AD8655/AD8656

Rev. A | Page 15 of 20

THEORY OF OPERATION

The AD8655/AD8656 amplifiers are voltage feedback, rail-to-

rail input and output precision CMOS amplifiers, which operate

from 2.7 V to 5.0 V of power supply voltage. These amplifiers

use the Analog Devices DigiTrim technology to achieve a

higher degree of precision than is available from most CMOS

amplifiers. DigiTrim technology, used in a number of ADI

amplifiers, is a method of trimming the offset voltage of the

amplifier after it is packaged. The advantage of post-package

trimming is that it corrects any offset voltages caused by the

mechanical stresses of assembly.

The AD8655/AD8656 are available in standard op amp pinouts,

making DigiTrim completely transparent to the user. The input

stage of the amplifiers is a true rail-to-rail architecture, allowing

the input common-mode voltage range of the amplifiers to

extend to both positive and negative supply rails. The open-

loop gain of the AD8655/AD8656 with a load of 10 kΩ is

typically 110 dB.

The AD8655/AD8656 can be used in any precision op amp

application. The amplifier does not exhibit phase reversal for

common-mode voltages within the power supply. The

AD8655/AD8656 are great choices for high resolution data

acquisition systems with voltage noise of 2.7 nV/√Hz and

THD + Noise of –103 dB for a 2 V p-p signal at 10 kHz. Their

low noise, sub-pA input bias current, precision offset, and high

speed make them superb preamps for fast filter applications.

The speed and output drive capability of the AD8655/AD8656

also make them useful in video applications.