ADM1026JSTZ View Datasheet(PDF) - Analog Devices
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ADM1026JSTZ Datasheet PDF : 56 Pages
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VDD
I
N× I
IBIAS
REMOTE
SENSING
TRANSISTOR
D+
C1*
D–
BIAS
DIODE
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
VOUT+
TO ADC
VOUT–
ADM1026
The results of external temperature measurements are stored in
8-bit, twos complement format, as illustrated in Table 7.
Table 7. Temperature Data Format
Temperature
Digital Output
Hex
−128°C
1000 0000
80
−125°C
1000 0011
83
−100°C
1001 1100
9C
−75°C
1011 0101
B5
−50°C
1100 1110
CE
−25°C
1110 0111
E7
−10°C
11110110
F6
0°C
0000 0000
00
10°C
0000 1010
0A
25°C
0001 1001
19
50°C
0011 0010
32
75°C
0100 1011
4B
100°C
0110 0100
64
125°C
0111 1101
7D
127°C
0111 1111
7F
Layout Considerations
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.
• 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.
GND
D+
D–
GND
10MIL
10MIL
10MIL
10MIL
10MIL
10MIL
10MIL
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.
• 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.
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.
Rev. A | Page 21 of 56
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