ADT14GP AD [Analog Devices], ADT14GP Datasheet - Page 12

• Manufacturer Part Number: ADT14GP
• Description: Quad Setpoint, Programmable Temperature Monitor and Controller
• Manufacturer: AD [Analog Devices]

ADT14
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.
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
Figure 23. Temperature-to-Frequency Converter
ADT14
VPTAT
1.8k
500
R1
R2
4
3
ADT14
VPTAT
V
REF
8
V+
470k
R1
OP290
6
0.1 F
OSC
C1
AD654
5
7
2
V+
60µF
1
OP290
V+
Figure 24. Isolation Amplifier
5k
100
F
OUT
IN9148
4
2
1
3
–12–
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 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:
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:
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
until V
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.
V
I
ISOLATION
1
BARRIER
OUT
IL300XC
OUT
2.5V – I2R2 2.5V – 0.7
I
2
= 1.49 V as well. Both the REF43 and the OP90
2.5V
604k
6
I1
OP90
REF43
680pF
V+
100k
2.5V –VPTAT
2
4
V+
470 k
2.5V –VPTAT
470 k
1.16V TO 1.7V
644 k
VPTAT
REV. 0