FUNCTIONAL DESCRIPTION
The ALD2722E/ALD2722 utilizes EPADs as in-circuit
elements for trimming of offset voltage bias characteristics.
Each ALD2722E/ALD2722 operational amplifier 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
basic EPAD device is a monotonically adjustable (offset
voltage trimming) programmable device, the V
OS
of the
ALD2722E can be adjusted many times in both directions.
Once programmed, the set V
OS
levels are stored
permanently, even when the device 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 stand-alone, with user application-specific
programming or in-system programming conditions. User
application-specific circuit programming refers to a
situation where the Total Input Offset Voltage of the
ALD2722E can be trimmed with the actual intended operat-
ing conditions.
Take the example of an application circuit that uses + 5V
and -5V power supplies, an operational amplifier input
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 while it is being electrically trimmed. Any
error in V
OS
due to these bias conditions can be automati-
cally zeroed out. The Total V
OS
error, V
OST
, is now limited
only by the adjustable range and the stability of V
OS
, and
the input noise voltage of the operational amplifier. This
Total Input Offset Voltage now includes V
OS
, as V
OS
is
traditionally specified; plus the V
OS
error contributions from
PSRR, CMRR, TCV
OS
, and noise. Typically, V
OST
ranges
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 both normal
mode and in programming mode. One of the benefits of in-
system programming is that not only the ALD2722E offset
voltage from operating bias conditions has been accounted
for, any residual errors introduced by other circuit
components, such as resistor or sensor induced voltage
errors, can also be programmed and corrected. In this way,
the “in-system” circuit output can be adjusted to a desired
level eliminating need for another trimming function.
Functional Description of ALD2722
The ALD2722 is pre-programmed at the factory under
standard operating conditions for minimum equivalent input
offset voltage. The ALD2722 offers similar programmable
features as the ALD2722E, but with more limited offset
voltage program 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 ALD2722E/ALD2722 has four additional pins,
compared 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
1 to 2 Volts. 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 1 Volt to 4 Volts.
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 either 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.
2
Advanced Linear Devices
ALD2722E/ALD2722
ALD [ ADVANCED LINEAR DEVICES ]
