USER PROGRAMMABLE OFFSET VOLTAGE (VOS) OF
EPAD® OPERATIONAL AMPLIFIERS
APPLICATION NOTE AN1700
ADVANCED
LINEAR
DEVICES, INC.
INTRODUCTION
The ALD172xE is a family of monolithic single CMOS rail-
to-rail operational amplifiers with on-chip Electrically Pro-
grammable Analog Device (EPAD). They provide user
electrical offset voltage adjustment capability. With its pro-
grammable features, the programmed target VOS is not
necessarily zero, but can be defined as any desired value
to account for other system requirements, such as compen-
sation for external sensor errors, or changing desired out-
put v oltage r anges. These devices have industry standard
8 lead pinout, and in many instances can be used as direct
substitutes for a variety of operational amplifiers circuits.
ALD272xE is also available from Adv anced Linear Devices
as a family of dual EPAD operational amplifiers housed in
14 pin PDIP, SOIC, and Hermetic Ceramic DIP packages.
ALD EPAD operational amplifiers provide the user with
precision operational amplifiers that can be electrically
trimmed with user application-specific programming or in-
system programming.
Figure 1 shows the distribution of the Total Input Offset
Voltage, VOST, before and after EPAD programming. VOST
includes VOS as VOS is traditionally specified; plus the VOS
error contributions from PSRR, CMRR, TCVOS, and noise,
plus any external system level equivalent VOS error. The
ALD1722E, for example, typically has VOST equal to
approximately ±25µV.
ALD EPAD operational amplifiers are designed for low
voltage, low power systems where a precision offset
voltage trimming function is desirab le. They are used where
for economic, convenience, functionality, or for access fea-
sibility reasons, a computer controlled and automated trim-
ming capability is required. These operational amplifiers can
be programmed before being placed into a circuit, or they
can be designed into a circuit function so that “In-System
Programming” can be performed after the amplifiers and
other components have been installed onto the printed
circuit board.
For some applications, EPAD operational amplifiers are an
alternative to chopper stabilized operational amplifiers that
require expensive extra components and add extra board
space. EPAD operational amplifiers are internally DC biased
and do not contain any internal frequency clocking circuitry
that may introduce clock noise or interference . Consequently ,
there is no AC power consumed by any clocking circuitry
associated with chopper stabilized operational amplifiers.
There is also no internal null loop that can cause output
overload conditions. Furthermore, EPAD operational
amplifiers are completely self contained and require no
external components for functionality. Once they are
initially programmed by the user, EPAD operational
amplifiers do not require any periodic recalibration.
The application circuits discussed herein are intended for
demonstration of applications used in telecommunications,
instrumentation, medical devices, and industrial process
control systems. Although automation can be an important
end goal, a primary goal may be to simplify the manufactur-
ing and control process, by electrically altering an analog
circuit transfer function without resorting to a system of
micro-controllers, Rams and ROMs, EPROMs, data
converters and an overhead of circuit and system functions.
In many applications, where there is a need to eliminate
moving mechanical parts, or where access to a trimmer
potentiometer is no longer available, such as in an epoxy
potted module, adjustment of circuit parameters using EPAD
operational amplifiers is a simple and economical solution.
GENERAL DESCRIPTION
The ALD172xE and ALD272xE are monolithic CMOS
operational amplifiers capable of input offset voltage
adjustment by the user. They utilize Complementary Metal
Oxide Semiconductor Field Effect Transistor (CMOS FET)
with electrically settable threshold voltages to adjust and
set the amplifier input offset voltages . Their on-chip offset
voltage trimming circuits employ differential temperature
effect matching and error cancellation design techniques.
As a result of using these design techniques, the program-
ming of input offset voltage from one level to a different lev el
-2500 -2000 -1500 -1000 -500 0500 1000 1500 2000 2500
TOTAL INPUT OFFSET VOLTAGE (µV)
100
80
60
40
20
0
DISTRIBUTION OF TOTAL INPUT OFFSET VOLTAGE
BEFORE AND AFTER EPAD PROGRAMMING
EXAMPLE B:
V
OST
AFTER EPAD
PROGRAMMING
V
OST
TARGET = -750µV
EXAMPLE A:
V
OST
AFTER EPAD
PROGRAMMING
V
OST
TARGET = 0.0µV
V
OST
BEFORE EPAD
PROGRAMMING
PERCENTAGE OF UNITS (%)
Figure 1
NOTICE: Advanced Linear Devices (ALD) reserves the right to make changes and to discontinue any product and or services as identified in this pub lication without notice. Current specifications f or any product and or services
should be verified by customer before placing any orders. ALD warrants its products to current specifications in effect at time of manufacture in accordance to its standard warranty. Unless mandated by government
requirements, ALD performs certain, but not necessarily all, specific testing and procedures as ALD deems necessary to support this warranty.
ALD assumes no liability for any circuitry described herein. Applications for any circuits contained herein is for illustrative purposes only. No representation of continued operation of said circuits under any operating conditions
are implied. Any use of such circuits are the responsibility of the user. No circuit licenses, copyrights or patents of any kind is implied or granted. ALD does not authorize or warrant any of its products or designs for use in life
support applications, and hereby expressly prohibit any such use. All rights reserved.
