10

Advanced Linear Devices

ALD2724E/ALD2724

DEFINITIONS AND APPLICATION NOTES:

1. Initial Input Offset Voltage is the initial offset voltage of the

ALD2724E/ALD2724 operational amplifier when shipped from

the factory. The device has been pre-programmed and tested

for programmability.

2. Offset Voltage Program Range is the range of adjustment of

user specified target offset voltage. This is typically an adjust-

ment in either the negative or positive direction of the input offset

voltage from an initial input offset voltage. The input offset

programming pins, VE1A/VE1B or VE2A/VE2B change the

input offset voltages in the negative or positive direction, for

each of the amplifier A or B, respectively. User specified target

offset voltage can be any offset voltage within this programming

range.

3. Programmed Input Offset Voltage Error is the final offset

voltage error after programming when the Input Offset Voltage

is at target Offset Voltage. This parameter is sample tested.

4. Total Input Offset Voltage is the same as Programmed Input

Offset Voltage, corrected for system offset voltage error. Usu-

ally this is an all inclusive system offset voltage, which also

includes offset voltage contributions from input offset voltage,

PSRR, CMRR, TCVOS and noise. It can also include errors

introduced by external components, at a system level. Pro-

grammed Input Offset Voltage and Total Input Offset Voltage is

not necessarily zero offset voltage, but an offset voltage set to

compensate for other system errors as well. This parameter is

sample tested.

5. The Input Offset and Bias Currents are essentially input

protection diode reverse bias leakage currents. This low input

bias current assures that the analog signal from the source will

not be distorted by it. For applications where source impedance

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

through proper shielding.

6. Input Voltage Range is determined by two parallel comple-

mentary input stages that are summed internally, each stage

having a separate input offset voltage. While Total Input Offset

Voltage can be trimmed to a desired target value, it is essential

to note that this trimming occurs at only one user selected input

bias voltage. Depending on the selected input bias voltage

relative to the power supply voltages, offset voltage trimming

may affect one or both input stages. For the ALD2724E/

ALD2724, the switching point between the two stages occur at

approximately 1.5V above negative supply voltage.

7. Input Offset Voltage Drift is the average change in Total Input

Offset Voltage as a function of ambient temperature. This

parameter is sample tested.

8. Initial PSRR and initial CMRR specifications are provided as

reference information. After programming, error contribution to

the offset voltage from PSRR and CMRR is set to zero under the

specific power supply and common mode conditions, and

becomes part of the Programmed Input Offset Voltage Error.

9. Average Long Term Input Offset Voltage Stability is based on

input offset voltage shift through operating life test at 125

°C

°C, assuming activation energy of

1.0eV. This parameter is sample tested.

DESIGN NOTES:

A. The ALD2724E/ALD2724 is internally compensated for unity

gain stability using a novel scheme which produces a single pole

role off in the gain characteristics while providing more than 70

degrees of phase margin at unity gain frequency. A unity gain

buffer using the ALD2724E/ALD2724 will typically drive 400pF

of external load capacitance.

B. The ALD2724E/ALD2724 has complementary p-channel

and n-channel input differential stages connected in parallel to

accomplish rail-to-rail input common mode voltage range. The

switching point between the two differential stages is 1.5V

above negative supply voltage. For applications such as invert-

ing amplifier or non-inverting amplifier with a gain larger than 2.5

(5V operation), the common mode voltage does not make

excursions below this switching point. However, this switching

does take place if the operational amplifier is connected as a rail-

to-rail unity gain buffer and the design must allow for input offset

voltage variations.

C. The output stage consists of class AB complementary output

drivers. The oscillation resistant feature, combined with the rail-

to-rail input and output feature, makes the ALD2724E/ALD2724

an effective analog signal buffer for high source impedance

sensors, transducers, and other circuit networks.

D. The ALD2724E/ALD2724 has static discharge protection.

Care must be exercised when handling the device to avoid

strong static fields that may degrade a diode junction, causing

increased input leakage currents. 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.

E. VExx are high impedance terminals, as the internal bias

currents are set very low to a few microamperes to conserve

power. For some applications, these terminals may need to be

shielded from external coupling sources. For example, digital

signals running nearby may cause unwanted offset voltage

fluctuations. Care during the printed circuit board layout to place

ground traces around these pins and to isolate them from digital

lines will generally eliminate such coupling effects. In addition,

optional decoupling capacitors of 1000pF or greater value can

be added to VExx terminals.

F. The ALD2724E/ALD2724 is designed for use in low voltage,

micropower circuits. The maximum operating voltage during

normal operation should remain below 10 Volts at all times. Care

should be taken to insure that the application in which the device

is used do not experience any positive or negative transient

voltages that will cause any of the terminal voltages to exceed

this limit.

G. All inputs or unused pins except VExx pins should be

connected to a supply voltage such as Ground so that they do

not become floating pins, since input impedance at these pins

is very high. If any of these pins are left undefined, they may

cause unwanted oscillation or intermittent excessive current

drain. As these devices are built with CMOS technology, normal

operating and storage temperature limits, ESD and latchup

handling precautions pertaining to CMOS device handling

should be observed.