AD592

REV. A

–5–

+V

AD592

R

100

Ω

950

Ω

Figure 4. Basic Voltage Output (Single Temperature Trim)

To trim the circuit the temperature must be measured by a ref-

erence sensor and the value of R should be adjusted so the out-

procedure should be implemented as close as possible to the

temperature highest accuracy is desired for. In most applications

if a single temperature trim is desired it can be implemented

where the AD592 current-to-output voltage conversion takes

place (e.g., output resistor, offset to an op amp). Figure 5 illus-

trates the effect on total error when using this technique.

AFTER SINGLE

TEMPERATURE

CALIBRATION

ACCURACY

WITHOUT TRIM

+1.0

+0.5

–25

+105

+25

0

–0.5

–1.0

Figure 5. Effect of Scale Factor Trim on Accuracy

If greater accuracy is desired, initial calibration and scale factor

errors can be removed by using the AD592 in the circuit of

Figure 6.

8.66k

Ω

R1

1k

Ω

97.6k

Ω

R2

5k

Ω

7.87k

Ω

AD741

+5V

AD1403

V–

AD592

Figure 6. Two Temperature Trim Circuit

With the transducer at 0

°C adjustment of R1 for a 0 V output

nulls the initial calibration error and shifts the output from K to

°C. Tweaking the gain of the circuit at an elevated temperature

by adjusting R2 trims out scale factor error. The only error

remaining over the temperature range being trimmed for is

nonlinearity. A typical plot of two trim accuracy is given in

Figure 7.

SUPPLY VOLTAGE AND THERMAL ENVIRONMENT

EFFECTS

The power supply rejection characteristics of the AD592 mini-

mizes errors due to voltage irregularity, ripple and noise. If a

supply is used other than 5 V (used in factory trimming), the

power supply error can be removed with a single temperature

trim. The PTAT nature of the AD592 will remain unchanged.

The general insensitivity of the output allows the use of lower

cost unregulated supplies and means that a series resistance of

several hundred ohms (e.g., CMOS multiplexer, meter coil

resistance) will not degrade the overall performance.

+2.0

+1.0

–25

+105

+25

0

–1.0

–2.0

0

+75

Figure 7. Typical Two Trim Accuracy

The thermal environment in which the AD592 is used deter-

mines two performance traits: the effect of self-heating on accu-

racy and the response time of the sensor to rapid changes in

temperature. In the first case, a rise in the IC junction tempera-

ture above the ambient temperature is a function of two vari-

ables; the power consumption level of the circuit and the

thermal resistance between the chip and the ambient environ-

ment (

θ

°C can be derived by multiply-

ing the power dissipation by

θ

vary widely for surroundings with different heat sinking capaci-

ties it is necessary to specify

θ

I shows how the magnitude of self-heating error varies relative

to the environment. In typical free air applications at +25

°C

with a 5 V supply the magnitude of the error is 0.2

°C or less. A

common clip-on heat sink will reduce the error by 25% or more

in critical high temperature, large supply voltage situations.

Table I. Thermal Characteristics

Medium

θ

τ (sec)*

Still Air

Without Heat Sink

175

60

With Heat Sink

130

55

Moving Air

Without Heat Sink

60

12

With Heat Sink

40

10

Fluorinert Liquid

35

5

Aluminum Block**

30

2.4

NOTES

*

τ is an average of five time constants (99.3% of final value). In cases where the

thermal response is not a simple exponential function, the actual thermal re-

sponse may be better than indicated.

**With thermal grease.