ADT14

–12–

REV. 0

By connecting the VPTAT output to the input of the AD654,

the 5 mV/K, temperature coefficient gives a sensitivity of

25 Hz/

°C, centered around 7.5 kHz at 25°C. The trimming

resistor, R2, is needed to calibrate the absolute accuracy of the

AD654. For more information on the converter, consult the

AD654 data sheet. An AD650 frequency-to-voltage converter

can be used to accurately convert the frequency back to a dc

voltage on the receiving end.

OSC

AD654

86

7

1

4

3

52

V+

C1

0.1 F

5k

Ω

V+

R1

1.8k

Ω

R2

500

Ω

ADT14

VPTAT

Figure 23. Temperature-to-Frequency Converter

Isolation Amplifier

In many industrial applications the sensor is located in an envi-

ronment that needs to be electrically isolated from the central

processing area. Figure 24 shows a simple circuit that uses an

8-pin opto-isolator (IL300XC) that can operate across a 5,000

V barrier. IC1 (an OP290 single-supply amplifier) is used to

drive the LED connected between Pins 1 to 2. The feedback

actually comes from the photodiode connected to Pins 3 to 4.

The OP290 drives the LED such that there is enough current

generated in the photodiode to exactly equal the current derived

from the VPTAT voltage across the 470 k

Ω resistor. On the

receiving end, an OP90 converts the current from the second

photodiode to a voltage through its feedback resistor R2. Note

that the other amplifier in the dual OP290 is used to buffer the

2.5 V reference voltage of the ADT14 for an accurate, low drift

LED bias level without affecting the programmed hysteresis

current. A REF43 (a precision 2.5 V reference) provides an

accurate bias level at the receiving end.

To understand this circuit, it helps to examine the overall equa-

tion for the output voltage. First, the current (I1) in the photo-

diode is set by:

I1

=

2.5V –VPTAT

470 k

Ω

Note that the IL300XC has a gain of 0.73 (typical) with a min

and max of 0.693 and 0.769 respectively. Since this is less than

1.0, R2 must be larger than R1 to achieve overall unity gain. To

show this the full equation is:

2.5V –VPTAT

470 k

Ω









×644 kΩ=VPTAT

A trim is included for R2 to correct for the initial gain accuracy

of the IL300XC. To perform this trim, simply adjust for an

output equal to VPTAT at any particular temperature. For

example, at room temperature, VPTAT = 1.49 V, so adjust R2

operate from a single supply, and contribute no significant error

due to drift.

In order to avoid the accuracy trim, and to reduce board space,

complete isolation amplifiers, such as the high accuracy AD202,

are available.

OP290

OP290

V+

ADT14

VPTAT

60µF

100

Ω

IN9148

IL300XC

1

2

3

4

OP90

ISOLATION

BARRIER

V+

2.5V

6

2

4

V+

604k

Ω

100k

Ω

680pF

1.16V TO 1.7V

R1

470k

Ω

REF43

Figure 24. Isolation Amplifier