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

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

but this is easily changed by varying C1. The circuit operates

well up to a few hundred hertz.

1/4

OP490

A

1/4

OP490

B

SQUARE

OUT

IN/OUT

IN/OUT

IN/OUT

IN/OUT

CONT

CONT

CONT

CONT

OUT/IN

OUT/IN

OUT/IN

OUT/IN

C1

75nF

TRIANGLE

OUT

R5

200k

Ω

R6

200k

Ω

+15V

7

6

5

R1

200k

Ω

R2

200k

Ω

+15V

R4

200k

Ω

1

4

11

2

3

R3

100k

Ω

S1

S2

S3

S4

R8

200k

Ω

+5V

+5V

+5V

R7

200k

Ω

114

213

312

411

510

69

78

Figure 22. Micropower Voltage Controlled Oscillator