REV. B

OP193/OP293/OP493

–13–

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 versus temperature is calibrated by adjusting the

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

the slope is correct.

Once the span trim has been completed, the zero trim can be made.

Remember that adjusting the zero trim will not affect the gain.

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

R5 until the output current equals:

IT

T

OUT

I

T

FS

OPERATING

AMBIENT

MIN

=−

+







∆

∆

4 mA

Table I shows the values of R6 required for various temperature

ranges.

Table I. R6 Values vs. Temperature

Temp Range

R6

0

°C to 70°C

10 k

Ω

–40

°C to +85°C

6.2 k

Ω

–55

°C to +150°C3 kΩ

A Micropower Voltage Controlled Oscillator

An OP293 in combination with an inexpensive quad CMOS

analog switch forms the precision VCO of Figure 12. This cir-

cuit 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 volts, set by

resistors R5, R6, and R7, and associated CMOS switches. The

resulting output of A1 is a triangle wave with upper and lower

levels of 3.33 and 1.67 volts. 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

OUT

CONTROL

=×

V

Hz

10

/

but this can easily be changed by varying C1. The circuit oper-

ates well up to 500 Hz.

8

4

1/2 OP293

1

2

3

R3

100k

R4

200k

TRIANGLE

OUT

A1

5V

R1

200k

R2

200k

1/2 OP293

6

5

7

A2

SQUARE

OUT

5V

R6

200k

R7

200k

S3

S4

S1

1 IN/OUT

2 OUT/IN

CONT 12

OUT/IN

10

3 OUT/IN

S2

CONT 13

4 IN/OUT

6 CONT

5 CONT

7

IN/OUT

8

IN/OUT 11

OUT/IN

9

14

5V

5V

CD4066

R8

200k

5V

R5

200k

C1

75nF

Figure 12. Micropower Voltage Controlled Oscillator

A Micropower, Single-Supply Quad Voltage Output 8-Bit DAC

The circuit of Figure 13 uses the DAC8408 CMOS quad 8-bit

DAC and the OP493 to form a single-supply quad voltage out-

put DAC with a supply drain of only 140

µA. The DAC8408 is

used in the voltage switching mode and each DAC has an out-

put resistance (

≈10 kΩ) independent of the digital input code.

The output amplifiers act as buffers to avoid loading the DACs.

The 100 k

Ω resistors ensure that the OP493 outputs will swing

to within 1/2 LSB of ground, i.e.:

1

2

256

×=

1.23 V

3 mV