Datasheet

OP490
Rev. E | Page 10 of 16
INPUT VOLTAGE PROTECTION
The OP490 uses a PNP input stage with protection resistors in
series with the inverting and noninverting inputs. The high
breakdown of the PNP transistors coupled with the protection
resistors provides a large amount of input protection, allowing
the inputs to be taken 20 V beyond either supply without
damaging the amplifier.
MICROPOWER VOLTAGE-CONTROLLED
OSCILLATOR
An OP490 in combination with an inexpensive quad CMOS
switch comprise the precision V
CO
of Figure 22. This circuit
provides triangle and square wave outputs and draws only 75 μA
from a 5 V supply. A acts as an integrator; S1 switches the
charging current symmetrically to yield positive and negative
ramps. The integrator is bounded by B, which acts as a Schmitt
trigger with a precise hysteresis of 1.67 V, set by Resistors R5,
R6, and R7, and the associated CMOS switches. The resulting
output of A is a triangle wave with upper and lower levels of
3.33 V and 1.67 V. The output of B is a square wave with almost
rail-to-rail swing. With the components shown, frequency of
operation is given by the equation
f
OUT
= V
CONTROL
(Volts) × 10 Hz/V
but this is easily changed by varying C1. The circuit operates
well up to a few hundred hertz.
V
CONTROL
00308-022
+
–
1/4
OP490
A
+
–
1/4
OP490
B
SQUARE
OUT
IN/OUT
IN/OUT
IN/OUT
IN/OUT
CONT
CONT
CONT
CONT
V
SS
V
DD
OUT/IN
OUT/IN
OUT/IN
OUT/IN
C1
75nF
TRIANGLE
OUT
R5
200k
R6
200k
+15V
7
6
5
R1
200k
R2
200k
+15V
R4
200k
1
4
11
2
3
R3
100k
S1
S2
S3
S4
R8
200k
+5V
+5V
+5V
R7
200k
114
213
312
411
510
69
78
Figure 22. Micropower Voltage Controlled Oscillator