AN1108 Advanced Linear Devices, AN1108 Datasheet - Page 10

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

EPAD CIRCUIT WITH BI-DIRECTIONAL Vt ADJUSTMENT
For applications where multiple adjustment of a voltage pa-
rameter is necessary, two EPADs can be employed in a cir-
cuit such as Figure 12. In this circuit, V
or down multiple times, by programming the respective
EPAD. Care should be used in the design such that the
amount of Vt adjustment step be kept small enough so that
the two EPADs can be programmed upwards many times
alternately. On the other hand, the Vt adjustment step should
be large enough to minimize error due to other various error
terms. The resistor value of Rf should be high relative to the
value of R to avoid excessive loading down resistor R and
affecting the bias current supplied to the EPAD.
To isolate the outputs of the EPAD stages from the output
amplifier in Figure 12, the same circuit can be buffered with
unity gain buffer amplifiers, as shown in Figure 13. In this
circuit, the resistor value R can be set to produce a zero
tempco bias current of 68 A, or any other desired current
level. Due to differential EPAD pairs feeding the same out-
put amplifier stage at the output, temperature effect due to
one of the EPAD circuits cancels out that of the other. The
resulting circuit is temperature-stable. The resistors R
R
eliminates the EPAD bias current level sensitivity to the
resistor values of R
APPLICATION NOTE AN1108
B
can also be set to other values as needed, as this circuit
A
and R
B.
O
can be adjusted up
Advanced Linear Devices
A
and
GN1
EPAD Circuit With Bi-directional Vt Adjustment
DN1
V+
GN1
EPAD With Buffered Outputs Providing
S12
R
V-
1/2 ALD1108E
DN1
V+
V+
Bi-directional Vt Adjustment
S12
R
R
DN2
1/2 ALD1108E
GN2
V+
R
DN2
Figure 12
Figure 13
GN2
-
-
+
+
V+
V+
R
R
R
f
f
f
1/4 ALD4701
1/4 ALD4701
R = 47K
R
R
f
f
= 1M
>> R
-
+
R
R
A
A
V+
R
f
R
ALD 1701
B
+
-
R
V+
B
1/4 ALD4701
V
O
10
V
O