© 1998 Advanced Linear De vices, Inc. 415 Tasman Drive, Sunnyv ale, California 94089 -1706 Tel: (408) 747-1155 F ax: (408) 747-1286 http://www .aldinc.com
2 Advanced Linear Devices APPLICATION NOTE AN1700
does not appreciably alter the temperature coefficient of the
input offset voltage or other electrical characteristics.
For simplicity, the ALD1722E operational amplifier is
chosen as an e xample f or the description in this Application
Note. This descr iption is applicable to the entire ALD EPAD
operational amplifier family.
The ALD1722E has been pre-programmed at the factory
and tested for guaranteed input offset voltage program
range. For applications where pre-programming at the
factory under standard operating conditions suffice and
where little or no electrical programming by the user is
necessary, a version of ALD1722E is available as ALD1722.
The ALD1722E operational amplifier is based on the
standard ALD1702 operational amplifier, with the added
feature of user offset voltage trimming. This added feature
is built with ALD EPAD technology, using electrically
programmable device technology refined for analog circuit
applications. The ALD1722E uses EPADs as an internal
circuit element for “trimming or setting” a bias voltage
characteristic. This bias voltage can be programmed
remotely and automatically via software control using a
personal computer.
In addition to offering high precision and electrically program-
mable V OS, EPAD operational amplifiers also offer rail-to-
rail input and output voltage ranges, low voltage and low
power operation, tolerance to overvoltage input spikes, unity
gain stability, extremely low input bias and offset currents,
and high slew rate per unit of power consumption.
The basic EPAD is a monotonically increasing voltage
adjustable device. The ALD1722E has a pair of EPAD
circuits connected such that one EPAD is utiliz ed to adjust
VOS in one direction and the other is used to adjust V OS in
the other direction. The EPAD circuits can be adjusted many
times to control the VOS in both directions. Once
programmed the set VOS le vels are stored permanently, even
when the EPAD power is removed, but can be reprogrammed
if desired.
The ALD1722E provides the user with an operational
amplifier that can be trimmed with Application Specific
Programming or In-System Programming conditions.
Application Specific Programming refers to the situation
where the ALD1722E can be trimmed as a unit to the actual
intended application circuit operating conditions. In-System
Programming refers to the condition where the EPAD
adjustment can be accomplished after the ALD1722E has
been inserted into the application circuit board. Examples of
Application Specific Programming and In-System Program-
ming are illustrated in Figures 14 and 15 respectively.
INPUT OFFSET VOLTAGE (VOS) ADJUSTMENT
General System Requirements
EPAD oper ational amplifiers offset voltages are programmed
using an ALD E100 EPAD programmer unit and the
appropriate Adapter Module. The user provides a personal
computer , a parallel printer cable, and an external DC power
supply. The entire system can be set up and ready to
program in a matter of minutes.
Adapter Modules
In conjunction with the ALD E100 EPAD Programmer, two
specific Adapter Modules are available for programming
EPAD operational amplifiers . The EA103 Adapter Module is
used to program single operational amplifiers such as
ALD1721E, ALD1722E and ALD1726E. The EA104 Adapter
Module is configured to program ALD dual operational
amplifiers , such as ALD2721E, ALD2722E and ALD2726E.
Within these Adapter Modules, an operational amplifier loop
with a gain of 100 is used. Both of these Adapter Modules
can be readily modified for In-System Programming by the
user to suit a specific operating condition.
Control Software
Software is supplied with an EPAD Adapter Module to
control the programming routines and to send commands to
the EPAD Programmer to control, measure and compute
programming conditions f or the specific Adapter Module. With
EA103 and EA104 Adapter Modules, the prog ram is set to
meet the VOS specified by the user. In many cases, the
recommended custom adaptation of this control software is
to input a different desired VOS value. The control software
algorithm then takes over. In programming mode, the
programming terminals VE1 and VE2 are pulsed in a
controlled fashion along with an optimal sequence of
voltage biasing conditions. VE1 and VE2 must not be
connected to a low impedance source that could interfere
with the programmer pulses in order for the programming
system to function properly.
Each ALDE100 EPAD Programmer Adapter Module is
controlled by a different control program, which is included
with each Adapter Module shipment. EPAD control programs
are available as follows:
• Standard EA series adapter version
• Standard WINDOW’S version (WIN 3.X, WIN 95)
• Standard E100 programmable version
• Optional developer’s version, in Quick Basic 4.5
(for fully automated and integrated software
development).
• Optional developer’s version, in Turbo C++ 4.5
(for fully automated and integrated software
development)
For further information on software availability and
description, consult the factory applications department.
APPLICATION NOTE AN1700 Advanced Linear Devices 3
FUNCTIONAL DESCRIPTION
Total Input Offset Voltage
The Total Input Offset Voltage, VOST, is the sum total of all
the equivalent input offset voltage errors. This includes input
offset voltage, equivalent input offset v oltage errors due to
PSRR, CMRR, ambient temperature TA, equivalent input
noise voltage (due to noise voltages and noise currents),
and external equivalent input offset voltage error. For appli-
cations where source impedance is high, or where variation
of exter nal circuit equivalent input offset voltage is greater
than the operational amplifier internal offset voltages, the
VOST can be significantly different from that of the sum of
equivalent input offset voltages as operational amplifiers have
been traditionally specified.
