Application Note AN1108 - Advanced Linear Devices

INTRODUCTION

The ALD1108E/ALD1110E are the first members in a gro w-

ing family of Electrically Programmable Analog Device

(EPAD) integrated circuits from Advanced Linear Devices.

An EPAD is a new class of analog IC, like the operational

amplifier and the A/D converter.

The ALD 1108E/ALD110E are based on CMOS MOSFET

devices with precision electrically programmable threshold

voltage, which sets their on-resistance characteristics. This

programmed on-resistance is in turn used to set a voltage

or a current level with the use of external, inexpensive fixed

precision resistors.

When used in combination with an inexpensive ALD E100

EPAD Programmer and Adapter Module, the EPAD allows

analog circuit trimming in up to 20,000 discrete steps with

0.1mV increments. In engineering environments, the EPAD

is quickly and easily programmed to fit the needs of a spe-

cific application. In test environments, it can be programmed

in-system, that is, as an integral part of a circuit board as-

sembly — thereby providing precise control over the entire

system. The EPAD technology is designed for use in low

voltage, low power circuits where a precision voltage or cur-

rent trimming function is necessary. In addition, it provides

a simple, inexpensive method for achieving remote trimming

of circuits within user-controlled environments.

The ALD1108E is a quad (4-channel) EPAD device; the

ALD1110E is dual (2-channel) EPAD device. Both EPADs

can be configured to operate in single or dual channel mode,

enabling unlimited variations in functionality. Both EPADs

are supplied as stand-alone, individually programmable de-

vices encased in standard IC packaging. They can be pre-

programmed before being placed into a circuit (application-

specific programming), providing control of standard linear

functions such as operational amplifiers and voltage com-

parators. Alternatively, they can be integrated directly into a

circuit, enabling in-system programming of the entire assem-

bly after it has been built (in-system programming).

The ALD1108E/ALD1110E EPADs do not need other active

circuitry to enable functionality. As such, these devices are

extremely versatile as both circuit element and design com-

ponent. EPAD technology presents the user with a wealth

of possible applications, limited only by the imagination of

the user and the many ways an adjustable bias voltage/cur-

rent device can be used as a circuit design element.

The applications listed below outlines in a general way some

of the applications f or which EPAD can be used in telecom-

munications, instrumentation, medical devices and indus-

trial process control systems. Note that although automa-

tion can be an important end goal, a primary benefit of EPAD

technology is that it simplifies the manufacturing and control

process, by enabling electrical programming of an analog

circuit transfer function without resor ting to difficult and ex-

pensive trim pots or an even more expensive system of

microcontrollers, RAMs, ROMs, EPROMs, data converters

and an entire overhead of system functions. The EPAD is

also an ideal solution for trimming of packaged modules

where trim pot access is no longer readily available.

GENERAL DESCRIPTION

The ALD1108E/ALD1110E devices are based on monolithic

CMOS FETs (Field Effect Transistors) with electrically

programmed threshold voltages that can be precisely con-

trolled via stored, non-volatile charges. Control of these

threshold voltages is enabled through a DOS-based user

interf ace, an EPAD Programmer and control software sup-

plied by Advanced Linear Devices.

Each channel in the EPAD can be individually progr ammed

to a diff erent threshold, or bias, voltage. The range of thresh-

old voltage adjustment is 2000mV; programming voltage

resolution is 0.1mV respectively. For a given input voltage,

precisely changing the threshold voltage of a channel

(MOSFET) also changes its drain on-current, resulting in an

on-resistance characteristic that can be precisely con-

trolled and set.

ELECTRICALLY PROGRAMMABLE ANALOG DEVICE (EPAD®) APPLICATIONS

APPLICATION NO TE AN1108

ADVANCED

LINEAR

DEVICES, INC.

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 b y 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

APPLICATION NOTE AN1108 Advanced Linear Devices 2

EPAD programming is monotonic: that is, the channel can

only be programmed to an increase in threshold voltage and

a decrease in drain-on current as a function of a given input

bias voltage. However, each channel can be programmed

many times before the threshold limit is reached. In addi-

tion, the baseline current level can also be adjusted lower,

creating the same effect as lower ing the bias voltage.

A pair of EPAD channels can also be connected differen-

tially such that one channel is used to adjust a parameter in

one direction and the other channel is used to adjust the

same parameter in the other direction, thereby providing bi-

directional control capability. EPADs can be prog rammed in

stand-alone programming mode, application-specific pro-

gramming mode, or in-system programming mode, after the

entire assembly has been built. In all these cases, a combi-

nation of the ALD E100 EPAD Programmer and an Adapter

Module is required.

