ADT7466
Rev. 2 | Page 19 of 48 | www.onsemi.com
Table 13. Temperature Measurement Registers
Register Description Default
0x0D Remote temperature 0x00
0x0E Local temperature 0x00
0x08 Extended Resolution 1 0x00
Bits 1:0 remote temperature LSBs
0x09 Extended Resolution 2 0x00
Bits 1:0 local temperature LSBs
Associated with each temperature measurement channel are
high and low limit registers. Exceeding the programmed high or
low limit causes the appropriate status bit to be set. Exceeding
either limit can also generate
ALERT
interrupts.
Table 14. Temperature Measurement Limit Registers
Register Description Default
0x1A Remote1 temperature low limit 0x00
0x1B Remote1 temperature high limit 0x7F
0x1C Local temperature low limit 0x00
0x1D Local temperature high limit 0x7F
0x14 Thermistor 1/Remote 2 low limit 0x00
0x15 Thermistor 1/Remote 2 high limit 0xFF
0x16 Thermistor 2 low limit 0x00
0x17 Thermistor 2 high limit 0xFF
All temperature limits must be programmed in the same format
as the temperature measurement. If this is offset binary, add 64
(0x40 or 01000000) to the actual temperature limit in degrees
Celsius.
Layout Considerations
Digital boards can be electrically noisy environments. Take the
following precautions to protect the analog inputs from noise,
particularly when measuring the very small voltages from a
remote diode sensor.
Place the ADT7466 as close as possible to the remote sensing
diode. Provided that the worst noise sources, such as clock
generators, data/address buses and CRTs, are avoided, this
distance can be 4 inches to 8 inches.
If the distance to the remote sensor is more than 8 inches, the
use of twisted-pair cable is recommended. This works from
about 6 feet to 12 feet.
For very long distances (up to 100 feet), use shielded twisted
pair, such as Belden #8451 microphone cable. Connect the
twisted pair to D+ and D− and the shield to GND close to the
ADT7466. Leave the remote end of the shield unconnected to
avoid ground loops.
Because the measurement technique uses switched current
sources, excessive cable and/or filter capacitance can affect the
measurement. When using long cables, the filter capacitor
could be reduced or removed.
Route the D+ and D− tracks close together, in parallel, with
grounded guard tracks on each side. Provide a ground plane
under the tracks if possible.
Use wide tracks to minimize inductance and reduce noise
pickup. A 5 mil track minimum width and spacing is
recommended.
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
5MIL
GND
D+
GND
D–
04711-044
Figure 25. Arrangement of Signal Tracks
Try to minimize the number of copper/solder joints, which can
cause thermocouple effects. Where copper/solder joints are
used, make sure that they are in both the D+ and D− paths and
are at the same temperature.
Thermocouple effects should not be a major problem because
1°C corresponds to about 240 μV, and thermocouple voltages
are about 3 μV/°C of temperature difference. Unless there are
two thermocouples with a big temperature differential between
them, thermocouple voltages should be much less than 200 mV.
Place a 0.1 μF bypass capacitor close to the ADT7466.
TEMPERATURE MEASUREMENT USING
THERMISTORS
The analog input channels, AIN1 and AIN2, can be used to
measure temperature by using negative temperature coefficient
(NTC) thermistors. NTC thermistors have a nonlinear transfer
function of the form
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−×=
12
t1t2
t
B
t
B
eRR
where:
R
t2
is the resistance at temperature t2.
R
t1
is the resistance at temperature t1 (usually 25°C).
e = 2.71828.
B is the B constant of the thermistor (typically between 3000
and 5000).
A thermistor can be made to give a voltage output that is fairly
linear over a limited range by making it part of a potential
divider as shown in Figure 26.
