ALD2701A/ALD2701B

Advanced Linear Devices

4

ALD2701

Design & Operating Notes:

1. The ALD2701 CMOS operational amplifier uses a 3 gain stage

architecture and an improved frequency compensation scheme to

achieve large voltage gain, high output driving capability, and better

frequency stability. In a conventional CMOS operational amplifier

design, compensation is achieved with a pole splitting capacitor

together with a nulling resistor. This method is, however, very bias

dependent and thus cannot accommodate the large range of supply

voltage operation as is required from a stand alone CMOS operational

amplifier. The ALD2701 is internally compensated for unity gain

stability using a novel scheme that does not use a nulling resistor. This

scheme produces a clean single pole roll off in the gain characteristics

while providing for more than 70 degrees of phase margin at the unity

gain frequency.

2. The ALD2701 has complementary p-channel and n-channel input

differential stages connected in parallel to accomplish rail to rail input

common mode voltage range. This means that with the ranges of

common mode input voltage close to the power supplies, one of the

two differential stages is switched off internally. To maintain com-

patibility with other operational amplifiers, this switching point has

been selected to be about 1.5V below the positive supply voltage.

Since offset voltage trimming on the ALD2701 is made when the input

voltage is symmetrical to the supply voltages, this internal switching

does not affect a large variety of applications such as an inverting

amplifier or non-inverting amplifier with a gain larger than 2.5 (5V

operation), where the common mode voltage does not make excur-

sions above this switching point. The user should however, be aware

that this switching does take place if the operational amplifier is

connected as a unity gain buffer, and should make provision in his

design to allow for input offset voltage variations.

3. The input bias and offset currents are essentially input protection

diode reverse bias leakage currents, and are typically less than 1pA

at room temperature. This low input bias current assures that the

analog signal from the source will not be distorted by input bias

currents. Normally, this extremely high input impedance of greater

Ω would not be a problem as the source impedance would

limit the node impedance. However, for applications where source

impedance is very high, it may be necessary to limit noise and hum

pickup through proper shielding.

4. The output stage consists of class AB complementary output drivers,

capable of driving a low resistance load. The output voltage swing is

limited by the drain to source on-resistance of the output transistors as

determined by the bias circuitry, and the value of the load resistor.

When connected in the voltage follower configuration, the oscillation

resistant feature, combined with the rail to rail input and output feature,

makes an effective analog signal buffer for medium to high source

impedance sensors, transducers, and other circuit networks.

5. The ALD2701 operational amplifier has been designed to provide full

static discharge protection. Internally, the design has been carefully

implemented to minimize latch up. However, care must be exercised

when handling the device to avoid strong static fields that may

degrade a diode junction, causing increased input leakage currents.

In using the operational amplifier, the user is advised to power up the

circuit before, or simultaneously with, any input voltages applied and

to limit input voltages not to exceed 0.3V of the power supply voltage

levels.

6. The ALD2701, with its micropower operation, offers numerous benefits

in reduced power supply requirements, less noise coupling and

current spikes, less thermally induced drift, better overall reliability

due to lower self heating, and lower input bias current. It requires

practically no warm up time as the chip junction heats up to only 0.2

°C

above ambient temperature under most operating conditions.

TYPICAL PERFORMANCE CHARACTERISTICS

SUPPLY CURRENT AS A FUNCTION

OF SUPPLY VOLTAGE

SUPPLY VOLTAGE (V)

±800

±400

±600

0

±200

0

±1

±2

±3

±4

±5

±6

-25

°C

+25

°C

+70

°C

+125

°C

INPUTS GROUNDED

OUTPUT UNLOADED

COMMON MODE INPUT VOLTAGE RANGE

AS A FUNCTION OF SUPPLY VOLTAGE

SUPPLY VOLTAGE (V)

±7

±6

±5

±4

±3

±2

±1

0

0

±1

±2

±3

±4

±5

±6

±7

OPEN LOOP VOLTAGE GAIN AS A

FUNCTION OF LOAD RESISTANCE

10M

LOAD RESISTANCE (

Ω)

10K

100K

1M

1000

100

10

1

INPUT BIAS CURRENT AS A FUNCTION

OF AMBIENT TEMPERATURE

AMBIENT TEMPERATURE (

°C)

1000

100

10

0.1

1.0

100

-25

0

75

125

50

25

-50

10000