AN1108 Advanced Linear Devices, AN1108 Datasheet - Page 9

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

Select V
EPADs WITH ACTIVE CIRCUITS
The EPAD threshold voltage Vt can be scaled, ratioed and
shifted with operational amplifier circuits. Many of these cir-
cuits are common operational amplifier circuits. A few of
these circuits are illustrated below in Figure 9, Scaling Ad-
justable Vt; Figure 10, Select V
ing Resistor Ratios and Figure 11, Amplify and Shift EPAD
Adjustment range with an operational amplifier. Obviously
many more types of operational amplifier circuits can also
be used with an EPAD as an adjustment vehicle.
APPLICATION NOTE AN1108
Vt
Vt
o
Adjustment Range by Using Resistor Ratios
R
R
GN1
A
B
GN1
= R
= R
2
2
Scaling Adjustable Vt
V+
V+
R
DN1
S12
1/4 ALD1108E
Figure 10
DN1
S12
Figure 9
1/4 ALD1108E
R
A
O
Adjustment Range by us-
+
-
-
+
ALD1702
NOTE: For R A = R B = R
V+
For R A = R B = R
V O = (V+) + (Vt)
R
V+
B
V
ALD1702
O
= 2V t
2
V
Advanced Linear Devices
2
O
V
O
USEFUL HINTS
A couple of useful hints in designing with EPADs would be
appropriate here. First, in general, a relatively large EPAD
Vt adjustment range should be used. A recommended ad-
justment range would be from a 1V range such as Vt =1.000V
to 2.000V range. This minimizes error budget due to pro-
gramming voltage errors and temperature effects causing a
shift to Vt.
For example, assume a Vt adjustment range of 1.000V to
2.600V range, giving a voltage programming range of
1600mV. Assume further that the maximum system error is
estimated to be 32mV. The error voltage in this case would
be 32mV/1600mV, or 2 percent. Matched EPAD circuit tech-
niques and zero tempco biasing can further reduce the er-
ror. For more limited temperature ranges, such as may be
the case for some laboratory equipment operating environ-
ments, the errors can be brought down to about 10mV with
careful design and error budgeting. For example, using
a 2V adjustment range of Vt =1.000V to 3.000V would yield
an error of 10 mV/2000mV, or 0.5 percent.
If a lower adjusted absolute error voltage is desired, voltage
scaling can be used. Using the above example, an adjust-
ment range of 2000mV with an error range of 10mV can be,
with the help of some active circuitry, scaled down to an
adjustment range of 200mV with an error range of 1mV. Note
that additional errors may be introduced by active scaling of
the circuit. However, keep in mind that if scaling-induced
adjustment error is fixed and temperature and time invari-
ant, it can be adjusted through EPAD programming.
Vt
Amplify and Shift EPAD Adjustment Range
with an Operational Amplifier
GN1
V+
R
1/4 ALD1108E
DN1
S12
Figure 11
R
B
For V+ = +5V, R
V
O
= (V+) -
-
+
R
[ (
A
(V+) - Vt
V+
ALD1702
47K
)
R
V
O
A
R
+ R
B
B
9
]