PC87365 datasheet(183/215 Pages) NSC

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10.0 Temperature Sensor (TMS) (Continued)

183

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10.5.4

Twisted Pair and Shielded Cables

For remote sensor distances longer than 8 in. or in particularly noisy environments, a twisted pair is recommended. Its prac-

tical length is 6 feet to 12 feet (typical) before noise becomes a problem, as tested in a noisy electronics laboratory. For

longer distances, the best solution is a shielded twisted pair like that used for audio microphones. Connect the twisted pair

to DXP and DXN and the shield to GND and leave the shield’s remote end unterminated.

Excess capacitance at DX_ limits practical remote sensor distances. For very long cable runs, the cable’s parasitic capaci-

tance often provides noise filtering, so the 2200 pF capacitor can often be removed or reduced in value.

Cable resistance also affects remote sensor accuracy: 1

Ω series resistance introduces about +1/2˚C error.

10.5.5

Obtaining the Specified VLM/TMS Accuracy

To obtain the specified VLM/TMS accuracy, use the following sequence on VSB power-up:

1. Set the TMS Logical Device base address.

2. Enable the TMS Logical Device.

3. Write 00h to index 08h of the TMS Logical Device.

4. Write 0Fh to index 09h of the TMS Logical Device.

5. Write 08h to index 0Ah of the TMS Logical Device.

6. Write 04h to index 0Bh of the TMS Logical Device.

7. Write 35h to index 0Ch of the TMS Logical Device.

8. Write 05h to index 0Dh of the TMS Logical Device.

9. Write 05h to index 0Eh of the TMS Logical Device.

PC87365 датащи(PDF) 183 Page - National Semiconductor (TI)