External circuit equivalent input offset voltage error is
traditionally not accounted for in an operational amplifier
specification. External equiv alent input offset voltage errors ,
such as that resulting from an e xternal sensor, or from other
circuit components, are usually trimmed by the user after
the circuit has been built. However, in an EPAD operational
amplifier, this system level equivalent input offset voltage
error can be compensated for by programming (trimming)
the operational amplifier at in-system level. At that point, a
specific sensor component has been paired with a specific
oper ational amplifier . Therefore any equiv alent input offset
voltage error caused by sensor component to component
variation can be trimmed by user programming.
Examples of applications in different application specific
operating conditions are shown in Figure 2. Note that the
input offset voltage can be progr ammed to any user speci-
fied value. In many cases the target input offset voltage of
prog ramming is not equal to z ero offset voltage, but instead
equals to a diff erent input offset voltage that achieves other
goals as well.
Figure 2. Application specific/ in-system programming examples of applications where accumulated total
input offset voltage from various contributing sources is minimized under different sets of user-specified operating conditions
Total Input V
OS
after EPAD
Programming
+
Device input V
OS
PSRR equivalent V
OS
CMRR equivalent V
OS
T
A
equivalent V
OS
Noise equivalent V
OS
External Error equivalent V
OS
X
EXAMPLE A
TOTAL INPUT OFFSET VOLTAGE (µV)
2500
2000
1500
1000
500
0
-500
-1000
-1500
-2000
-2500
V
OS
BUDGET BEFORE
EPAD PROGRAMMING
V
OS
BUDGET AFTER
EPAD PROGRAMMING
+
X
EXAMPLE B
TOTAL INPUT OFFSET VOLTAGE (µV)
2500
2000
1500
1000
500
0
-500
-1000
-1500
-2000
-2500
+
X
V
OS
BUDGET BEFORE
EPAD PROGRAMMING
V
OS
BUDGET AFTER
EPAD PROGRAMMING
EXAMPLE C
TOTAL INPUT OFFSET VOLTAGE (µV)
2500
2000
1500
1000
500
0
-500
-1000
-1500
-2000
-2500
+
X
V
OS
BUDGET BEFORE
EPAD PROGRAMMING
V
OS
BUDGET AFTER
EPAD PROGRAMMING
EXAMPLE D
TOTAL INPUT OFFSET VOLTAGE (µV)
2500
2000
1500
1000
500
0
-500
-1000
-1500
-2000
-2500
+
X
V
OS
BUDGET AFTER
EPAD PROGRAMMING
V
OS
BUDGET BEFORE
EPAD PROGRAMMING
4 Advanced Linear Devices APPLICATION NOTE AN1700
Dual EPAD operational amplifiers have separate VE1 and
VE2 pins for each amplifier. Please refer to the individual
datasheet for the pinouts and functions of each device.
EPAD PROGRAMMING
Figure 3 represents a block diagram of a standard stand-
alone EPAD programming system. Using this configuration,
the setup and user programming can be accomplished in a
simple and straightforward manner. EPAD operational
amplifiers with VOS adjustment can be programmed using
the EA103 or the EA104 Adapter Module , as interface mod-
ules to the ALD E100 EPAD programmer unit.
In order to program an EPAD operational amplifier as an
Application Specific or In-System element, V+, VE1 and VE2
pins need to be pulsed at 12V during the programming
process. The application circuit in which the EPAD
operational amplifier is used can be powered by any user
selected voltage, within the limits of specification. Ho wever,
it must allow the VE1 and VE2 pins to be pulsed without a
low impedance path to other circuit nodes , so that the EPAD
programmer pulsing circuit can function properly. After
programming, VE1 and VE2 pins must also be left open
without any low impedance path to any other circuit nodes.
Both of the above stated conditions, either during program-
ming or during normal operation, can be met simply by
leaving the VE nodes in a high impedance state, shielded
by ground traces, as necessary.
FUNCTIONAL DESCRIPTION OF EPAD FEATURE FOR
VOS CORRECTION
Each ALD1722E EPAD operational amplifier has two
additional pins named VE1 and VE2. Each pin is internally
connected to an EPAD circuit. VE1 and VE2 have initial
typical v alues of approximately 1 to 2 Volts. The voltages on
these terminals can be programmed using the ALD E100
EPAD Programmer and an appropriate Adapter Module. The
useful programming r ange of VE1 and VE2 is 1 to 4 Volts in
0.1mV steps.
VE1 and VE2 pins ha ve tw o functions. The first function is to
set a bias voltage during normal operation. Each VEx pin is
biased at an internal bias voltage that controls and adjusts
the offset voltage of the operational amplifier. An increases
in VE1 results in a decrease in the input offset v oltage of the
operational amplifier, while increases of VE 2 increases the
input offset v oltage of the operational amplifier . (In the case
of ALD2721E and ALD2726E, increases of VE1 increases
the input offset voltage, whereas increases of VE2 decreases
the input offset voltage.)
The second function of VE1 and VE2 is to perform as
programming pins, used during device programming. The
programming pin is used during electrical programming to
inject charge into the internal EPADs bias circuit. The
injected charge is then permanently stored. This stored
charge results in an increased threshold voltage of the EPAD ,
which in turn deter mines the input offset voltage of the op-
erational amplifier. After EPAD programming the VE1 and
VE2 pins must be left open to settle on a voltage determined
by the internal bias currents.
