TMP01FS datasheet(15/20 Pages) AD

15 / 20 page

TMP01

Rev. E | Page 15 of 20

TEMPERATURE

SENSOR AND

VOLTAGE

REFERENCE

VREF

VPTAT

ISOLATION

BARRIER

HYSTERESIS

GENERATOR

WINDOW

COMPARATOR

TMP01

OP290

V+

604k

Ω

100k

Ω

470k

Ω

V+

IN4148

I1 I2

2.5V

V+

REF43

1.16V TO 1.7V

100k

Ω

680pF

OP290

V+

OP90

IL300XC

680pF

Figure 29. Isolation Amplifier

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 29 shows a simple circuit that uses an

8-pin optoisolator (IL300XC) that can operate across a 5,000 V

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

the LED connected between Pin 1 and Pin 2. The feedback

actually comes from the photodiode connected from Pin 3 to

Pin 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 kΩ 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 TMP01 for an accurate, low

drift LED bias level without affecting the programmed hyster-

esis 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

equation for the output voltage. First, the current (I1) in the

photodiode is set by

kΩ

470

VPTAT

−

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

minimum and maximum of 0.693 and 0.769, respectively.

Because this is less than 1.0, R2 must be larger than R1 to

achieve overall unity gain. To show this, the full equation is

−

OUT

VPTAT

⎜

⎝

⎛

−

kΩ

470

⎟

⎠

⎞

kΩ

644

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 voltage equal to VPTAT at any particular temperature.

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

R2 until VOUT = 1.49 V as well. Both the REF43 and the OP90

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 are available, such as the high

accuracy AD202.

OUT-OF-RANGE WARNING

By connecting the two open-collector outputs of the TMP01

together into a wired-OR configuration, a temperature out-

of-range warning signal is generated. This can be useful in

sensitive equipment calibrated to work over a limited temper-

ature range.

R1, R2, and R3 in Figure 30 are chosen to give a temperature

range of 10°C around room temperature (25°C). Thus, if the

temperature in the equipment falls below 15°C or rises above

35°C, the OVER or UNDERoutput, respectively, goes low and

turns the LED on. The LED may be replaced with a simple pull-

up resistor to give a logic output for controlling the instrument,

or any of the switching devices discussed above can be used.

TEMPERATURE

SENSOR AND

VOLTAGE

REFERENCE

VREF

VPTAT

LED

HYSTERESIS

GENERATOR

WINDOW

COMPARATOR

TMP01

47.5k

Ω

4.99k

Ω

71.5k

Ω

V+

200

Ω

Figure 30. Out-of-Range Warning

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