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ADM1026

Rev. A | Page 21 of 56

C1*

D+

D–

REMOTE

SENSING

TRANSISTOR

× I

IBIAS

VDD

VOUT+

TO ADC

VOUT–

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

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

−125°C

1000 0011

−100°C

1001 1100

−75°C

1011 0101

−50°C

1100 1110

−25°C

1110 0111

−10°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

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.

10MIL

GND

D+

GND

D–

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.

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