REV. A

OP290

–9–

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 offset trim will not affect the gain.

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

I

I

T

TT

mA

OUT

FS

OPERATING

AMBIENT

MIN

= 





+

∆

∆

–

4

Table I shows the values of R6 required for various tempera-

ture ranges.

VARIABLE SLEW RATE FILTER

The circuit shown in Figure 6 can be used to remove pulse noise

from an input signal without limiting the response rate to a genu-

ine signal. The nonlinear filter has use in applications where

the input signal of interest is known to have physical limitations.

An example of this is a transducer output where a change of

temperature or pressure cannot exceed a certain rate due to

physical limitations of the environment. The filter consists of a

comparator which drives an integrator. The comparator com-

pares the input voltage to the output voltage and forces the

integrator output to equal the input voltage. A1 acts as a com-

parator with its output high or low. Diodes D1 and D2 clamp

the voltage across R3 forcing a constant current to flow in or

out of C2. R3, C2, and A2 form an integrator with A2’s output

slewing at a maximum rate of:

Maximum slew rate

V

RC

V

RC

D

=≈

32

06

32

.

For an input voltage slewing at a rate under this maximum slew

rate, the output simply follows the input with A1 operating in its

linear region.

1/2

OP290EZ

GND

6

3

2

4

R1

10k

REF-43BZ

2N1711

V+

8V TO 40V

1N4002

SPAN TRIM

R10

100

1%, 1/2W

R9

100k

R8

1k

R3

100k

R5

5k

R2

1k

4

8

2

ZERO

TRIM

R4

20k

R6

3k

R7

5k

7

6

5

1

1/2

OP290EZ

Figure 5. Temperature to 4-20 mA Transmitter

Table I.

Temperature Range

R6 (k )

0

°C to +70°C10

–40

°C to +85°C

6.2

–55

°C to +150°C3