cFP-RTD-122

[Ahemer]

We have PT100 sensors directly connected to a cFP-RTD-122 and have the problem of instable temperatures while calibrating the complete system and noise on the signal. With a ferrite filter on the powersupply cable we partly eliminated the noise but still have instable measurements while calibrating, i.e.. yesterday adjusted to 1°C now giving 2°C. Even differences are noticed between inputs on 1 board! What can be the problem?

[Matthew_Williams]

Hi Ahemer.  A couple of questions:

1) Are you using shielded cables to the RTD sensors?  Grounded only at the cFP-RTD-122 end?  How long are your wires?

Any connections in the wires, other than at the CB-1 and the sensor?

2) Are you seeing only long term instability now?  It sounds from your post like you did see some noise (short term

instability) but that was resolved with the power supply ferrite.

3) Is the backplane well grounded (grounding screw on the bottom, below slot 2 on a 4 slot backplane)?  Power supply grounded?

4) Are you taking readings from MAX, are the temperatures stable there?

5) What production revision of the cFP-RTD-122 do you have (letter code in the part number, i.e. 185078D-02 is

a revision D)?

Matt

[Ahemer]

Hi,

In reaction too your questions:

ad 1. Asfar I can see these RTD sensors-cables are not shielded. Approx. 3 m. long. No other connections in the wiring, direct from RTD to the NI CB-1 module.

ad 2. The short term noise is still there but with the ferrite the noise on the signal is lower!

ad 3. The backplate is not grounded at the screw. But I checked the ground signal between the screw and the ground in 19" rack and powersocket and these make connection. The powersupply is grounded via the powersocket. The supply is made out of plastic and further the are no possibilities to ground it.

ad 4. Yes, no also noise on the signal

ad 5. RTD-122 version 185078D-02, CB-1 188989D-01

Ahemer