA4856-3 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 ADA4856-3 extends from 200 mV below the negative

rail to about 1 V from the positive rail. This feature makes the

ADA4856-3 ideal for low voltage single-supply applications. In

addition, this new input stage does not sacrifice noise performance

for slew rate. At 14 nV/√Hz, the ADA4856-3 is one of the

lowest noise rail-to-rail output video amplifiers in the market.

Besides a novel input stage, the ADA4856-3 employs the Analog

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

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

op amp to drive up to three video loads to within 300 mV from

both rails. In addition, this output stage provides the amplifier

with very fast overdrive characteristics, an important property

in video applications.

The ADA4856-3 comes in a 16-lead LFCSP that has an exposed

thermal pad for lower operating temperature. This pad is connected

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

layout problems, the ADA4856-3 features a new pinout flow that

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

feedback and gain resistors are on-chip, which minimizes the

number of components needed and improves the design layout.

The ADA4856-3 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 32. High Level Design Schematic