In stand-alone programming, the exact adapter module may

vary depending on the intended application. For example,

the EA101 Adapter Module programs EPAD threshold volt-

ages with a fixed resistor. The EA102 Adapter Module is

specially designed to program the EPAD with current sources

set at a Zero Temperature Coefficient (ZTC or Zero tempco)

current level of 68µA.

Both of these Adapter Modules can be readily modified f or

other types of application-specific programming. For in-

stance, to enable direct control of an operational amplifier,

the user changes the resistor component values on the

adapter module, which creates a corresponding change in

bias drain current level. Entire lots of EPADs can be pro-

grammed in this way with a single adapter module. These

programmed EPADs are then inserted into a user circuit

board.

With In-System Progr amming, the ALD1108E or ALD1110E

device is designed into an application circuit where it be-

comes an integr al par t of the circuit function. The device is

inserted into a finished circuit and accessed or programmed

through the use of a special programming cable or a num-

ber of preassigned edge connector pins. For more informa-

tion please refer to In-System Programming section de-

scribed later .

The E100 EPAD Programming System contains voltage

measurement and voltage programming capabilities to mea-

sure the baseline voltage and program it to the desired value.

To set diff erent current lev els , the bias resistor values within

the Adapter Module circuit board must be changed. Alter-

natively , the user can switch in current setting resistors which

can be placed under software control and implemented with

analog or relay switches.

DEFINITION OF TERMS

Terms that are used in this article are defined as follows:

Baseline voltage level.

The initial threshold voltage Vt, defined as drain and gate

diode-connected voltage at a drain current of 1µA, when

source voltage is at 0.0 Volt. The initial threshold voltage is

also a def ault factory setting. Howev er, the user can change

this default baseline voltage and set the drain current value

of the threshold voltage to a desired value, thereby allowing

the device to be “zeroed out”, or “set” to a new bias level.

Bias voltage, Vbias

Same as threshold voltage, except at a drain-source bias

current diff erent from that specified at threshold voltage . F or

e xample, the threshold voltage of the ALD1108E/ALD1110E

is specified at drain source bias current of 1µA. The useful

bias current range of the ALD1108E/ALD1110E is from 4nA

to 3mA.

Drain Current, Ids

Drain Current is the current that flows from drain to source.

Drain Voltage, Vds.

Drain voltage is the voltage across drain and source pins.

Gate Voltag e, Vgs.

Gate voltage is the voltage across gate and source pins.

Threshold voltage, Vt.

Threshold voltage of an EPAD is the voltage across Gate

and Source when 1µA is forced into the Drain terminal as

the Drain and Gate are connected together. The term thresh-

old voltage is used interchangeably with “bias voltage.”

Threshold voltage programming.

Encompasses all combinations of programmed gate bias

voltages, drain bias voltages and drain current levels.

Zero Temperature Coefficient, ZTC.

Operating point of an EPAD where voltage and current

changes as a function of temperature is zero.

APPLICATION NOTE AN1108 Advanced Linear Devices 3

FUNCTIONAL DESCRIPTION

Each EPAD’s input and output functional characteristics can

be modeled as a MOSFET with adjustable threshold volt-

age. Each EP AD has fiv e terminals, namely , the Gate, Drain,

Source, Programming and Substrate terminals. The pro-

gramming terminal is used only during electrical program-

ming to inject charge into the device. The injected charge is

permanently stored and raises the threshold voltage of the

device. For a given current level and a gate voltage, the

drain voltage var ies according to the threshold voltage. Af-

ter programming, the programming terminal can be left open

or connected to the source voltage.

A basic connection configuration is to connect gate and drain

terminals together. At a selected drain current, this gate-drain

connected voltage can be precisely set by using the EPAD

Programmer. After electrical programming, the EPAD Pro-

grammer can be disconnected, and the device retains the

set voltage indefinitely under normal operating conditions,

even when power is tur ned off. This connection configura-

tion is referred to as a diode-connected configuration.

The ALD E100 EPAD programmer injects a small, measured

amount of charge by pulsing the programming terminal. An

EPAD is in progr amming mode when both the gate terminal

and the progr amming terminal are pulsed together, through

the EPAD progr ammer, at approximately 12V. The number

of pulses required to bring the threshold voltage to a given

voltage value is determined by the EPAD programmer con-

trol software. All that the user needs to do is to input the

desired threshold voltage value. If a drain current other than

1µA is desired, that current level can be set by the user by

changing the value of the set-current resistor on the EPAD

Adapter Module. This drain current can also be any bias

current supplied by the user from an application circuit, such

as that from an external current source.

