AN1108 Advanced Linear Devices, AN1108 Datasheet - Page 3

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

FUNCTIONAL DESCRIPTION
Each EPAD’s input and output functional characteristics can
be modeled as a MOSFET with adjustable threshold volt-
age. Each EPAD has five 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 varies 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 turned 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 programming mode when both the gate terminal
and the programming terminal are pulsed together, through
the EPAD programmer, 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
1 A 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 fed in either as a keyboard entry, or as a variable 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-
APPLICATION NOTE AN1108
Advanced Linear Devices
ferent drain bias voltage at a given circuit bias current, be-
fore 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 A. 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 68 A 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 trimming, 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
low resolution trimming and circuit stability. However, this is
also a quick, low-cost and easy voltage adjustment applica-
3