TMP01
Rev. E | Page 15 of 20
TEMPERATURE
SENSOR AND
VOLTAGE
REFERENCE
VREF
VPTAT
ISOLATION
BARRIER
1
2
3
4
8
7
6
5
HYSTERESIS
GENERATOR
WINDOW
COMPARATOR
TMP01
OP290
R1
R2
R3
V+
604kΩ
100kΩ
R1
470kΩ
V+
IN4148
I
1
I
2
6
5
3
4
1
2
2.5V
V+
REF43
4
6
2
1.16V TO 1.7V
100kΩ
680pF
OP290
7
4
3
6
2
V+
OP90
7
4
3
6
2
IL300XC
680pF
00333-029
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 k 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
k470
V5.2
1
VPTAT
I
−
=
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
=−=
22
5.2 RIVV
OUT
VPTAT
VPTATV
V
⎜
⎜
⎝
⎛
−
−
k470
5.2
7.05.2 =
⎟
⎟
⎠
⎞
k644
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 V
OUT
= 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
UNDER
output, 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
VPTAT
LED
1
2
3
4
8
7
6
5
HYSTERESIS
GENERATOR
WINDOW
COMPARATOR
TMP01
R1
7.5kΩ
R2
.99kΩ
R3
71.5kΩ
+
200Ω
00333-030
Figure 30. Out-of-Range Warning
