TMP01

REV. C

–14–

Figure 21. 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 21 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 Pins 1 to 2. The feedback actually

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

OP290 drives the LED such that there is enough current gener-

ated 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 TMP01 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:

I

2.5 V – 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:

VOUT = 2. 5 V – I2R2 = 2. 5 V –0. 7

2. 5 V – VPTAT

470 k

Ω









644 k

Ω= 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 out-

put voltage 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 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 sen-

sitive equipment calibrated to work over a limited temperature

range. R1, R2, and R3 in Figure 22 are chosen to give a tem-

perature 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 Undertemp Output or Overtemp Output re-

spectively will go low and turn 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 dis-

cussed above can be used.

LED

VPTAT

V+

R1

47.5k

Ω

R2

4.99k

Ω

R3

71.5k

Ω

200

Ω

TEMPERATURE

SENSOR &

VOLTAGE

REFERENCE

1

2

3

4

HYSTERESIS

GENERATOR

WINDOW

COMPARATOR

TMP01

VPTAT

VREF

7

8

5

6

Figure 22. Out-of-Range Warning

IN4148

I

1

I

2

6

5

3

4

1

2

2.5V

V+

REF43

4

6

2

1.16V TO 1.7V

ISOLATION

BARRIER

OP290

V+

V+

R1

R2

R3

100

Ω

TEMPERATURE

SENSOR &

VOLTAGE

REFERENCE

1

2

3

4

HYSTERESIS

GENERATOR

WINDOW

COMPARATOR

TMP01

VPTAT

VREF

7

8

5

6

7

6

4

3

2

OP290

680pF

R1

470k

Ω

V+

7

6

4

3

2

OP90

604k

Ω

100k

Ω

IL300XC

680pF