During programming, when connected to an EPAD
programmer, the voltages on VE1 or VE2 are increased
incrementally to set the input offset voltage, VOS, of the
operational amplifier to the desired level. Note that the
desired VOS can be any value within the offset voltage ad-
justment ranges, and can be either equal to zero, a posi-
tive value or a negative value. Once programmed, the V OS
value is retained indefinitely, regardless of whether the
device is powered on. This VOS value can also be repro-
grammed to a different value at a later time, provided that
the useful VE1 or VE2 programming voltage range has not
been exceeded.
VE1 and VE2 are high impedance terminals, as the internal
bias currents are set very low, typically to a few microam-
peres to conserve power . For certain applications, these high
impedance terminals may need to be shielded from
external coupling sources. For example, digital signals
running nearby ma y cause unwanted offset voltage fluctua-
tions. Care in isolating the pins from digital lines during the
printed circuit board layout would generally eliminate such
coupling effects . When necessary, ground traces should be
placed around these pins to further isolate them. In
addition, optional decoupling capacitors can be added to
VE1 and VE2 pins.
Figure 3.
Standard Stand Alone
EPAD Programming System
EPAD
OP AMP
EA SERIES
CONTROL
SOFTWARE PC
E100 EPAD
PROGRAMMER
PARALLEL
PRINTER
CABLE EA SERIES
INTERFACE
ADAPTER
INTERFACE
ADAPTER
CABLE
APPLICATION NOTE AN1700 Advanced Linear Devices 5
APPLICATION SPECIFIC PROGRAMMING
For applications where custom setup and special conditions
are desired, such as operating at different power supply
voltages, different bias current or reference voltage condi-
tions, and/or different temperature environments, the
supplied Interface Adapter Module can be readily modified
by the user to reflect these conditions.
For example, an application circuit may have +6V and -2.5V
power supplies, and the operational amplifier input biased
at +0.7V due to a sensor connection, with the average
operating temperature at 55ºC. This circuit can be wired up
to these operating conditions in an environmental chamber,
and the ALD1722E can be inserted into a test socket
connected to this circuit and trimmed as required. Any VOS
error due to these biasing and environmental conditions will
be zeroed out by user programming. The VOS error, VOST,
is limited only by the adjustable range and the stability of
VOS, and the input noise voltage of the operational
amplifier . VOST now includes VOS as traditionally specified
plus the VOS error term contributions from PSRR, CMRR,
TCVOS and noise. The typical VOST is appro ximately ±25µV
for the ALD1722E. The block diagram of this setup is
illustrated in Figure 4.
An EA series Adapter Module can be modified to desired
user operating conditions and placed inside a remote
environmental chamber, along with each EPAD operational
amplifier to be programmed. In this way, each operational
amplifier can be programmed optimally for a specific
application and its particular environment.
Figure 4. EA Series Interface Adapter Module Custom Tailored to Application
Specific Conditions and Environment
* EA Series Control Software, Interface Adapter Cable And Interface Adapter can All be customized by User for specific application, if necessary.
In using the EA series Adapter Module, especially when
using an environmental chamber, care must be taken to
insure that the v oltage and temperature ratings of the Adapter
Modules are not exceeded. For continued high temperature
applications, the user is advised to set up a custom adapter
circuit board to meet those conditions, with a connection to
a separate socket for the operational amplifier.
Entire lots of ALD1722E operational amplifiers can be
programmed to a user’ s specific application conditions with
a single modified adapter module. These ALD1722E units
are then available for assembly use.
IN-SYSTEM PROGRAMMING
One of the benefits of In-System Programming is that not
only the ALD1722E operating bias conditions have been
accounted for, any residual errors introduced by other
circuit components, such as resistor or sensor induced
voltage errors, can also be accounted for and programmed
out. In this way, the “in-system” circuit output can be
adjusted to a desired “ calibrated” level.
For In-System Programming, any of the EPAD operational
amplifiers can be designed into an application circuit where
they are embedded as an integral part of the circuit
function. Each EPAD operational amplifier is programmed
through the use of a n in-system programming cable or a
number of pre-assigned edge connector pins.
EA SERIES
CONTROL
SOFTWARE PC
E100 EPAD
PROGRAMMER
PARALLEL
PRINTER
CABLE EA SERIES
INTERFACE
ADAPTER
INTERFACE
ADAPTER
CABLE
REMOTE
ENVIRONMENTAL
CHAMBER
EPAD
OP AMP
6 Advanced Linear Devices APPLICATION NOTE AN1700
Figure 5. In-System Programming With EPAD Incorporated into User Application Circuit.
For in-system applications, the EPAD operational amplifier
is soldered onto the printed circuit board before EPAD
programming. In-system programming is accomplished by
designing the application circuit to accommodate the two
diff erent modes of operation, the normal operating mode and
the programming mode.
During in-system programming mode, the VEx, the input,
output and V+ pins are pulsed with programming pulse
sequences by the EPAD programmer. Other low imped-
ance nodes in the application circuit can be isolated from
these pulses by adding fixed resistors as isolation resistors.
A typical isolation resistor is a 1/4W , 50KΩ (10KΩ to 100KΩ)
5% carbon resistor.
In normal operating mode, the designer only needs to
ensure that these isolation resistors do not interfere with
circuit functionality or cause added errors. For CMOS
analog circuits, extremely high input impedance of the CMOS
inputs and normal CMOS output loading in the 10KΩ to
100KΩ range would be compatible with using these
isolation resistors. Once such a circuit has been designed,
a special in-system programming cable can be built to imple-
ment In-System Programming as referred to in Figure 5.
