transfer analog signal via RJ45 cable

mozobata

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I want to use a TMP36 temperature sensor, so I thought to transfer the signal through an RJ45 cable. (Is that equal to a cat5 cable? I just used an old ethernet cable)

When I use the sensor next to the MCU (an Arduino) with some common jumper wires, I read a value of about 25oC.

25oC equals to 25*10 + 500 = 750mV

When I use the sensor a few meters away from the MCU and transfer the signal with an RJ45 cable, I get readings as low as 3oC.

3oC equals to 3*10 + 500 = 530mV

I use an RJ45 cable (8 wires inside) about 1.8m (6ft) long with terminals

Here is the datesheet of ds18b20


I wonder how a voltage drop of about 200mV occures in less than 2 meters of cable? On the same cable (different wires) I transfer the digital signal of a DS18B20 temperature sensor as well as the signal of a DHT humidity sensor, and both seem to work fine.

Is this voltage drop reasonable? If so why? In any case what kind of cable should I use in order to transfer a signal for just a few meters?

Any thoughts would be appreciated.
 
mozobata":2msjcltm said:
I want to use a TMP36 temperature sensor, so I thought to transfer the signal through an RJ45 cable. (Is that equal to a cat5 cable? I just used an old ethernet cable)

When I use the sensor next to the MCU (an Arduino) with some common jumper wires, I read a value of about 25oC.

25oC equals to 25*10 + 500 = 750mV

When I use the sensor a few meters away from the MCU and transfer the signal with an RJ45 cable, I get readings as low as 3oC.

3oC equals to 3*10 + 500 = 530mV

I use an RJ45 cable (8 wires inside) about 1.8m (6ft) long with terminals

Here is the datesheet of ds18b20



mozobata":2msjcltm said:
I wonder how a voltage drop of about 200mV occures in less than 2 meters of cable?

It occurs because the output of the sensor isn't buffered, so the impedance looking out of the chip is poorly matched to the impedance looking into the device. If you look at the datasheet, they say you must have a 5k resistor going to VDD acting as a pullup resistor on the RX line. Since the impedance of the cable and the chip are in parallel, you'll see a drop across the lower impedance - which is your cable - effectively loading the output down of the sensor. You should find that as you shorten the cable, the output voltage will continue to rise at some point and then no longer rise, even though you're still shortening the cable.

To fix the issue, you need to drive the output using a wideband op-amp wired as a buffer on the output of the sensor, or look into using a pullup resistor if you're communicating over a 1 wire bus. Better yet, wire the sensor to the arduino and code the arduino to run the length of wire you desire. Usually, the arduino can supply about 20mA of current or so - enough to do what you want for the most part. Buffering the output of the sensor itself will provide a high input impedance to the sensor (input of the op-amp) and a low impedance at the output of the op-amp such that your voltage will stay constant over a smaller bandwidth. As mentioned earlier, you can also try using a pullup resistor depending on how you have the chip connected.
 
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