Please refer to the individual EPAD specifications for more

details on operating voltage and current ranges. With the

control software supplied with the EPAD Programmer

Adapter Module, the desired target threshold voltage value

can be f ed in either as a keyboard entry, or as a variab le by

a user-written program.

The gate voltage Vgs of an EPAD can be set as an input

voltage and the drain voltage Vds can be used as an output

voltage. When the Gate and the Drain terminals of the EPAD

are shorted together, in the diode-connected configuration,

Vds, equal to Vgs, is the output voltage. For many applica-

tions, it may be desirable to force Vds of an EPAD to a dif-

ferent drain bias voltage at a given circuit bias current, be-

f ore programming. The user then determines the final thresh-

old voltage necessary to adjust the transfer function of the

application circuit to a desired final value. The final bias

voltage is then the Vds voltage where the EPAD needs to

be set for that particular circuit.

SPECIAL DESIGN FEATURES

Each channel in the ALD1108E/ALD1110E EPAD devices

can be programmed separately or in matched pairs. For

certain applications , effects such as temperature coefficients

can be canceled out by using the matched pair capability.

Matched pairs are also ideal for applications where variables

must be adjusted in both directions. In this case, a circuit

parameter or variable can be adjusted in both directions al-

lowing “trimming” or “calibration” of the parameter variable

by the user to minimize temperature effects .

ZERO TEMPERATURE COEFFICIENT

Each channel within an ALD1108E/ALD1110E has a posi-

tive temperature coefficient current range and a negative

temperature coefficient current range. The crossover cur-

rent level is the zero tempco current level.

For the ALD1108E/ALD1110E EPADs, this current level is

approximately 68 µA. When biased at this current level, the

drain voltage and the drain current have significantly reduced

temperature coefficients, minimizing variation of the drain

voltage and related parameters throughout a given tempera-

ture range.

In applications where the system must have minimal varia-

tions due to changes in temperature, matched pair circuits

biased at 68µA is recommended. Many of the applications

mentioned in this application note can be biased at zero

tempco current. When biased at zero tempco current, the

typical single channel voltage tempco is 50ppm and the

matched pair relative voltage tempco is less than 10ppm.

COARSE OR FINE TRIMMING

EPADs can be used for both coarse and fine trimming appli-

cations. In coarse tr imming, a single EPAD can be used in

conjunction with a single resistor to form a resistor network.

In this configuration, and depending on the resistor value

and the bias current level of the EPAD, the tempco of the

circuit may not be well controlled, translating to a relatively

lo w resolution trimming and circuit stability. Howe ver, this is

also a quick, low-cost and easy voltage adjustment applica-

APPLICATION NOTE AN1108 Advanced Linear Devices 4

tion that can be readily automated. In fine trimming applica-

tions, in general, tw o EPADs are used in a matched circuit

with temperature effect cancellation. As two EPADs within

an IC package are matched by design and in close proxim-

ity to each other, their respective relative temperatures are

closely matched. As ambient temperature changes, both

EPAD channels have equal temperature effects. The two

matched EPAD channels can be used in a circuit with tem-

perature effect subtraction, resulting in a circuit that is tem-

perature-stable. As a general r ule, the temperature coeffi-

cient effects of a circuit with matched EPAD temperature

cancellation are an order of magnitude less than single

EPAD circuits.

Furthermore, in the zero tempco current mode, the corre-

sponding temperature coefficient effects of a dual EPAD-

based circuit can be reduced by one and a half orders of

magnitude, resulting in a circuit that is temperature-stable

and suitable for fine trimming applications. Note that zero

tempco bias mode can be used in both single and matched

pair EPAD applications, resulting in either coarse or fine trim-

ming of circuits that have improved temperature charac-

teristics.

RELAXATION V OLT A GE & RELAXATION TIME CONSTANT

After an EPAD has been programmed, the bulk of the elec-

trons are stored on a polysilicon gate capacitor with high

barrier energy and remains stored indefinitely. However, a

small number of residual electrons at a few oxide sites near

the oxide-to-silicon interface can gain sufficient energy to

become detrapped, causing a small loss of electrons. This

loss of electrons decreases exponentially with a time con-

stant.

This loss of electrons is shown as the difference between

the initial measured threshold v oltage of an EPAD and its

subsequently measured threshold voltage. After a short time

period, the threshold voltage stabilizes and reaches a final

threshold voltage that remains fixed indefinitely thereafter.

The amount of this residual charge loss causes a small volt-

age drop, referred to as the relaxation of the threshold volt-

age. The Relaxation Voltage is defined as the voltage drop

after three Relaxation Time Constants.