This special in-system programming cable can be designed
to temporarily connect the EPAD Programmer to the in-
system EPAD operational amplifier directly within the
application circuit when progr amming is desired. After pro-
gramming, the special in-system programming cable can be
disconnected and used again for the next circuit.
CHANGE IN POWER SUPPLY VOLTAGE
The Power Supply Rejection Ratio (PSRR) of an operational
amplifier specifies its equivalent input offset v oltage change
as a result of change in power supply voltage. Other
precision operational amplifiers currently available on the
market are trimmed at the factory under a specific set of
power supply conditions.
For example, a specific operational amplifier may be laser
trimmed at ±15 V. If a user plans to use such a de vice in an
application circuit where the power supplies are actually at
±4 V, and the PSRR specification is 35µV/V, then the
equiv alent input offset voltage error due to PSRR is equal to
±11V multiplied by the PSRR.
Equivalent input offset voltage error due to the power
supply voltage change is given by:
∆VOS = 11V x 2 x 35µV/V
= 770µV
Consider another example with the use of an operational
amplifier that was calibrated at the factory using a single 5V
supply. The fact that this operational amplifier may have a
PSRR of 100 µV/V actually works out to have a lower equiva-
lent input offset voltage error due to power supply voltage
change because the actual change in power supply
voltages from factory calibration conditions is smaller.
Figure 6
0123456789 10
500
400
300
200
100
0
EQUIVALENT INPUT OFFSET VOLTAGE DUE TO
CHANGE IN SUPPLY VOLTAGE (µV)
TWO EXAMPLES OF EQUIVALENT INPUT OFFSET VOLTAGE DUE TO
CHANGE IN SUPPLY VOLTAGE vs. SUPPLY VOLTAGE
SUPPLY VOLTAGE (V)
PSRR = 80 dB
EXAMPLE B:
V
OS
EPAD
PROGRAMMED
AT V
SUPPLY
= +8V
EXAMPLE A:
V
OS
EPAD PROGRAMMED
AT V
SUPPLY
= +5V
USER CUSTOMIZED
CONTROL
SOFTWARE PC
E100 EPAD
PROGRAMMER
PARALLEL
PRINTER
CABLE EA SERIES
INTERFACE
ADAPTER
USER
APPLICATION
CIRCUIT
INTERFACE
ADAPTER
CABLE
IN-SYSTEM
PROGRAMMING
CABLE
EPAD
OP AMP
REMOTE
ENVIRONMENTAL
CHAMBER
REMOTE
USER
PROGRAM
SOFTWARE
INTERFACE
CUSTOMIZED
BY USER
USER CUSTOMIZED
APPLICATION NOTE AN1700 Advanced Linear Devices 7
In this case, equivalent input offset voltage error due to power
supply voltage change can be calculated as follows:
∆VOS = [ (4 x 2) - 5 ]V x 100µV/V
= 300µV
All ALD EPAD operational amplifiers off er the benefit of user
offset voltage programming under the actual operating
voltage conditions. Examples of equivalent input offset
voltage error with offset voltage trimmed at two separate
supply voltages, 5V and 8V, are illustrated in Figure 6.
Take the case of Example A in Figure 6, where the opera-
tional amplifier has its offset voltage trimmed at 5V, then at
8V the equivalent input offset voltage error is 300µV. How-
ever, if the EPAD operational amplifier is programmed at
8V, as shown in example B, then the equivalent input offset
voltage error at 8V due to power supply voltage change is
zero.
The user now only has to contend with a further change in
power supply voltage from 8V in the application and its
effect on the equiv alent input offset voltage error. For a w ell
controlled state-of-the-art regulated power supply, as one
may find in a precision measurement system, the variation
in supply voltages due to power supply regulation is very
small, in the order of less than 100mV. Using a typical PSRR
value of 100µV/V, this translates into an equivalent input
offset voltage error due to power supply fluctuations of 20µV.
Equivalent input offset voltage error due to power supply
voltage change is calculated as:
∆VOS = ( 0.1 x 2 )V x 100µV/V
= 20µV
In this way , the po wer supply voltage effects causing equiva-
lent input offset v oltage errors at the input of the operational
amplifier can be minimized or eliminated in many applications.
CHANGE IN COMMON MODE VOLTAGE
ALD EPAD operational amplifiers are designed for a wide
range of input signal voltage levels. For large input voltage
signals, the rail-to-rail feature of the operational amplifiers
offers the largest possible input voltage range available for
a given power supply voltage. For small signal amplifica-
tion, and for very small DC input voltage levels, the com-
mon mode voltage range for a specific application can be
determined. Examples of three different input voltage DC
bias levels are illustrated in Figure 7. Assuming a Common
Mode Rejection Ratio (CMRR) of 80 dB for the operational
amplifier, the benefits associated with user programming of
the equivalent input offset voltage error due to common mode
voltages become apparent.
In comparison with factory trimmed operational amplifiers,
which may be set up at common mode voltages of 0.0V,
EPAD operational amplifiers can be user programmed at
supply voltage at any common mode voltage, which in this
case is any voltage between V+ and V-.
