AN1108 Advanced Linear Devices, AN1108 Datasheet - Page 2

• Manufacturer Part Number: AN1108
• Description: Application Note For Electrically Programmable Analog Device Applications
• Manufacturer: Advanced Linear Devices

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 lowering 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 programmed 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 68 A.
Both of these Adapter Modules can be readily modified for
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 Programming, the ALD1108E or ALD1110E
device is designed into an application circuit where it be-
comes an integral part 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 different current levels, the bias resistor values within
APPLICATION NOTE AN1108
Advanced Linear Devices
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 1 A, when
source voltage is at 0.0 Volt. The initial threshold voltage is
also a default factory setting. However, 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, V
Same as threshold voltage, except at a drain-source bias
current different from that specified at threshold voltage. For
example, the threshold voltage of the ALD1108E/ALD1110E
is specified at drain source bias current of 1 A. 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 Voltage, 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 1 A 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.
bias
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