AD8651/AD8652

Rev. C | Page 14 of 20

APPLICATIONS

THEORY OF OPERATION

The AD865x family consists of voltage feedback, rail-to-rail

input and output precision CMOS amplifiers that operate from

2.7 V to 5.5 V of power supply voltage. These amplifiers use

Analog Devices, Inc. DigiTrim technology to achieve a higher

degree of precision than is available from most CMOS

amplifiers. DigiTrim technology, used in a number of Analog

Devices 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 AD865x family is 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 op amp to extend

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

of the AD865x with a load of 1 kΩ is typically 115 dB.

The AD865x can be used in any precision op amp application.

The amplifiers do 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 AD865x is a great choice for high resolution data

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

precision offset, and high speed make them superb preamps for

fast photodiode applications. The speed and output drive

capabilities of the AD865x also make the amplifiers 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 AD865x 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, the

AD865x, shown in Figure 54, 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

Figure 53. Input Offset Distribution over Common-Mode

Voltage for the OPA350

COMMON-MODE VOLTAGE (V)

0

–600

–200

200

600

400

0

–400

2

14

35

6

Figure 54. Input Offset Distribution over Common-Mode

Input Protection for the AD865x