FUNCTIONAL DESCRIPTION
The ALD2722E/ALD2722 uses EPADs as in-circuit ele-
ments for trimming of offset voltage bias characteristics.
Each ALD2722E/ALD2722 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. While each of the EPAD devices is a
monotonically adjustable programmable device, the V
OS
of
the ALD2722E can be adjusted many times in both direc-
tions. Once programmed, the set V
OS
levels are stored
permanently, even when the device power is removed.
Functional Description of ALD2722E
The ALD2722E 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.
The ALD2722E 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 ALD2722E can be trimmed with
the actual intended operating conditions.
For example, an application circuit may have +5V and -5V
power supplies, and the operational amplifier input is biased
at +1V, and an average operating temperature at +85
°
C.
The circuit can be wired up to these conditions within an
environmental chamber with the ALD2722E inserted into a
test socket connected to this circuit while it is being electri-
cally 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 (V
OST
) is approximately
±25µV
for the ALD2722E.
In-System Programming refers to the condition where the
EPAD adjustment is made after the ALD2722E has been
inserted into a circuit board. In this case, the circuit design
must provide for the ALD2722E to operate in normal mode
and in programming mode. One of the benefits of in-system
programming is that not only is the ALD2722E 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.
Functional Description of ALD2722
The ALD2722 is pre-programmed at the factory under stan-
dard operating conditions for minimum equivalent input off-
set voltage. The ALD2722 offers similar programmable
features as the ALD2722E, but with a more limited offset
voltage program range. In is intended for standard opera-
tional amplifier applications, where little or no electrical
porggramming by the user is necessary.
USER PROGRAMMABLE V
OS
FEATURE
Each ALD2722E/ALD2722 has four additional pins, com-
pared to a conventional dual operational amplifier which has
eight pins. These four additional pins are named VE1A,
VE2A for op amp A and VE1B, VE2B for op amp B. Each of
these pins VE1A, VE2A, VE1B, VE2B (represented by VExx)
are connected to a separate, internal offset bias circuit. VExx
pins have initial internal bias voltage values of approximately
1V to 2V. The voltage on these pins can be programmed
using the ALD E100 EPAD Programmer and the appropriate
Adapter Module. The useful programming range of voltages
on VExx pins are 1V to 4V.
VExx pins are programming pins, used during electrical
programming mode to inject charge into the internal EPADs.
Increasing voltage on VE1A/VE1B decreases the offset
voltage whereas increasing voltage on VE2A/VE2B in-
creases the offset voltage of op amp A and op amp B,
respectively. The injected charge is then permanently
stored. After programming, VExx pins must be left open in
order for these voltages to remain at the programmed levels.
During programming, voltages on VExx pins are increased
incrementally to program the offset voltage of the operational
amplifier to the desired V
OS
. Note that desired V
OS
can be
any value within the offset voltage programmable ranges,
and can be equal 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 VE1x or VE2x
programming voltage range has not been exceeded. VExx
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 ALD2722E 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.
ALD2722E/ALD2722
Advanced Linear Devices
2 of 13
ALD [ ADVANCED LINEAR DEVICES ]
