ADM1026
V
DD
I
N × I
I
BIAS
D+
C1*
D–
V
OUT+
TO ADC
REMOTE
SENSING
TRANSISTOR
V
BIAS
DIODE
OUT–
LOW-PASS FILTER
fC = 65kHz
* CAPACITOR C1 IS OPTIONAL. IT IS ONLY NECESSARY IN NOISY ENVIRONMENTS.
C1 = 2.2nF TYPICAL, 3nF MAX.
Figure 34. Signal Conditioning for Remote Diode Temperature Sensors
The results of external temperature measurements are stored in
8-bit, twos complement format, as illustrated in Table 7.
10MIL
10MIL
10MIL
10MIL
10MIL
10MIL
10MIL
GND
D+
Table 7. Temperature Data Format
Temperature
−±28°C
−±25°C
−±00°C
−75°C
−50°C
−25°C
−±0°C
0°C
±0°C
25°C
50°C
75°C
Digital Output
±000 0000
±000 00±±
±00± ±±00
±0±± 0±0±
±±00 ±±±0
±±±0 0±±±
±±±±0±±0
0000 0000
0000 ±0±0
000± ±00±
00±± 00±0
0±00 ±0±±
0±±0 0±00
0±±± ±±0±
0±±± ±±±±
Hex
80
83
9C
B5
CE
E7
F6
00
0A
±9
32
4B
64
7D
7F
D–
GND
Figure 35. 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.
±00°C
±25°C
±27°C
Layout Considerations
•
•
Place a 0.1 µF bypass capacitor close to the ADM1026.
If the distance to the remote sensor is more than eight
inches, the use of twisted-pair cable is recommended.
This works from about 6 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 ADM1026. Leave the remote end of the
shield unconnected to avoid ground loops.
Digital boards can be electrically noisy environments. Take
these precautions to protect the analog inputs from noise,
particularly when measuring the very small voltages from a
remote diode sensor.
•
•
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.
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 may
be reduced or removed. Cable resistance can also introduce
errors. A 1 Ω series resistance introduces about 0.5°C error.
•
•
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 10 mil track minimum width and spacing is
recommended.
Rev. A | Page 2± of 56
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