FUNCTIONAL DESCRIPTION
The ALD2721E/ALD2721 utilizes EPADs as in-circuit
elements for trimming of offset voltage bias characteristics.
Each ALD2721E/ALD2721 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 devices is monotonically adjustable, the V
OS
of the
ALD2721E 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 ALD2721E
The ALD2721E 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 ALD2721E 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 ALD2721E can be trimmed with the actual
intended operating conditions.
Take the example of an application circuit that uses + 1V and
-1V 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 environ-
mental chamber with the ALD2721E inserted into a test socket
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 opera-
tional 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, this Total V
OS
error term ranges approximately
±35µV
for the ALD2721E.
In-System Programming refers to the condition where the
EPAD adjustment is made after the ALD2721E has been
inserted into a circuit board. In this case, the circuit design must
provide for the ALD2721E to operate in both normal mode and
in programming mode. One of the benefits of in-system
programming is that not only the ALD2721E 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 ALD2721
The ALD2721 is pre-programmed at the factory under
standard operating conditions for minimum equivalent input
offset voltage. The ALD2721 offers similar programmable
features as the ALD2721E, but with more limited offset
voltage program range. It is intended for standard opera-
tional amplifier applications where little or no electrical
programming by the user is necessary.
USER PROGRAMMABLE V
OS
FEATURE
Each ALD2721E/ALD2721 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 3 Volts.
VExx pins are programming pins, used during electrical
programming mode to inject charge into the internal EPADs.
Increasing voltage on VE1A/VE1B increases the offset volt-
age whereas increasing voltage on VE2A/VE2B decreases
the offset voltage of op amp A and op amp B, respectively.
The injected charge is then permanently stored. After pro-
gramming, 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 ex-
ceeded. 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 ALD2721E 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
ALD2721E/ALD2721
ALD [ ADVANCED LINEAR DEVICES ]