The EPAD Programmer control software supplied with the

Adapter Module compensates for the Relaxation Voltage by

programming the threshold (bias) voltage to a higher initial

threshold voltage. The control software contains a default

Relaxation Voltage compensation setting which can be al-

tered by the user. For the ALD1108E/ALD1110E, typical

Relaxation Time Constant is 2 hours and the Relaxation

Voltage is -0.3% of the threshold voltage.

DESIGN PRECAUTIONS

The ALD1108E/ALD1110E EPAD devices are designed for

use in low voltage, micropower circuits. The maximum op-

erating voltage during normal operation should remain be-

lo w 10 V olts at all times . Care should be taken to insure that

the application in which the devices are used does not ex-

perience any positive or negative transient voltages that

cause any of the terminal voltages to exceed this limit.

All inputs or unused pins except the programming pins should

be connected to V - so that the y do not become floating pins,

since input impedance at these pins is very high. If any of

these pins are left undefined, they may cause unwanted os-

cillation or intermittent e xcessive current dr ain. As these de-

vices are built with CMOS technology, normal ESD and

latchup handling precautions, operating and storage tem-

perature limits as stated in the data sheet must be observed.

APPLICATION NOTE AN1108 Advanced Linear Devices 5

APPLICATIONS

The application circuits as described in this application note

have been functionally tested by Advanced Linear Devices’

application staff. Ho wever, these circuits are for illustrative

purposes only, and should not be used for production or for

a specific application unless independently verified by the

user and found to be in conformance with all other user

requirements.

BASIC CIRCUITS

The basic circuit connection is shown in Figure 1. In this

connection, the gate terminal of the device is connected to

the drain terminal of the device. This connection forces the

gate and drain voltage to be equal at all times. As the gate

input has extremely high input resistance — at a few pA

input leakage current at room temperature — most or nearly

all of the bias current through the device flows from the drain

to the source terminal. This bias current can be set exter-

nally by a combination of Vt programming, selection of a

bias reference voltage and a bias resistor R. For some ap-

plications, it may be appropriate to replace R with a current

source so that the current Ids is relatively constant before

and after EPAD programming. The simplest bias method is

just to use a single resistor with value R ohms.

To calculate the value of the resistor R, first ref er to the data

sheet to insure that a desired current level is within the de-

vice specification. The resistor value R is simply given by

the following Ohm’s Law equation:

R = ( V+ - Vtm ) / Ids

where Vtm is the nominal midpoint value of the desired Vt

adjustment range,

V+ is the power supply voltage, and

Ids is the desired drain source current (equal to the

current through resistor R)

For example, with the ALD1108E, if a current level of 68µA

is chosen for Ids for minimum tempco, and if the supply volt-

age is 8.3V, and the Vt adjustment range is expected to be

between 1.000V and 2.000V, then Vtm is 1.5V.

With these values in the equation, we get:

R = ( 8.3 - 1.5 ) V/ 68µA = 100 Kohms

Note that for the above circuit the current Ids does not re-

main constant for different Vt values, as the current varies

with the value of the programmed Vt. In addition, the mag-

nitude of the Ids current value and the tempco of the resistor

R can also cause variation of Ids. These effects can be

reduced with a circuit such as the one shown in Figure 2.

When a current source is connected to an EPAD instead of

a biasing resistor R, the Ids of the EPAD is set by the cur-

rent source and its variation depends primarily on the cur-

rent source variations.

F or diff erent Vt v alues, if the current from the current source

is constant, then the Ids value also remains constant. The

Vt values could change due to programming, but if the Ids is

constant, the tempco of the Vds voltage also remains con-

stant. In this case, Vt programmed to any desired target Vt

value setting, is capable of remaining relatively temperature

stable. In many applications, this advantage may be worth

the extra circuitry required to produce a current source.

Figure 1

Basic Circuit Connection

1/4 ALD1108E

S12

GN1

V+

DN1

Ids

VO = Vt

APPLICATION NOTE AN1108 Advanced Linear Devices 6

VOLTAGE CONTROLLED RESISTOR

One of the basic characteristics of the EPAD is that its

MOSFET output on-resistance can be controlled with a ref-

erence bias voltage . When the EPAD is biased off, as when

the reference bias voltage (the input gate voltage) is set to

more than 0.5V below Vt, the output resistance is greater

than 10 Gigaohm. When the bias reference voltage is in-

creased to Vt, the EPAD is biased on, and its on-resistance

is 1 Mohm. As the reference bias v oltage is set to abo ve Vt,

the output resistance decreases.

