AD8601/AD8602/AD8604

Rev. G | Page 15 of 24

THEORY OF OPERATION

The AD8601/AD8602/AD8604 family of amplifiers are rail-to-rail

input and output, precision CMOS amplifiers that operate from

2.7 V to 5.0 V of the power supply voltage. These amplifiers use

Analog Devices, Inc., DigiTrim® technology 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 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 technology is

scalable and used with every package option, including the 5-lead

SOT-23, providing lower offset voltages than previously achieved in

these small packages.

The DigiTrim process is completed at the factory and does not

add additional pins to the amplifier. All AD860x amplifiers are

available in standard op amp pinouts, making DigiTrim completely

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

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

limit how close the output voltage can get to the supply rail,

which 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 within 15 mV of the

negative rail. At light loads of >100 kΩ, the output swings

within ~1 mV of the supplies.

The open-loop gain of the AD860x is 80 dB, typical, with a load

of 2 kΩ. Because of the rail-to-rail output configuration, the gain

of the output stage and the open-loop gain of the amplifier are

dependent on the load resistance. Open-loop gain decreases with

smaller load resistances. Again, this is a characteristic inherent

to all rail-to-rail output amplifiers.

RAIL-TO-RAIL INPUT STAGE

The input common-mode voltage range of the AD860x extends

to both the 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.

The NMOS and PMOS input stages 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. The input offset voltage shifts slightly in this transition

region, as shown in Figure 9 and Figure 10 .The common-mode

rejection ratio is also slightly lower when the input common-

mode voltage is within this transition band. Compared to the

Burr-Brown OPA2340UR rail-to-rail input amplifier, shown in

Figure 54, the AD860x, shown in Figure 55, exhibits lower

offset voltage shift across the entire input common-mode

range, including the transition region.

0.7

0.4

0.1

–0.2

–0.5

–0.8

–1.1

–1.4

01

2

3

45

Figure 54. Burr-Brown OPA2340UR Input Offset Voltage vs.

Common-Mode Voltage, 24 SOIC Units @ 25°C

0.7

0.4

0.1

–0.2

–0.5

–0.8

–1.1

–1.4

01

2

3

45

Figure 55. AD8602AR Input Offset Voltage vs. Common-Mode Voltage,

300 SOIC Units @ 25°C