ALD1726E Advanced Linear Devices 2 of 13
FUNCTIONAL DESCRIPTION
The ALD1726E uses EPADs as in-circuit elements for trim-
ming of offset voltage bias characteristics. Each ALD1726E
has a pair of EPAD-based circuits 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. While each
of the EPAD devices is a monotonically adjustable program-
mable 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 is pre-programmed at the factory under
standard operating conditions for minimum equivalent input
offset voltage. It also has a guaranteed offset voltage pro-
gram range, which is ideal for applications that require
electrical offset voltage programming.
The ALD1726E is an operational amplifier that can be
trimmed with user application-specific programming or in-
system programming conditions. User application-specific
circuit programming refers to the situation where the Total
Input Offset Voltage of the ALD1726E can be trimmed with
the actual intended operating conditions.
For example, an application circuit may have +6V and -2.5V
power supplies, and the operational amplifier input is biased
at +0.7V, and an average operating temperature at 55
°C.
The circuit can be wired up to these conditions within an
environmental chamber with ALD1726E 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 contributions 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
trimming is able to provide much improved system perfor-
mance by reducing these other sources of error to provide
significantly 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
resistor or sensor induced voltage errors, can also be cor-
rected. In this way, the “in-system” circuit output can be
adjusted to a desired level, eliminating the need for another
trimming function.
USER PROGRAMMABLE V
OS
FEATURE
Each ALD1726E has two pins named VE1 and VE2 which are
internally connected to an internal offset bias circuit. VE1/
VE2 have initial typical values of 1.0V to 1.5V. The voltage
on these pins can be programmed using the ALD E100 EPAD
Programmer and the appropriate Adapter Module. The
useful programming range of VE1 and VE2 is 1.2V to 3.0V.
VE1 and VE2 pins are programming pins, used during
programming mode to inject charge into the internal EPADs.
Increasing voltage on VE1 decreases the offset voltage
whereas increasing voltage on VE2 increases 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 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 program-
ming 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 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.