For the ALD1108E/ALD1110E devices, the output on-resis-

tance decreases from 1 Mohm to approximately 1.5 Kohm

at a reference voltage of 5 Volts. This represents many or-

ders of magnitude of change in on-resistance. For a given

reference bias voltage, the on-resistance of an EPAD is ad-

justable within a cer tain range. This may be useful in appli-

cations where a fixed reference voltage is available, and

where many different on-resistance values are desired. In

Figure 3, with the addition of a fixed resistor, the circuit be-

comes a voltage divider. The output voltage varies accord-

ing to the resistance ratios between the fixed resistor and

the EPAD on-resistance. As the on-resistance of the EPAD

is adjusted, the output voltage is changed, producing the

desired VO permanently.

MATCHED EPAD PAIRS

Matched EPAD pairs used in application circuits hav e many

significant circuit advantages over single EPAD circuits.

Within a pair of EPADs, many parameters can be matched

and the parasitic and temperature effects of these devices

can be manipulated to cancel each other. The net result is a

much more balanced and temperature-stable circuit.

For ALD1108E, the first and second channels form one EPAD

pair and the third and fourth channels form another. Note

that the matched EPAD pairs share common source con-

nections. When connected in a differential configuration,

tw o EPAD channels also provide f or bidirectional parameter

adjustment capabilities, as illustrated in Figure 4 and Figure 5 .

The output in these circuits, V O, can be adjusted up or down

by programming the respective EPAD channel. If the circuit

is designed into an application where an adjustment is nec-

essary, EPAD1 and EPAD2 Vt can be progressively pro-

grammed, allowing multiple up and down adjustments to the

VO.

One noteworthy aspect of this capability is that the overall

gain of the circuit depends on the value of R. For larger

values of R, the gain of the circuit is greater, and the mis-

matched offset voltage is amplified by the gain. For best

results, the drain current of the EPADs should be set to

68 µA, which effectively minimiz es the temper ature depen-

dence of the output voltage VO. If the resistors are also

matched, as may be the situation when using resistor packs,

then the respective resistors also experience tempco

cancellation, further enhancing the overall circuit stability.

Note: VREF can be

connected to any relatively

stable voltage, such as a

reference voltage or V+

Figure 3

Voltage Controlled Resistor

Figure 2

Basic Circuit Connection with Current Source

1/4 ALD1108E

V+

VREF

S12

GN1

DN1

Ids

1/4 ALD1108E

S12

GN1

V+

DN1

Ids

VO = Vt

APPLICATION NOTE AN1108 Advanced Linear Devices 7

Figure 6

EPAD Output With Buffer Amplifier

Figure 4

Mached EPAD Pair With A Single Supply EPAD VOLTAGE OUTPUT WITH BUFFER

A simple voltage follower is shown in Figure 6. The voltage

follower isolates the output and prevents it from loading the

EPAD bias resistor and causing loading effects. Again, for

low tempco, set R such that the current I is 68µA.

BASIC CURRENT SOURCE

A basic current source with output IO is shown in Figure 7.

V alue of resistor R is selected such that current through R is

approximately 68µA for low temperature effects. Resistor

Rset is biased with the same voltage registered across the

EPAD. This voltage is set by bias resistor value R and the

EPAD programmed threshold voltage. The tempco of out-

put current IO is directly proportional to the tempco of Rset.

This tempco can be minimized by selection of an appropri-

ate resistor type. In this circuit, the output current can be

set by programming the EPAD threshold voltage.

In order to set the output current IO, a current meter can be

set up to monitor IO through ALD1106 N-Channel MOSFET

while a voltage source forces a voltage on Vbias. Once a

desired current level is attained, then that voltage which pro-

duces this desired current, Vbias, can be noted. The EPAD

Vt can now be programmed to Vbias, using the same sup-

ply voltage and resistor R. This entire process can be fully

automated with the appropriate equipment, setup and cus-

tomized control software.

For low tempco set Ids ≈ 68µA

For V+ = +5V, R ≈ 47KΩ

Figure 5

Matched EPAD Pair With Dual Supplies

For low tempco set Ids ≈ 68µA

For V+ = +5V & V- = -5V, R ≈ 47KΩ

1/2 ALD1108E

S12

DN2DN1

GN2

GN1

V-

V+

V-

Ids

Ids x 2

1/2 ALD1108E

S12

DN2DN1

GN2

GN1

V-

V+

Ids Ids

For low tempco set Ids ≈ 68µA

For V+ = +5V, R ≈ 47KΩ

ALD1702

V+

1/4 ALD1108E

V+

GN1

DN1

S12

APPLICATION NOTE AN1108 Advanced Linear Devices 8

HIGH OUTPUT IMPEDANCE CURRENT MIRROR

A calibrated cascode current mirror is shown in Figure 8A.

