ADM1026
PRELIMINARY TECHNICAL DATA
– 1 6 –
REV. PrL
PRELIMINAR
Y
TECHNICAL
DA
TA
TABLE 3. TEMPERATURE DATA FORMAT
Temperature
Digital Output
-128 °C
1000 0000
-125 °C
1000 0011
-100 °C
1001 1100
-75 °C
1011 0101
-50 °C
1100 1110
-25 °C
1110 0111
-10
o
C
11110110
0 °C
0000 0000
+10 °C
0000 1010
+25 °C
0001 1001
+50 °C
0011 0010
+75 °C
0100 1011
+100 °C
0110 0100
+125 °C
0111 1101
+127 °C
0111 1111
To prevent ground noise interfering with the measure-
ment, the more negative terminal of the sensor is not ref-
erenced to ground, but is biased above ground by an
internal diode at the D- input. As the sensor is operating
in a noisy environment, C1 is provided as a noise filter.
See the section on layout considerations for more infor-
mation on C1.
To measure
∆
V
be
, the sensor is switched between operat-
ing currents of I and N x I. The resulting waveform is
passed through a 65kHz lowpass filter to remove noise,
thence to a chopper-stabilized amplifier that performs the
functions of amplification and rectification of the wave-
form to produce a DC voltage proportional to
∆
V
be
. This
voltage is measured by the ADC to give a temperature
output in 8-bit two’s complement format. To further re-
duce the effects of noise, digital filtering is performed by
averaging the results of 48 measurement cycles. A remote
temperature measurement takes nominally 2.14ms.
The results of external temperature measurements are
stored in 8 bit, twos-complement format, as illustrated in
Table 3.
LAYOUT CONSIDERATIONS
Digital boards can be electrically noisy environments, and
care must be taken to protect the analog inputs from
noise, particularly when measuring the very small voltages
from a remote diode sensor. The following precautions
should be taken:
1. Place the ADM1026 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 to 8 inches.
2. 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.
3. Use wide tracks to minimize inductance and reduce
noise pickup. 10 mil track minimum width and spacing
is recommended.
GND
D+
D-
GND
10 mil.
10 mil.
10 mil.
10 mil.
10 mil.
10 mil.
10 mil.
Figure 9. Arrangement of Signal Tracks
4. 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- path and at the same temperature.
Thermocouple effects should not be a major problem as
1
o
C corresponds to about 240µV, and thermocouple
voltages are about 3µV/
o
C of temperature difference.
Unless there are two thermocouples with a big tempera-
ture differential between them, thermocouple voltages
should be much less than 200mV.
5. Place 0.1µF bypass and 1000pF input filter capacitors
close to the ADM1026.
6. If the distance to the remote sensor is more than 8
inches, the use of twisted pair cable is recommended.
This will work up to about 6 to 12 feet.
7. For really 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 ADM1026. 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 ca-
pacitor may be reduced or removed.
Cable resistance can also introduce errors. 1
⍀
series resis-
tance introduces about 0.5
o
C error.
LIMIT VALUES
Limit values for analog measurements are stored in the
appropriate limit registers. In the case of voltage measure-
ments, high and low limits can be stored so that an inter-
rupt request will be generated if the measured value goes
above or below acceptable values. In the case of tempera-
ture, a Hot Temperature or High Limit can be pro-
grammed, and a Hot Temperature Hysteresis or Low
Limit, which will usually be some degrees lower. This can
be useful as it allows the system to be shut down when the
hot limit is exceeded, and re-started automatically when it
has cooled down to a safe temperature.
