AD8601/AD8602

–13–

REV. 0

SETTLING TIME – ns

5

5

3

0

1

3

4

2

1

0

1,000

200

400

600

800

0.1%

0.01%

0.01%

0.1%

2

4

TPC 49. Output Swing vs. Settling Time

THEORY OF OPERATION

The AD8601/AD8602 family of amplifiers are rail-to-rail input and

output precision CMOS amplifiers specified from 2.7 V to 5.0 V of

power supply voltage. These amplifiers use Analog Devices’ propri-

etary technology called DigiTrim™ to achieve a higher degree of

precision than available from most CMOS amplifiers. DigiTrim

technology is a method of trimming the offset voltage of the

amplifier after it has already been assembled. The advantage in

post-package trimming lies in the fact that it corrects any offset

voltages due to the mechanical stresses of assembly. This tech-

nology is scalable and utilized with every package option, including

SOT23-5, providing lower offset voltages than previously achieved in

these small packages.

The DigiTrim process is done at the factory and does not add

additional pins to the amplifier. All AD860x amplifiers are avail-

able in standard op amp pinouts, making DigiTrim completely

transparent to the user. The AD860x can be used in any preci-

sion op amp application.

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 voltage swing

of the output stage is also rail-to-rail and is achieved by using an

NMOS and PMOS transistor pair connected 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 supply rail. This is a characteristic of

all rail-to-rail output amplifiers. With 1 mA of output current, the

output voltage can reach within 20 mV of the positive rail and

15 mV of the negative rail.

The open-loop gain of the AD860x is 100 dB, typical, with a load

of 2 k

Ω. Because of the rail-to-rail output configuration, the gain

of the output stage, and thus the open-loop gain of the amplifier,

is dependent on the load resistance. Open-loop gain will decrease

with smaller load resistances. Again, this is a characteristic inher-

ent to all rail-to-rail output amplifiers.

Rail-to-Rail Input Stage

The input common-mode voltage range of the AD860x 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 stage are separately trimmed using

DigiTrim to minimize the offset voltage in both differential pairs.

Both NMOS and PMOS input differential pairs are active in a

500 mV transition region, when the input common-mode voltage

is between approximately 1.5 V and 1 V below the positive supply

voltage. Input offset voltage will shift slightly in this transition

region, as shown in Figures 5 and 6. Common-mode rejection

ratio will also be slightly lower when the input common-mode

voltage is within this transition band. Compared to the Burr

Brown OPA2340 rail-to-rail input amplifier, shown in Figure 1,

the AD860x, shown in Figure 2, exhibits lower offset voltage shift

across the entire input common-mode range, including the transi-

tion region.

0.7

0.4

1.4

0

5

1

23

4

0.2

0.5

0.8

1.1

0.1

Figure 1. Burr Brown OPA2340UR Input Offset Voltage

vs. Common-Mode Voltage, 24 SOIC Units @ 25

°C

0.7

0.4

1.4

0

5

1

23

4

0.2

0.5

0.8

1.1

0.1

Figure 2. AD8602AR Input Offset Voltage vs.

Common-Mode Voltage, 300 SOIC Units @ 25

°C

DigiTrim is a trademark of Analog Devices.