OP193/OP293

Rev. C | Page 17 of 20

From the formulas, it can be seen that if the span trim is adjusted

before the zero trim, the two trims are not interactive, which

greatly simplifies the calibration procedure.

Calibration of the transmitter is simple. First, the slope of the

output current vs. temperature is calibrated by adjusting the

span trim, R7. A couple of iterations may be required to be sure

the slope is correct.

When the span trim has been adjusted, the zero trim can be

made. Adjusting the zero trim does not affect the gain.

The zero trim can be set at any known temperature by adjusting

R5 until the output current equals:

(

4

+

−













∆

∆

=

MIN

AMBIENT

OPERATING

FS

OUT

T

T

T

I

I

Table 7 shows the values of R6 required for various temperature

ranges.

Table 7. R6 Values vs. Temperature

Temp Range

R6

0°C to 70°C

10 kΩ

−40°C to +85°C

6.2 kΩ

−55°C to +150°C

3 kΩ

A MICROPOWER VOLTAGE CONTROLLED

OSCILLATOR

The OP293 CMOS analog switch forms the precision VCO of

Figure 36. 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 Resistor R5, Resistor R6, and Resistor R7, and

associated CMOS switches. The resulting output of A1 is a

triangle 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:

However, the frequency can easily be changed by varying C1.

The circuit operates well up to 500 Hz.

8

4

1

2

3

6

5

7

5V

S3

S4

S1

1

IN/OUT

2

OUT/IN

3

OUT/IN

S2

4

IN/OUT

6

CONT

5

CONT

7

14

5V

5V

CD4066

5V

–

+

–

+

5V

SQUARE

OUT

TRIANGLE

OUT

A1

1/2 OP293

A2

1/2 OP293

C1

75nF

R5

200k

Ω

R1

200k

Ω

R7

200k

Ω

R6

200k

Ω

R8

200k

Ω

R4

200k

Ω

R3

100k

Ω

R2

200k

Ω

Figure 36. Micropower Voltage Controlled Oscillator