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RTD noise filtering

Mikhail Asiatici
Prodigy 50 points

Other Parts Discussed in Thread: ADS1247

Hi all,

I'm designing and testing a circuit for 3-wire hardware-compensated RTD measurement with an ADS1247. I noticed that measures are affected by a very intense noise when I connect a Pt100 resistor through a quite long cable - about 4 meters long. Can you please suggest me some design rules to reduce the noise reaching the ADC? I think that I need to place some decoupling capacitors but I don't know where exactly - also because I'm using the ADS1247 IDACs to excite the RTD and I don't know if the current sunk by capacitors could invalid my measure - and which should be their value.

Thanks in advance,

Mikhail Asiatici

  • 0
    TI__Guru**** 183061 points

    Hi Mikhail,

    This is not necessarily an easy question to answer.  What sort of 'noise' are you seeing with your setup of the ADS1247?  Are you picking up 50/60 Hz?  Is it switcher noise from a local power supply?  Noise from nearby equipment?  Can you provide a schematic of your board an a description of the working environment along with any particular details of the noise (magnitude, frequency content, etc) that you are seeing?

  • 0 in reply to Tom Hendrick
    Prodigy 50 points

    Hi Tom,

    thanks for your answer. Unfortunately I had no time to analyze very well the noise, and 'till next Monday I will not be able to go back to the laboratory to take more accurated measures; I just noticed that my samples were characterized by a standard deviation about 5-6 times greater than the samples collected by measuring a normal through hole resistor. As far as 50 Hz noise however I suppose that it should be adequately rejected by the ADS1247 internal filter since I'm working at 5 SPS, don't it?

    This is my schematic: http://img528.imageshack.us/img528/9417/16052012623.jpg (sorry if it isn't so clear)

    where Rx is the resistor to be measured (Pt100) and RL its leads resistance. Rx is connected to the rest of the circuit through a screw connector. If you need it I can also upload my PCB layout.

    My testing environment is an electronic laboratory whose electric equipment consist in a PC with CRT monitor, standard electronic test equipment (DC power supply, oscilloscope, multimeter) and a fluorescent light, whereas the working environment for which my circuit is being desinged will be the outside environment, on a pole about two meters from ground and a few meters from nearby buildings.

    Anyway, by doing some experiments I found that placing two 47 nF capacitors, one between pin 1 of the screw connector and GND (ground plane) and the other between pin 2 of the same connector and the ground plane, as close as possible to the connector itself, seems to reduce quite well the noise; I only hope that this solution will not cause other problems in the future.