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AD694BRZ

4–20 mA Transmitter

Analog Devices 

AD694

Figure 4. Using Optional Pass Transistor to Minimize Self-Heating Errors; Dual Supply Operation Shown

POWER DISSIPATION CONSIDERATIONS

The AD694 is rated for operation over its specified temperature

without the use of an external pass transistor. However, it is

possible to exceed the absolute maximum power dissipation,

with some combinations of power supply voltage and voltage

reference load. The internal dissipation of the part can be calcu-

lated to determine if there is a chance that the absolute maxi-

mum dissipation may be exceeded. The die temperature must

never exceed 150°C.

Total power dissipation (PTOT), is the sum of power dissipated

by the internal amplifiers, P (Standing), the voltage reference,

P(VREF) and the current output stage, P(IOUT) as follows:

PTOT = P (Standing) + P (VREF) + P (IOUT)

where:

P (Standing) = 2 mA (max) × VS

P (VREF) = (VS – VREF) × IVREF

P(IOUT) (VS – VOUT) × IOUT (max):

IOUT (max) may be the max expected operating cur-

rent, or the overdriven current of the device.

P(IOUT) drops to (2 V × IOUT) if a pass transistor

is used.

Definitions:

VREF = output voltage of reference

IVREF = output current of reference

VS = supply voltage

VOUT = voltage at IOUT pin.

An appropriate safety factor should be added to PTOT.

The junction temperature may be calculated with the following

formula:

TJ = PTOT (θJC + θCA) + TAMBIENT

θJC is the thermal resistance between the chip and the package

(case), θCA is the thermal resistance between the case and its

surroundings and is determined by the characteristics of the

thermal connection of the case to ambient.

For example, assume that the part is operating with a VS of 24 V

in the CERDIP package at 50°C, with a 1 mA load on the 10 V

reference. Assume that IOUT is grounded and that the max IOUT

would be 20 mA. The internal dissipation would be:

P(TOT) = 2 mA × 24 V + (24 V – 10 V) × 1 mA + (24 V – 0 V) × 20 mA

= 48 mW + 14 mW + 480 mW = 542 mW

Using θJC of 30°C/W and θCA of 70°C/W (from specifications

page), the junction temperature is:

TJ = 542 mW (30°C/W + 70°C/W) + 50°C = 104.2°C

The junction temperature is in the safe region.

Internal power dissipation can be reduced either by reducing the

value of θCA through the use of air flow or heat sinks, or by re-

ducing PTOT of the AD694 through the use of an external pass

transistor. Figure 5 shows the maximum case and still air tem-

peratures for a given level of power dissipation.

Figure 5. Internal Power Dissipation in mW

ADJUSTMENT PROCEDURES

The following sections describe methods for trimming the out-

put current offset, the span, and the voltage reference.

ADJUSTING 4 mA ZERO

The 4 mA zero current may be adjusted over the range of 2 mA

to 4.8 mA to accommodate large input signal offsets, or to allow

small adjustment in the zero current. The zero may be adjusted

by pulling up or down on Pin 6 (4 mA Adj) to increase or de-

crease the nominal offset current. The 4 mA Adj. (Pin 6) should

not be driven to a voltage greater than 1 V. The arrangement of

REV. B

–7–

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