The input reference current can be set by means of a refer-

ence bias voltage and a fixed resistor by programming the

EPAD connected to the input reference current. The output

current IOUT of the current mirror can be set to within 1% of

the desired current mirror value. Although the mirrored cur-

rent can be matched to the input reference current, it can

also be set to a ratioed current value, where IOUT is propor-

tional to the current source. The output impedance of this

programmable current mirror is approximately 20 Mohms.

The current mirror in Figure 8A can be easily modified to

become an electronically calibrated current source that

can be programmed and set at 68µA, as shown in Figure

8B. Note that a reference voltage VREF or a relatively stable

voltage source is required. The tempco of RREF is the key

factor that determines the tempco of the current source.

Figure 7

Current Source With EPAD Trimming

For low tempco set Ids ≈ 68µA

For V+ = +5V, R ≈ 47KΩ

1/4 ALD1108E

SET

V+

1/4 ALD1106

S12

ALD1701

V+

bias

GN1

DN1

SN1

GN1

DN1

Ids

Figure 8A

High Output Impedance Current Mirror

Figure 8B

High Output Impedance Current Source

1/2 ALD1106

SN1

DN2DN1

GN2

GN1

V-

1/2 ALD1108E

REF

OUT

S12

DN2DN1

GN2

GN1

V-

SN2

REF

1/2 ALD1106

SN1

DN2DN1

GN2

GN1

V-

1/2 ALD1108E

REF

OUT

S12

DN2DN1

GN2

GN1

V-

CURRENT

SOURCE

SN2

APPLICATION NOTE AN1108 Advanced Linear Devices 9

1/4 ALD1108E

V+

= R

ALD1702

V+

GN1

DN1

S12

V+

1/4 ALD1108E

ALD1702

-V+

GN1

DN1

S12

Figure 11

Amplify and Shift EPAD Adjustment Range

with an Operational Amplifier

VO = (V+) - [ ((V+) - Vt) ]

For V+ = +5V, R ≈ 47K

RA + RB

1/4 ALD1108E

V+

ALD1702

-V+

GN1

DN1

S12

Figure 9

Scaling Adjustable Vt

NOTE: For RA = RB = R

VO = 2Vt

Figure 10

Select Vo Adjustment Range by Using Resistor Ratios

EPADs WITH ACTIVE CIRCUITS

The EPAD threshold voltage Vt can be scaled, ratioed and

shifted with operational amplifier circuits. Many of these cir-

cuits are common operational amplifier circuits. A few of

these circuits are illustrated below in Figure 9, Scaling Ad-

justable Vt; Figure 10, Select VO Adjustment Range by us-

ing Resistor Ratios and Figure 11, Amplify and Shift EPAD

Adjustment range with an operational amplifier. Obviously

many more types of operational amplifier circuits can also

be used with an EPAD as an adjustment vehicle.

USEFUL HINTS

A couple of useful hints in designing with EPADs would be

appropriate here. First, in general, a relatively large EPAD

Vt adjustment range should be used. A recommended ad-

justment range would be from a 1V range such as Vt =1.000V

to 2.000V range. This minimizes error budget due to pro-

gr amming v oltage errors and temperature effects causing a

shift to Vt.

For example, assume a Vt adjustment range of 1.000V to

2.600V range, giving a voltage programming range of

1600mV. Assume further that the maxim um system error is

estimated to be 32mV. The error voltage in this case would

be 32mV/1600mV, or 2 percent. Matched EPAD circuit tech-

niques and zero tempco biasing can further reduce the er-

ror. For more limited temperature ranges, such as may be

the case for some laboratory equipment operating environ-

ments, the errors can be brought down to about 10mV with

careful design and error budgeting. For example, using

a 2V adjustment range of Vt =1.000V to 3.000V would yield

an error of 10 mV/2000mV, or 0.5 percent.

If a lower adjusted absolute error voltage is desired, voltage

scaling can be used. Using the above example, an adjust-

ment range of 2000mV with an error range of 10mV can be,

with the help of some active circuitry, scaled down to an

adjustment range of 200mV with an error range of 1mV. Note

that additional errors may be introduced by active scaling of

the circuit. However, keep in mind that if scaling-induced

adjustment error is fixed and temperature and time invari-

ant, it can be adjusted through EPAD programming.

