ALD1722/ALD1722G

Advanced Linear Devices

4 of 9

Design & Operating Notes:

1. The ALD1722/ALD1722G 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 ALD1722/ALD1722G 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. A unity gain buffer using the

ALD1722/ALD1722G will typically drive 400pF of external load ca-

pacitance without stability problems. In the inverting unity gain con-

figuration, it can drive up to 800pF of load capacitance. Compared to

other CMOS operational amplifiers, the ALD1722/ALD1722G has

shown itself to be more resistant to parasitic oscillations.

2. The ALD1722/ALD1722G 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

compatibility with other operational amplifiers, this switching point has

been selected to be about 1.5V above the negative supply voltage.

Since offset voltage trimming on the ALD1722/ALD1722G 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 excursions below this switching point. The user should how-

ever, 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. ALD1722/ALD1722G 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 to not exceed 0.3V of the power

supply voltage levels.

6. The ALD1722/ALD1722G has an internal design architecture that

provides robust high temperature operation. Contact factory for

custom screening versions.

TYPICAL PERFORMANCE CHARACTERISTICS

INPUT BIAS CURRENT AS A FUNCTION

OF AMBIENT TEMPERATURE

AMBIENT TEMPERATURE (

°C)

100

10

1.0

0.01

0.1

100

-25

0

75

125

50

25

-50

1000

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

0S

OPEN LOOP VOLTAGE GAIN AS A FUNCTION

OF SUPPLY VOLTAGE AND TEMPERATURE

SUPPLY VOLTAGE (V)

1000

100

10

1

0

±2

±4

±6

°C

°C

°C

±8

SUPPLY CURRENT AS A FUNCTION

OF SUPPLY VOLTAGE

SUPPLY VOLTAGE (V)

2.5

2.0

1.0

1.5

0

0.5

0

±1

±2

±3

±4

±5

±6

-25

°C

+25

°C

+80

°C

+125

°C

INPUTS GROUNDED

OUTPUT UNLOADED

TA = -55ºC