ADT14GP AD [Analog Devices], ADT14GP Datasheet - Page 11

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

Buffering the Temperature Output Pin
The VPTAT sensor output is a low impedance dc output volt-
age with a 5 mV/K temperature coefficient, and is useful in a
number of measurement and control applications. In many
applications, this voltage may need to be transmitted to a central
location for processing. The unbuffered VPTAT voltage output
is capable of 500 A drive into 50 pF (max). As mentioned in
the discussion regarding buffering circuits for the V
is useful to consider external amplifiers for interfacing VPTAT
to external circuitry to ensure accuracy, and to minimize load-
ing, which could create dissipation-induced temperature sensing
errors. An excellent general-purpose buffer circuit using the
OP177, which is capable of driving over 10 mA and will remain
stable under capacitive loads of up to 0.1 F, is shown in Figure
20. Other interface circuits are shown below.
Differential Transmitter
In noisy industrial environments, it is difficult to send an accu-
rate analog signal over a significant distance. However, by send-
ing the signal differentially on a wire pair, these errors can be
significantly reduced. Since the noise will be picked up equally
on both wires, a receiver with high common-mode input rejec-
tion can be used very effectively to cancel out the noise at the
receiving end. Figure 21 shows two amplifiers being used to
send the signal differentially, and an excellent differential re-
ceiver, the AMP03, (SSM2141 or SSM2143 are two other
options), which features a common-mode rejection ratio of
95 dB at dc and very low input and drift errors.
4 mA to 20 mA Current Loop
Another very common method of transmitting a signal over long
distances is to use a 4 mA-to-20 mA loop (see Figure 22). An
advantage of using a 4 mA-to-20 mA loop is that the accuracy of
a current loop is not compromised by voltage drops across the
line. One requirement of 4 mA-to-20 mA circuits is that the
remote end must receive all of its power from the loop, meaning
that the circuit must consume less than 4 mA. Operating from
+5 V, the quiescent current of the ADT14 is 600 A max, and
REV. 0
Figure 21. Send the Signal Differentially for Noise
Immunity
ADT14
Figure 20. Buffer VPTAT to Handle Difficult Loads
VPTAT
10k
ADT14
VPTAT
V+
V–
4.9k
10k
1/2 OP297
1/2 OP297
V+
V–
0.1 F
OP177
50
50
10k
100
V
REF
OUT
V+
V–
AMP03 OR
SSM2143
output, it
V
OUT
–11–
the OP90s is 20 A max, totaling much less than 4 mA. Although
not shown, the open collector outputs and temperature setting
pins can be connected to do any local control of switching.
The current is proportional to the voltage on the VPTAT out-
put, and is calibrated to 4 mA at a temperature of –40 C, to
20 mA for +85 C. The main equation governing the operation
of this circuit gives the current as a function of VPTAT:
To determine the resistor values in this circuit, first note that
V
over R2 must give a variation of I
VPTAT varies from 1.165 V at –40 C to 1.79 V at +85 C. The
absolute value of the resistors is not important, only the ratio.
For convenience, 100 k is chosen for R5. Once R2 is calcu-
lated, the value of R3 and R1 is determined by substituting
4 mA for I
values are shown in the circuit. The OP90 is chosen for this
circuit because of its ability to operate on a single supply and its
high accuracy. For initial accuracy, a 10 k trim potentiometer
can be included in series with R3, and the value of R3 lowered
to 95 k . The potentiometer should be adjusted to produce an
output current of 12.3 mA at 25 C.
Temperature-to-Frequency Converter
Another common method of transmitting analog information is
to convert a voltage to the frequency domain. This is easily done
with any of the available low cost monolithic Voltage-to-Fre-
quency Converters (VFCs) that feature an open-collector digital
output. A digital signal is immune to noise and voltage drops
because the only important information is the frequency. As
long as the conversions between temperature and frequency are
accurately performed, the temperature data can be accurately
transmitted.
A simple circuit to do this combines the ADT14 with an AD654
VFC and is shown in Figure 23. The AD654 outputs a square
wave that is proportional to the dc input voltage according to
the following equation:
REF
243k
100k
remains constant over temperature. Thus the ratio of R5
R1
R3
I
OUT
Figure 22. 4 mA to 20 mA Current Loop
OUT
39.2k
and 1.165 V for VPTAT and solving. The final
R6
1
R2
VPTAT
F
14
OUT
5
2
3
R2
V
GND
REF
OP90
10 (R1 R2) C
100k
ADT14
R5
R5
7
4
OUT
VPTAT
V
IN
V+
6
V
from 4 mA to 20 mA as
R3 R1
REF
13
6
100
R6
T
2N1711
R3
R
1
ADT14
+5V TO +13.2V
4-20mA
L
R5
R2