For RA = RB = R

VO = (V+) + (Vt)

APPLICATION NOTE AN1108 Advanced Linear Devices 10

Figure 13

EPAD With Buffered Outputs Pr oviding

Bi-directional Vt Adjustment

V+

1/2 ALD1108E

1/4 ALD4701

-V+

S12

DN2DN1

GN2

GN1 +

V+

1/4 ALD4701

V+

R = 47KΩ

Rf = 1MΩ

Rf >> R

Figure 12

EPAD Circuit With Bi-directional Vt Adjustment

ALD 1701

RV+

1/2 ALD1108E

S12

V+V+

DN2DN1

GN2

GN1

V-

EPAD CIRCUIT WITH BI-DIRECTIONAL Vt ADJUSTMENT

For applications where multiple adjustment of a voltage pa-

r ameter is necessary, two EPADs can be employed in a cir-

cuit such as Figure 12. In this circuit, VO can be adjusted up

or down multiple times, by programming the respective

EPAD. Care should be used in the design such that the

amount of Vt adjustment step be kept small enough so that

the two EPADs can be programmed upwards many times

alternately . On the other hand, the Vt adjustment step should

be large enough to minimize error due to other various error

terms . The resistor value of Rf should be high relativ e to the

value of R to avoid excessive loading down resistor R and

affecting the bias current supplied to the EPAD.

To isolate the outputs of the EPAD stages from the output

amplifier in Figure 12, the same circuit can be b uffered with

unity gain buffer amplifiers, as shown in Figure 13. In this

circuit, the resistor value R can be set to produce a zero

tempco bias current of 68µA, or any other desired current

level. Due to differential EPAD pairs feeding the same out-

put amplifier stage at the output, temperature effect due to

one of the EPAD circuits cancels out that of the other. The

resulting circuit is temperature-stable. The resistors RA and

RB can also be set to other values as needed, as this circuit

eliminates the EPAD bias current level sensitivity to the

resistor values of R A and RB.

APPLICATION NOTE AN1108 Advanced Linear Devices 11

Vout = ∑ Vin (X) • w(X) = RL • ISUM = RL • ∑ I(X)

Figure 14

Neural Network Circuit

NEURAL NETWORK CIRCUIT

For applications where neural network circuit function is de-

sired, or where multiple inputs and their respective weights

are summed at the output, an EPAD-based neural circuit

can be constructed as shown in Figure 14.

The precision and resolution of the circuit depends on the

reference voltage, the output resistor and the accuracy and

resolution available from the EPAD. The w eights of the neu-

ral network, in this case, are the corresponding resultant

drain currents at the output as a result of programming of

the Vt values. The output node is at the same time also a

summing node, where all the currents are summed through

the output resistor, RL.

This circuit may be most suitably used where the “learning”

is already accomplished, and the “learned” function requires

hardware implementation. The key consideration here is sim-

plicity of the circuitry involved. This circuit is especially ap-

propriate for low cost, stand-alone hardware implementa-

tion of neural networks.

The circuit shown in Figure 14 implements the following

equation: Vout = ∑ { Vin (x) * w (x) }

= RL * I sum

= RL * ∑{ I(x) }

where

Vin(x) = input to each neural channel

w(x) = weight of each neural channel

RL= summing resistor at the output node

Isum = sum of the currents through the EPADs

VREF = reference voltage

Once the weights of a given neural network circuit have been

derived through a learning process, these values can be

progr ammed into each EPAD in each respective neural net-

work channel.

V+ = +3V

REF

= +3.000V

ALD1721E

50K

in1

in2

in3

in4

ALD4201

50K

V+

+3V

SUM

1K V

out

V- S ALD1108E

APPLICATION NOTE AN1108 Advanced Linear Devices 12

Figure 15. Standard Stand alone EPAD Programming System

Figure 15C. In-System Programming With EPAD Incorporated

into User Application Circuit

* EA Series Control Software, Interface Adapter Cable And Interface Adapter can All

be customized by User for specific application, if necessary.

Figure 15B. EA Series Interface Adapter Module Custom Tailored

to Application-Specific Conditions and Environment

EPAD

EA SERIES

CONTROL

SOFTWARE PC

E100 EPAD

PROGRAMMER

PARALLEL

PRINTER

CABLE EA SERIES

INTERFACE

ADAPTER

INTERFACE

ADAPTER

CABLE

EPAD

EA SERIES

CONTROL

SOFTWARE PC

E100 EPAD

PROGRAMMER

PARALLEL

PRINTER

CABLE EA SERIES

INTERFACE

ADAPTER

INTERFACE

ADAPTER

CABLE

REMOTE

ENVIRONMENTAL

CHAMBER

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

REMOTE

ENVIRONMENTAL

CHAMBER

REMOTE

USER

PROGRAM

SOFTWARE

INTERFACE

CUSTOMIZED

BY USER

USER CUSTOMIZED

APPLICATION NOTE AN1108 Advanced Linear Devices 13

V+

1/4 ALD1108E

ALD1702

-V+

GN1

DN1

S12

RISO

VO = (V+) - [ (V+) - Vt) (RA + RB) ]

For V+ = +5V,

R ≈ 47K

Figure 16

In-System Programming: Amplify and Shift EPAD

Adjustment Range with an Operational Amplifier

EPAD PROGRAMMING

Figure 15A shows a standard EPAD programming system.