Note that while the CMRR of the operational amplifier does
not change appreciably at different input common mode
voltages, the resultant effect of user programming is quite
dramatic. In the example illustrated in Figure 7, an
operational amplifier trimmed at a common mode voltage of
0.0V (as may be the case in a factory trimmed operational
amplifier) would have a common mode equivalent input
offset v oltage error of 500µV when the input common mode
voltage is at +5V. This equivalent input offset voltage error
due to CMRR can be eliminated when the user program-
ming of the input offset v oltage error tak es place when input
is set at +5V and the equivalent input offset voltage error
becomes part of the VOST.
Using example A of figure 7, an operational amplifier that
was trimmed at the factory with the power supplies at ±5V
and the common mode input v oltage set at 0V, with a CMRR
specification of 80dB, the equivalent input offset voltage
error due to common mode voltage when input common
mode voltage is a +5V is:
∆VOS = 5 x 100 µV/V
= 500µV
Examples B and C in Figure 7 show different equivalent
input offset voltage errors at various common mode
voltages with input offset voltage tr immed at different input
voltages.
-5 -4 -3 -2 -1 012345
COMMON MODE VOLTAGE (V)
500
400
300
200
100
0
EQUIVALENT INPUT OFFSET VOLTAGE DUE TO
CHANGE IN COMMON MODE VOLTAGE (µV)
EXAMPLE A:
V
OS
EPAD PROGRAMMED
AT V
IN
= 0V
EXAMPLE B:
V
OS
EPAD
PROGRAMMED
AT V
IN
= -4.3V
EXAMPLE C:
V
OS
EPAD PROGRAMMED
AT V
IN
= +5V
V
SUPPLY
= ±5V
CMRR = 80dB
THREE EXAMPLES OF EQUIVALENT INPUT OFFSET VOLTAGE DUE TO
CHANGE IN COMMON MODE VOLTAGE vs. COMMON MODE VOLTAGE
Figure 7
8 Advanced Linear Devices APPLICATION NOTE AN1700
For input signals that have an input common mode voltage
range, a specific optimized input common mode voltage
within that range can be selected by the user when program-
ming. For example, Figure 8 shows the equivalent input
offset voltage error throughout an input voltage range of 0.5V.
This input offset v oltage can be minimized by user prog ram-
ming at a common mode input voltage of 0.25V. This is
accomplished in application-specific or in-system mode by
simply programming the EPAD operational amplifiers with
the input common mode voltage set at 0.25V.
For applications that utilize not only part of the input voltage
range, but also have large input signals that essentially
utilize the full rail-to-rail input voltage range, user offset
programming would not offer the benefits of a predetermined
common mode voltage. However, in this case, since the
input signal voltage is a large signal, the available gain of
the closed loop amplifier has to be limited to a low value so
as not to saturate the output transistors. Hence the relative
percentage error of the output due to CMRR equivalent input
offset voltage errors are kept small.
One example described below illustrates how user offset
voltage programming can still be used to an advantage for
large rail-to-rail signal range. In a unity gain amplifier
application with rail-to-rail input signals, the maximum input
and output voltages are the same, and they are equal to the
power supply voltage range. There may exist, within this
input voltage range, a common mode voltage point where
an offset voltage adjustment is desirable, and where an off-
set voltage shift at that point would significantly enhance the
ov erall system accuracy throughout the range. The user can,
upon analyzing the situation, determine where that voltage
point should be and proceed to progr am the offset voltage at
that common mode voltage. That voltage point is where
“system calibration” can be performed.
-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5
COMMON MODE VOLTAGE (V)
50
40
30
20
10
0
EQUIVALENT INPUT OFFSET VOLTAGE DUE TO
CHANGE IN COMMON MODE VOLTAGE (µV)
V
OS
EPAD
PROGRAMMED
AT COMMON MODE
VOLTAGE OF 0.25V
CMRR = 80dB
EXAMPLE OF MINIMIZING EQUIVALENT INPUT OFFSET VOLTAGE
FOR A COMMON MODE VOLTAGE RANGE OF 0.5V
COMMON MODE VOLTAGE RANGE OF 0.5V
Figure 8
V
OUT
-
+
R2
R1
V
OS
V
IN
VOUT = - (R2/ R1) * VIN+(1+R2/R1) * (VOS)
Figure 9
EPAD Inverting Amplifier
BASIC INVERTING AND NON-INVERTING AMPLIFIER
CIRCUITS
The best way to visualize the effect of electrically
programming of VOS is by modeling the VOS term as a
separate voltage source, as shown in Figure 9 and Figure
10.
In the case of the inv erting amplifier , the VOS v oltage source
is a voltage source at the input of the non-inverting input.
The virtual ground of the inverting amplifier is directly affected
by the magnitude of the VOS voltage source. When VOS is
electrically programmed to +1.00 mV, for example, the
virtual ground is shifted to +1.00 mV.
The relationship between VIN and VOUT is given by:
( VOUT - VOS ) / R2 = - ( V IN - VOS ) / R1
assuming leakage currents to be negligible
Therefore
VOUT = - ( R2 / R1 ) * VIN + ( 1 + R2 / R1 ) * VOS
For the case where R2 / R1 > > 1,
VOUT = - ( R2 / R1 ) * ( VIN - VOS)
For positive values of VOS, the output VOUT is shifted to a
positive value. For negative VOS values, VOUT is shifted in
the negative direction.
As VOS is programmed internally based on differential and
matched circuits in close proximity to each other, the net
temperature eff ect on VOS change due to electrical program-
ming is typically less than the case when external
components such as a trimmer potentiometers are introduced
into the circuit for VOS adjustment.
