OP90

–10–

Micropower Voltage-Controlled Oscillator

Two OP90s in combination with an inexpensive quad CMOS

switch comprise the precision VCO of Figure 10. This circuit

provides triangle and square wave outputs and draws only 50

µA

from a single 5 V supply. A1 acts as an integrator; S1 switches

the charging current symmetrically to yield positive and negative

ramps. The integrator is bounded by A2 which acts as a Schmitt

trigger with a precise hysteresis of 1.67 V, set by resistors R5,

R6, and R7, and associated CMOS switches. The resulting output

of A1 is a triangular wave with upper and lower levels of 3.33 V

and 1.67 V. The output of A2 is a square wave with almost

rail-to-rail swing. With the components shown, frequency of

operation is given by the equation:

fV

V

Hz V

OUT

CONTROL

=

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

well up to a few hundred hertz.

Micropower Single-Supply Instrumentation Amplifier

The simple instrumentation amplifier of Figure 11 provides over

110 dB of common-mode rejection and draws only 15

µA of

supply current. Feedback is to the trim pins rather than to the

inverting input. This enables a single amplifier to provide differ-

ential to single-ended conversion with excellent common-mode

rejection. Distortion of the instrumentation amplifier is that of a

differential pair, so the circuit is restricted to high gain applica-

3

7

C1

75nF

6

4

2

R3

100k

R4

200k

R1

200k

R2

200k

+5V

TRIANGLE

OUT

R8

200k

+5V

R5

200k

3

7

6

4

2

SQUARE

OUT

R6

200k

R7

200k

IN/OUT

OUT/IN

IN/OUT

CONT

CONT

CONT

1

IN/OUT

IN/OUT

+5V

CONT

2

3

4

5

6

7

14

13

12

11

10

9

8

CD4066

+5V

+5V

+5V

OUT/IN

OUT/IN

OUT/IN

OP90

A2

S1

S2

S3

S4

OP90

A1

Figure 10. Micropower Voltage Controlled Oscillator

tions. Nonlinearity is less than 0.1% for gains of 500 to 1000

over a 2.5 V output range. Resistors R3 and R4 set the voltage

gain and, with the values shown, yield a gain of 1000. Gain

tempco of the instrumentation amplifier is only 50 ppm/

°C.

Offset voltage is under 150

µV with drift below 2 µV/°C. The

OP90’s input and output voltage ranges include the negative

rail which allows the instrumentation amplifier to provide true

“zero-in, zero-out” operation.

R1

4.3M

R4

3.9M

R3

1M

R2

500k

GAIN

ADJUST

3

7

6

4

2

5

–IN

1

+IN

0.1 F

+5V

OP90

Figure 11. Micropower Single-Supply Instrumentation

Amplifier

REV. C