AN1108 Advanced Linear Devices, AN1108 Datasheet - Page 4

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

tion that can be readily automated. In fine trimming applica-
tions, in general, two 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 rule, 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 VOLTAGE & 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 voltage 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
APPLICATION NOTE AN1108
Advanced Linear Devices
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-
low 10 Volts 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 they 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 excessive current drain. 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.
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