ALD1726PA datasheet(2/10 Pages) ALD

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Advanced Linear Devices

ALD1726E/ALD1726

Functional Description of ALD1726E

While each of the EPAD devices is a monotonically adjust-

able programmable device, the VOS of the ALD1726E can be

adjusted many times in both directions. Once programmed,

the set VOS levels are stored permanently, even when the

device power is removed.

The ALD1726E provides the user with an operational ampli-

fier that can be trimmed with user application-specific pro-

gramming or in-system programming conditions. User appli-

cation-specific circuit programming refers to the situation

where the Total Input Offset Voltage of the ALD1726E (VOST)

can be trimmed with the actual intended operating conditions.

The ALD1726E is pre-programmed at the factory under

standard operating conditions for minimum equivalent input

offset voltage. It also

has

a guaranteed offset voltage

program range, which is ideal for applications that require

electrical offset voltage programming.

For example, an application circuit may have +6V and -2.5V

power supplies, and the operational amplifier input is biased

at +0.7V, and the average operating temperature is at 55

°C.

The circuit can be wired up to these conditions within an

environmental chamber, and the ALD1726E can be inserted

into a test socket connected to this circuit while it is being

electrically trimmed. Any error in VOS due to these bias

conditions can be automatically zeroed out. The Total VOS

error is now limited only by the adjustable range and the

stability of VOS, and the input noise voltage of the operational

amplifier. Therefore, this Total VOS error now includes VOS

as VOS is traditionally specified; plus the VOS error contribu-

tions from PSRR, CMRR, TCVOS, and noise. Typically this

total VOS error term (VOST) is approximately ±50µV for the

ALD1726E.

The VOS contribution due to PSRR, CMRR, TCVOS and

external components can be large for operational amplifiers

without trimming. Therefore the ALD1726E with EPAD trim-

ming is able to provide much improved system performance

by reducing these other sources of error to provide signifi-

cantly reduced VOST.

In-System Programming refers to the condition where the

EPAD adjustment is made after the ALD1726E has been

inserted into a circuit board. In this case, the circuit design

must provide for the ALD1726E to operate in normal mode

and in programming mode. One of the benefits of in-system

programming is that not only is the ALD1726E offset voltage

from operating bias conditions accounted for, any residual

errors introduced by other circuit components, such as resis-

tor or sensor induced voltage errors, can also be corrected.

In this way, the “in-system” circuit output can be adjusted to

a desired level eliminating other trimming components.

Functional Description of ALD1726

The ALD1726 is pre-programmed at the factory under stan-

dard operating conditions for minimum equivalent input offset

voltage. The ALD1726 offers similar programmable features

as the ALD1726E, but with more limited offset voltage pro-

gram range. It is intended for standard operational amplifier

applications where little or no electrical programming by the

user is necessary.

USER PROGRAMMABLE VOS FEATURE

Each ALD1726E/ALD1726 has two pins named VE1 and

VE2 which are internally connected to an internal offset bias

circuit. VE1/VE2 have initial typical values of 1.0 to 1.5 Volt.

The voltage on these pins can be programmed using the ALD

E100 EPAD Programmer and the appropriate Adapter Mod-

ule. The useful programming range of VE1 and VE2 is 1.2

Volt to 3.0 Volts. VE1 and VE2 pins are programming pins,

used during programming mode. The Programming pin is

used during electrical programming to inject charge into the

internal EPADs. Increases of VE1 decrease the offset volt-

age while increases of VE2 increase the offset voltage of the

operational amplifier. The injected charge is permanently

stored and determines the offset voltage of the operational

amplifier. After programming, VE1 and VE2 terminals must

be left open to settle on a voltage determined by internal bias

currents.

During programming, the voltages on VE1 or VE2 are in-

creased incrementally to set the offset voltage of the opera-

tional amplifier to the desired Vos. Note that desired VOS can

be any value within the offset voltage programmable ranges,

and can be either zero, a positive value or a negative value.

This VOS value can also be reprogrammed to a different

value at a later time, provided that the useful VE1 or VE2

programming voltage range has not been exceeded. VE1 or

VE2 pins can also serve as capacitively coupled input pins.

Internally, VE1 and VE2 are programmed and connected

differentially. Temperature drift effects between the two

internal offset bias circuits cancel each other and introduce

less net temperature drift coefficient change than offset

voltage trimming techniques such as offset adjustment with

an external trimmer potentiometer.

While programming, V+, VE1 and VE2 pins may be alter-

nately pulsed with 12V (approximately) pulses generated by

the EPAD Programmer. In-system programming requires the

ALD1726E/ALD1726 application circuit to accommodate

these programming pulses. This can be accomplished by

adding resistors at certain appropriate circuit nodes. For

more information, see Application Note AN1700.

FUNCTIONAL DESCRIPTION

The ALD1726E/ALD1726 uses EPADs as in-circuit ele-

ments for trimming of offset voltage bias characteristics.

Each ALD1726E/ALD1726 has a pair of EPAD-based cir-

cuits connected such that one circuit is used to adjust VOS

in one direction and the other circuit is used to adjust VOS in

the other direction.

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