The setup and programming by the user is simple and

straight forward. The user provides a personal computer, a

parallel printer cab le and an external DC power supply. The

entire system can be set up in a laboratory environment and

be ready to start programming in a matter of minutes.

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 sup-

plied interface Adapter Module can be readily modified by

the user to reflect these conditions. In addition, the Adapter

Module, application circuit and EPAD can be set up inside

an environmental chamber f or EPAD adjustment under pre-

cise environmental conditions. The diagram of this setup is

illustrated in Figure 15B.

A simple e xample of Application-Specific Programming is to

change the operating voltage of an EPAD circuit. As an

Adapter Module has a factory preset operating voltage, if a

user application system voltage is at a different voltage lev el,

a small supply voltage induced error may be introduced into

the system. This small error can be eliminated by discon-

necting the V+ of the Adapter Module from the EPAD Pro-

grammer, and bringing in external supply into V+ terminal

instead. EPAD can now be programmed to user system

supply conditions. Often this kind of hardware change is

simple and effectiv e in eliminating system errors without any

control software programming change.

When using environmental chambers, care must be taken

to insure that the Adapter Module’s voltage and tempera-

ture ratings are not exceeded. For continued high tempera-

ture applications, the user is advised to set up a custom

adapter circuit board with a connection to a separate high

temperature socket for EPAD programming.

IN-SYSTEM PROGRAMMING (ISP)

For some applications, the user may desire to hav e the EPAD

soldered onto the production circuit board bef ore EPAD pro-

gramming. This is accomplished by designing the applica-

tion circuit to accommodate two modes of operation — a

normal operating mode and a programming mode. Once

such a circuit is designed, a special cable enables in-sys-

tem programming. Refer to Figure 15C. Many of the pre-

ceding circuits from Figure 1 through Figure 14 may also be

viewed as examples of EPAD progr ammable in-system

circuits.

In order to program an EPAD as an in-system element, the

gate terminal of the EPAD m ust be ab le to be pulsed b y the

EPAD Programmer. The application circuit in which the EPAD

is embedded can be powered by any user-selected voltage.

However, it must allow the gate voltages of the EPAD to be

pulsed without a low impedance path to other circuit nodes,

so that the EPAD programmer pulsing circuit can function

properly.

For many of the circuits, such as those in Figures 1 through

13, the bias resistor R should be set at greater than 20

Kohms. In Figure 12, in par ticular, the resistor value of Rf

should be much greater than resistor value of R. If setting R

value greater than 20 Kohms is not possible, a 50 Kohm

resistor or greater isolation resistor Riso can be used to con-

nect drain and gate terminals.

APPLICATION NOTE AN1108 Advanced Linear Devices 14

For example, the circuit in Figure 16, In-System Program-

ming, is a modified circuit based on Figure 11 to provide

conditions suitable for programming mode. In this case, this

is accomplished by the addition of a single resistor R (iso).

When the gate terminal is now pulsed during programming,

the dr ain terminal is effectively isolated from the gate termi-

nal. During normal operation, the gate terminal voltage is

not appreciably affected as a result of R(iso) because the

gate input impedance is very high.

This in-system approach to programming the EPAD is po-

tentially the most powerful in adjusting system level errors

involving other components, such as variations in unit to unit

sensor parameters and passive component parameters. If

integration of the EPAD programming is desired, while sys-

tem testing and/or calibration is taking place, then more cir-

cuit design and software programming may be necessary.

A special In-System Programming extension cable allows

temporary connection of the EPAD programmer to the in-

system EPAD element directly within the application circuit.

The cable is disconnected for future use after programming.

CONTROL SOFTWARE

Control software supplied with the EPAD Programmer

Adapter is used to control the programming routines and to

send commands to the EPAD Programmer to control, mea-

sure and compute programming conditions for the specific

adapter module. In most cases, the recommended custom

adaptation of this control software is to input a desired volt-

age value that can be measured at the drain terminal of the

EPAD. The control software algorithm then takes over. When

in programming mode, the gate and programming terminals

are pulsed in a controlled, optimal sequence of voltage bias-

ing conditions. These conditions m ust be preser ved by the

user in order for the programming system to function prop-

erly.

For more assistance in application-specific or in-system

EPAD programming, please contact an Advanced Linear De-

vices’ applications engineer. If you have a circuit question

or problem, Fax or e-mail us at:

FAX (408) 747-1286

Attention: Applications

E-mail: applications@aldinc.com