AN1108 Advanced Linear Devices, AN1108 Datasheet - Page 5

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

APPLICATIONS
The application circuits as described in this application note
have been functionally tested by Advanced Linear Devices’
application staff. However, 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 refer 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:
where
For example, with the ALD1108E, if a current level of 68 A
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:
APPLICATION NOTE AN1108
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)
R = ( 8.3 - 1.5 ) V/ 68 A = 100 Kohms
R = ( V+ - Vtm ) / Ids
Advanced Linear Devices
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.
For different Vt values, 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.
Basic Circuit Connection
GN1
Figure 1
V+
DN1
S12
R
1/4 ALD1108E
Ids
V
O
V
O
= Vt
5