˜
APPLICATION NOTE AN1700 Advanced Linear Devices 9
For non-inverting amplifier applications, the VOS voltage
source is a voltage source in series with the input, VIN, as
shown in Figure 10.
The relationship between input and output is:
VOUT = ( 1 + R2/ R1 ) * ( VIN + VOS )
For the case where R2 / R1 > > 1,
VOUT = ( R2 / R1 ) * ( VIN + VOS)
PRECISION LOW LEVEL VOLTAGE DETECTOR/
COMPARATOR
Using the circuit in Figure 10 above, a direct and immediate
application is to use the progr ammab le offset voltage f eature
of the EPAD operational amplifier to electrically set a
precision low level voltage detector, as shown in Figure 11.
In this example, the objective is to set up a 2.5 mV ± 50µV
threshold voltage detector on the ground line using a single 5
volt supply. The offset voltage is electrically programmed to
-2.5 mV.
V
OUT
-
+
R2
R1
V
OS
V
IN
Figure 10
EPAD Non-Inverting Amplifier
VOUT = (1+ R2/ R1) *
(VIN+VOS)
The input to the non-inverting input is now the ground
terminal. Any time the input exceeds 2.5 mV ( to within ± 50
uV), the output voltage turns positive and the gain of the
circuit is equal to ( 1 + R2 / R1 ). The output voltage is
therefore equal to the amount of the input voltage exceeding
2.5 mV, the overdrive voltage, multiplied by this gain. To use
this circuit as a voltage comparator, the gain is set very high
(R2 >> R1). The output voltage is then a highly amplified
signal, suitable as an input signal to a digital gate.
OUTPUT VOLTAGE LEVEL SHIFTER
Another e xample of ho w EPAD operational amplifiers can be
used as an output level shifter is descr ibed below.
In the inverting amplifier circuit shown in Figure 12, the output
offset voltage is given by:
Output offset voltage = [ 1 + R2/R1 ] x VOS
The input offset voltage of an ALD EPAD operational
amplifiers can be programmed to be equal 2.000mV ±25 uV.
If R2 = 2.5MΩ and R1=10K Ω, and VOS = 2mV, then VOUT is
equal to 500mV ±6.25mV for input voltage at 0V.
Figure 12
Low Offset Inverting Amplifier
V
OUT
V
IN
-
+
+2.5V .01µF
.01µF
- 2.5V
C
L
= 4000pF
* Circuit Drives Large Load
Capacitance ≤ 4000pF
10K
2.5M
R1
R2
ALD172XE
Figure 11
V
OUT
-
+
R2
R1
V
IN
+5V
Precision Low Level Voltage Detector/ Comparator
˜
10 Advanced Linear Devices APPLICATION NOTE AN1700
APPLICATION SPECIFIC PROGRAMMING
An example of an user specified Application Specific
Programming condition is shown in Figure 14. In this
example, the power supply voltage levels, the input
common mode voltage levels, output loading conditions, as
well as the nominal average operating ambient tempera-
tures, are all different from the standard factory default
trimming conditions.
Therefore the actual equivalent input offset voltage is
unknown. Ho w ever , it is e xpected that the actual equivalent
input offset voltage value will be different from that of the
factory trimmed conditions. This value can be readily
estimated by calculation using the data sheet information
on various parameters affecting equivalent input offset
voltage. The fact is that unless an operational amplifier is
custom trimmed at the factory, no operational amplifier
available on the market will pro vide equivalent input offset
voltage trimmed under the exact set of operating conditions
as shown in this example.
A standard factory trimmed EPAD operational amplifier,
however, w ould enable users to customize a setup at their
facilities and perform a secondary trimming of the input
offset voltage under these specific conditions. With EPAD
operational amplifiers, users now have the ability to
customize a standard operational amplifier in their own
facilities and tailor it to their own specific circuit environ-
ments.
In fact, a single EPAD operational amplifier type can be
customized for different circuits to many sets of operating
conditions using different user customized adapters. Note
that in Application Specific Programming mode, no change
in the user’s application circuit is required. Only a special
programming adapter is needed.
An interface adapter module such as EA103 can be
modified by the user to bias an EPAD oper ational amplifier
to a set of custom conditions, and then to proceed to trim
input offset voltage. Hence Application Specific Program-
ming can, in many cases, improve the final accuracy
required in the system by virtue of the fact that an EPAD
operational amplifier is capable of being electrically trimmed
under a particular user’s exact application conditions.
DEFAULT FACTORY-SETTING CONDITIONS
Standard default factory-setting conditions for input offset
voltage trimming is shown in Figure 13. This condition is
used for the ALD1722E input offset voltage programming,
with the input common mode voltage set at ground potential
and the fixture set at ambient temperature of 25ºC using
dual power supplies. All f actory trimmed operational amplifi-
ers, of course, would have to specify a particular set of
standard factory def ault trimming conditions . As input offset
voltage is trimmed under those conditions, all other
operating condition changes would entail some degree of
equivalent input offset voltage change. Common among
these changes would be changes in power supply voltages,
input source conditions, operating frequencies, common
mode voltages, and operating temperatures.
As many of these operating conditions change, the actual
impact of a particular operating condition change is not
necessarily well understood. In most cases, the specifica-
tions that appear on the data sheet of a given operational
amplifier reflect the most favorable operating conditions.
