AD8651/AD8652

Rev. B | Page 14 of 20

APPLICATIONS

THEORY OF OPERATION

The AD8651 amplifier is a voltage feedback, rail-to-rail input

and output precision CMOS amplifier that operates from 2.7 V

to 5.0 V of power supply voltage. This amplifier uses 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

has been assembled. The advantage of post-package trimming is

that it corrects any offset voltages caused by the mechanical

stresses of assembly.

The AD8651 is available in standard op amp pinout, making

DigiTrim completely transparent to the user. The input stage of

the amplifier is a true rail-to-rail architecture, allowing the

input common-mode voltage range of the op amp to extend to

both positive and negative supply rails. The open-loop gain of

the AD8651/AD8652 with a load of 1 kΩ is typically 115 dB.

The AD8651 can be used in any precision op amp application.

The amplifier does not exhibit phase reversal for common-

mode voltages within the power supply. With voltage noise of

4.5 nV/√Hz and –105 dB distortion for 10 kHz, 2 V p-p signals,

the AD8651/AD8652 is a great choice for high resolution data

acquisition systems. Its low noise, sub-pA input bias current,

precision offset, and high speed make it a superb preamp

for fast photodiode applications. The speed and output

drive capability of the AD8651 also make it useful in

video applications.

Rail-to-Rail Output Stage

The voltage swing of the output stage is rail-to-rail and is

achieved by using an NMOS and PMOS transistor pair con-

nected in a common source configuration. The maximum

output voltage swing is proportional to the output current, and

larger currents will limit how close the output voltage can get to

the proximity of the output voltage to the supply rail. This is a

characteristic of all rail-to-rail output amplifiers. With 40 mA of

output current, the output voltage can reach within 5 mV of the

positive and negative rails. At light loads of >100 kΩ, the output

swings within ~1 mV of the supplies.

Rail-to-Rail Input Stage

The input common-mode voltage range of the AD8651 extends

to both positive and negative supply voltages. This maximizes

the usable voltage range of the amplifier, an important feature

for single-supply and low voltage applications. This rail-to-rail

input range is achieved by using two input differential pairs, one

NMOS and one PMOS, placed in parallel. The NMOS pair is

active at the upper end of the common-mode voltage range,

and the PMOS pair is active at the lower end of the common-

mode range.

The NMOS and PMOS input stages are separately trimmed

using DigiTrim to minimize the offset voltage in both differen-

tial pairs. Both NMOS and PMOS input differential pairs are

active in a 500 mV transition region when the input common-

mode voltage is approximately 1.5 V below the positive supply

voltage. A special design technique improves the input offset

voltage in the transition region that traditionally exhibits a

ratio is improved within this transition band. Compared to the

Burr Brown OPA350 amplifier, shown in Figure 53 (A), the

AD8651, shown in Figure 53 (B), exhibits much lower offset

voltage shift across the entire input common-mode range,

including the transition region.

COMMON-MODE VOLTAGE (V)

0

–600

–200

200

600

400

0

–400

2

14

35

6

COMMON-MODE VOLTAGE (V)

0

–600

–200

200

600

400

0

–400

2

14

35

6

(A) OPA350 VOS vs. VCM

(B) AD8651 VOS vs. VCM

Figure 53. Input Offset Distribution over Common-Mode Voltage