4472 DSA board and Proximity probe

[vishalv]

I am trying to read output from the proximity probe using the PCI 4472 DSA board. For the grounded source(proximity probe) the connection given in the 4472 user manual(Page 2-7) is a pseudo-differential one.

The proximity probe(which i am sure is the industry norm) needs -24V power supply, so the +24V power supply is INVERTED to power the probe. The +24V lead become the COMMON and the GROUND lead becomes -24V.

The signal wire goes from 0 to -20V (Range) with respect to the COMMON(24V higher than the TRUE GROUND, since the power supply was inverted and +24V lead of the supply became COMMON for the probe).

According to the specs(signal ground and 4472 ground should not have potential diff. of more than 2.5V) and the pseudo-diff
erential configration I think it will blow the board as the 24V across the 50 ohm resistor will result in 0.5A of current.

Please help. Is there anyway out of this problem.

Thanks,
Vishal

[Photon_Dan]

You need signal conditioning. An inverting op-amp arrangement with 0.5 gain will map your 0 to -20 V range to 0 to +10 V thus matching the input range of the 4472.

Ah, the wonders of do-it-yourself.

Daniel L. Press
PrimeTest Corp.
www.primetest.com

[vishalv]

Thanks for your answer Dan, one issue has certainly been taken care of, what is bothering me now is that the potential difference of 24V between the signal and board ground might hurt the board, NI specifies a max limit of 2.5V and certainly NI cannot make a board which is not compatible with the proximity probe(-24V power supply norm), since this board is meant to be used with proximity probes, accelerometers etc.

Looking forward to your reply.

Thanks,
Vishal