Why does NI recommend 232 ohm resistors for current sensing.

[Mariahnova]

I am trying to set up current measurements (4-20 mA signals) on a 6229 DAQ. I need to add some shunt resistors to the SCB68 AI circuits as they explain in the manual. What I don't understand is why they recommend a 232 ohm resistor so that the scaling runs up 4.64 volts instead of 5? I was going to use 250 ohm because it seemed to make sense. So what am I missing?

Thanks,

Dave

[JB]

According to page 4-29 of this document (probably the one you read), this value is suggested.

Why 232 ? Good question ! I guess :

1. Whole range of current can be measured (with AI range = 0-5V) even if the max current of the 4-20mA transmitter is > 20.0mA (accuracy of transmitter)
2. Part of E96 (1% tolerance) decade table

Anyway, by the use of a DAQmx scale, it is very easy to convert the measured voltage to the corresponding current.

[AnalogKid2DigitalMan]

JB:

I concur, thanks for the link to the decade chart! It is now in my favorites.

-AK2DM