Temperature sensor & max distance

Hello all!

I would like to connect a temperature sensor I've lying around (an LM35) to a device I'm building to display the outside temperature. Because of physical constraints, I would like to connect the sensor to the device using a long cable (~2 meter). I know that this kind of "passive" sensors don't like long cables, or am I wrong? If not, what are my options?

Best regards,
Nelson

use teflon cable!

You can use standard, shielded cable. Just don't go too high with load resistor, keep current 0.5~1mA, put capacitor at input end on signal and bypass capacitor on power leads of sensor.
Do not ground sensor, you can put small heatsink on it to reduce self heating effects. If your cable and sensor are floating and connected to ground only on receiving end, there will be no significant loop and you should be fine.
Teflon cable is not needed and wont do much.

Many thanks Sinisa. That should put me on track. I'll post here later to tell how it went.

Best regards,
Nelson

try to use op-amp to amplify the signal
it is just idea!!!

I realize that it’s an old thread. But I’ll still post some comments anyways.

Astrolupa said:

I know that this kind of "passive" sensors don't like long cables, or am I wrong?

Click to expand...

I wouldn’t call LM35 a passive sensor. It’s an analog bandgap sensor with voltage output. How can cable degrade the signal from such sensor:

EMI. Cable will pick up more EMI. Use shielded cable. Make sure that the shield is grounded at exactly one end. Low-pass filter the signal after the cable. May be you will not need to worry about the EMI a lot. It depends on location. E.g. residential environment would have less EMI than industrial environment, cars.

Voltage drop across the ground lead. LM35 is has a single-ended output (as opposed to differential output). Signal is taken with respect to power ground. Adding a cable adds a small resistance in series with the ground. For the purposes of conversation, let’s assume that the resistance of the ground wire is 2Ω. LM35 consumes 60uA, according to the datasheet. So, the nominal voltage across the ground lead will be 120uV, which corresponds to 0.012C, which is negligible. Important caveat – the current through the ground lead is the combined current drawn by all of the devices, which share the ground lead.

Parasitic inductance and capacitance. p.7 in the datasheet has recommendations about using LM35 with cables.