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 V
OS
in one direction and the other circuit is used to adjust V
OS
in
the other direction.
Functional Description of ALD1726E
While each of the EPAD devices is a monotonically adjust-
able programmable device, the V
OS
of the ALD1726E can be
adjusted many times in both directions. Once programmed,
the set V
OS
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 (V
OST)
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 V
OS
due to these bias
conditions can be automatically zeroed out. The Total V
OS
error is now limited only by the adjustable range and the
stability of V
OS
, and the input noise voltage of the operational
amplifier. Therefore, this Total V
OS
error now includes V
OS
as V
OS
is traditionally specified; plus the V
OS
error contribu-
tions from PSRR, CMRR, TCV
OS
, and noise. Typically this
total V
OS
error term (V
OST
) is approximately
±50µV
for the
ALD1726E.
The V
OS
contribution due to PSRR, CMRR, TCV
OS
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 V
OST.
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 V
OS
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 V
OS
can
be any value within the offset voltage programmable ranges,
and can be either zero, a positive value or a negative value.
This V
OS
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.
2
Advanced Linear Devices
ALD1726E/ALD1726
ALD [ ADVANCED LINEAR DEVICES ]
