GENERAL
DESCRIPTION (cont'd)
an ultra-high input impedance voltage follower/buffer that
allows input and output voltage swings from positive to
negative supply voltages. This feature is intended to greatly
simplify systems design and eliminate higher voltage power
supplies in many applications.
Each ALD2724E/ALD2724 operational amplifier features
individual, user-programmable offset voltage trimming re-
sulting in significantly enhanced total system performance
and user flexibility. EPAD technology is an exclusive ALD
design which has been refined for analog applications where
precision voltage trimming is necessary to achieve a desired
performance. It utilizes CMOS FETs as in-circuit elements
for trimming of offset voltage bias characteristics with the aid
of a personal computer under software control. Once
programmed, the set parameters are stored indefinitely.
EPAD offers the circuit designer a convenient and cost-
effective trimming solution for achieving the very highest
amplifier/system performance.
FUNCTIONAL DESCRIPTION
The ALD2724E/ALD2724 utilizes EPADs as in-circuit ele-
ments for trimming of offset voltage bias characteristics.
Each ALD2724E/ALD2724 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 ALD2724E
can be adjusted many times in both directions. Once pro-
grammed, the set V
OS
levels are stored permanently, even
when the device power is removed.
Functional Description of ALD2724E/ALD2724
The ALD2724E 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 ALD2724E is an operational amplifier that can be
trimmed stand-alone, with user application-specific pro-
gramming or in-system programming conditions. User appli-
cation-specific circuit programming refers to a situation
where the Total Input Offset Voltage of the ALD2724E can be
trimmed with the actual intended operating 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 environ-
mental chamber with the ALD2724E 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, 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 ALD2724E.
In-System Programming refers to the condition where the
EPAD adjustment is made after the ALD2724E has been
2
inserted into a circuit board. In this case, the circuit design
must provide for the ALD2724E to operate in both normal
mode and in programming mode. One of the benefits of in-
system programming is that not only the ALD2724E offset
voltage from operating bias conditions has been accounted
for, any residual errors introduced by other circuit compo-
nents, 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.
The ALD2724 is pre-programmed at the factory under
standard operating conditions for minimum equivalent input
offset voltage. The ALD2724 offers similar programmable
features as the ALD2724E, 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 ALD2724E/ALD2724 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.
During programming, voltages on VExx pins are increased
incrementally to program the offset voltage of the opera-
tional 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. 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.
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. If needed, this requirement can be
accomplished by adding resistors at certain appropriate
circuit nodes.
Advanced Linear Devices
ALD2724E/ALD2724
ALD [ ADVANCED LINEAR DEVICES ]
