AN1108 Advanced Linear Devices, AN1108 Datasheet - Page 6

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

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 voltage is set to above 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 certain 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 V
APPLICATION NOTE AN1108
Basic Circuit Connection with Current Source
O
permanently.
GN1
Figure 2
V+
S12
DN1
1/4 ALD1108E
Ids
V
O
V
O
= Vt
Advanced Linear Devices
MATCHED EPAD PAIRS
Matched EPAD pairs used in application circuits have 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,
two EPAD channels also provide for bidirectional parameter
adjustment capabilities, as illustrated in Figure 4 and Figure 5.
The output in these circuits, V
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
V
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
dence of the output voltage V
matched, as may be the situation when using resistor packs,
then the respective resistors also experience tempco
cancellation, further enhancing the overall circuit stability.
68 A, which effectively minimizes the temperature depen-
O
.
VREF
Voltage Controlled Resistor
GN1
V+
Figure 3
O
S12
R
DN1
, can be adjusted up or down
O
1/4 ALD1108E
Ids
. If the resistors are also
V
O
connected to any relatively
stable voltage, such as a
reference voltage or V +
Note: V REF can be
6