For example, it is often assumed that a well designed and
well characterized operational amplifier has a rather uniform
and linear power supply rejection ratio at all power supply
voltages within the specified operating ranges. This, of
course, is not necessarily true at all. How ev er, if the change
in pow er supply rejection ratio at diff erent supply voltages is
non-linear, and if it does happen to have an impact on a
given application, the user will find out about it not from the
data sheet, but at the prototype stage or during production.
Again, the example quoted above illustrates the merit in
giving the user the ability to select an actual operating
supply voltage setting to perform input offset voltage
trimming, where fluctuations of the input offset voltage from
that nominal supply voltage level can be minimized,
approximated and linearized. EPAD operational amplifiers
with electrical programming are designed for this purpose
where the user can implement Application Specific
Programming described in the example below.
V
OUT
-
+C
L
SIGNAL
INPUT R
L
VE
1
VE
2
V
-
= -3V
V+ = +6V
R
f
Figure 14
Example of A User Application-Specific Trimming
Figure 13
Input Offset Voltage Trimming With
Factory-Setting Default Conditions
V
OUT
-
+
0.01µF
VE2
0.01µF
V- = - 2.5V
24.9KΩ 1%
249Ω 1%
V+ = + 2.5V
VE1
TA=25°C
APPLICATION NOTE AN1700 Advanced Linear Devices 11
IN-SYSTEM PROGRAMMING
In-System Programming takes the programmable feature of
the EPAD operational amplifier one step further than in
Application Specific Programming mode, at the expense of
making some modification to the application circuit. Now the
programmable feature is also used as an electrically trimmer
circuit not only for correcting the oper ational amplifier’s own
input offset voltage, but also for correcting other system
variables. System variables can be any kind of system errors
coming from other system components or changes in the
system desired for calibration purposes.
An example of In-System Programming is illustrated in
Figure 15, in which the output of the photo darlington is
calibrated by using a calibrated light source. VOUT is electri-
cally programmed to a corresponding desired voltage value,
which can be a calibrated voltage value. This calibrated
voltage can be precisely set for each individual unit of
different batches of photo darlington devices, each with greater
unit to unit output variation than desired. This calibrated v olt-
age can now be used as a precision system control threshold
voltage.
In this particular example, In-System Programming is used
to adjust for variation of individual sensor (photo darlington)
output current corresponding to a given light input intensity to
provide a calibrated output. This calibration is performed af-
ter a sensor and an EPAD operational amplifier have been
kitted and assembled into one assembly, along with other
circuit components. In the example shown in Figure 15, an-
other component value variation, that of the current sensing
resistor value of Rp, is also taken into account. Since the
input to the operational amplifier is a voltage, which is the
product of the sensor current and the sensing resistor value,
a voltage adjustment includes both sensor current value and
resistor resistance value variations. This example circuit can
be easily adapted to be used, for instance, in a ground-
sensing amplifier application.
Figure 15
Example of In-System Programming of an EPAD -
Based Operational Amplifier
VOUT
INPUT -
+
V+ =+5V
1K 1% 99K 1%
VE1VE2
RP
V+ from
EA103
adapter
Photo
Darlington
VBIAS
Gain=100
RISO ≥ 20KΩ
RISO
DESIGN PRECAUTIONS
ALD EPAD operational amplifiers are designed for use in
low voltage , micro-power circuits . The maximum oper ating
voltage during normal operation should remain below 10
Volts at all times. Care should be taken to insure that the
applications in which the devices are used would not
experience any positive or negative transient voltages that
cause any of the terminal voltages to exceed this limit.
EPAD programming tak es place at a v oltage determined b y
the EPAD programmer, typically at about 12V. In-System
Programming requires a programming mode where other
circuitry can accommodate EPAD prog ramming pulses and
conditions.
All inputs or unused pins except the programming pins (VE1
and VE2) should be connected to V- so that they do not
become floating pins, since input impedances at these pins
are very high. If any of these pins are left open, they may
cause unwanted oscillation or intermittent excessive current
drain. As these devices are built with CMOS technology,
normal ESD and latchup handling precautions should be
observed. Operating and storage temperature limits as stated
in the datasheet must also be observed.
CONCLUSION
EPAD VOS trimming is applied to a new generation of
operational amplifiers that empower the analog circuit
designers to customize their analog circuits with a new set
of capabilities that is simple and economical to use. The
precision achieved can be optimized for a unique individual
system requirement, where most of the system parameters
may be at different levels from that of the standard IC
factory environment. Under standard IC operating
conditions, offset voltage of a precision operational
amplifier would be minimized under only one factory
mandated, rather than user specified, setting.
Using EPAD operational amplifier programming, external
parameter variations introduced by other system compo-
nents can also be trimmed at the same time. Hence, EPAD
operational amplifiers are capable of performing the dual
role of a precision operational amplifier function and an all
solid-state electronic trimming function simultaneously. Once
the application circuit is designed, the task of programming
can be fully automated and controlled with the ubiquitous
and inexpensive PC.
F or further assistance on Application Specific Programming
or In-System Programming, please contact the ALD
applications department. Send a fax or e-mail to:
Fax (408) 747-1286 - Attention: Applications
E-Mail: Applications@aldinc.com
12 Advanced Linear Devices APPLICATION NOTE AN1700
