1303 current measurement configuration.

[Ralph@NES]

Hello

I am going to be using my SCXI-1102B/SCXI-1303 combination to measure a handful of thermocouples and would like to use some of the leftover channels to measure a 4-20ma current (or convert it to 0-5v with a 249 ohm resistor, either way is fine with me).  The units I have are Omega flow and pressure meters that output 4-20ma.

I realize the 1303 has a built-in 10Mohm resistor network for thermocouple meausurement, and I use them as shipped from the factory for thermocouple measurement.

For the channels that I wish to read current/voltage from, do I leave the resistor networks in or remove them?  Also, should I be referencing these sensors from my card or is an outside power supply acceptable?  Beyond a 249 ohm resistor to convert to voltage (if I have to), will I need external resistors at all?

Thanks in advance for any input!

Ralph

[David S.]

Hi Ralph -

The 1102 module is made for voltage inputs, so you will have to use that 249 Ohm resistor to measure current.  According to Table 2 of the 1303's Installation Guide, you can leave the 10 MOhm networks in when you connect the sensors.  This will prevent you from having to ground-reference them, too.

[Ralph@NES]

OK, thanks very much for that info!

One quick question, one of my 1303's has had the resistor networks removed, and sadly lost.  I have replacements on the way, but in the meantime what should I do about ground referencing?  Connect a 10m from my - terminal to the ground terminal?

Thanks again!

[Jordan_D]

Hi Ralph,

If you take a look at the 1303's Installation Guide under the following section

Configuring the Resistor Networks » Table 2. Selecting the Appropriate Resistor Networks

you will see that with no resistors in place, you are restricted to using Ground Referenced signals, and with a 10ohm resistor in place you are restricted to using a Floating signal source. I hope that answers your questions.

[Ralph@NES]

I guess I'm just confused as to what ground referencing and floating referencing are.

[Jordan_D]

Hi Ralph,

The following document should clarify the differences between grounded signals and floating signals and how each signal source should be wired.

Field Wiring and Noise Considerations for Analog Signals

Some typical examples of grounded signal sources include thermistors and power supplies.

Some typical examples of floating signal sources include thermocouples and batteries.

Message Edited by Jordan D on 08-08-2007 11:10 AM

[Gubby]

I too am using a 1303 terminal block with an 1102 module.  So, if I have a source that is sending a 4-20 mA signal over a twisted pair, can I simply attach one wire to the negative terminal and one wire to the positive terminal of a channel on the 1303 and that will give me a voltage reading at that channel?  I am using the stock resistor networks.

[EEsRULEtheWORLD]

Hi Gubby,

Yes, you should be able to wire the twisted pair to the analog input channel, connecting the positive wire to the positive analog input channel and the negative wire to the negative analog input channel.  If by "stock" resistor network, you are referring to the factory shipping configuration of 10 Mohms, then you will be configured correctly, whether your signal is floating or ground-referenced.

[Gubby]

Thanks for the response.  Is the current converted to voltage by the pullup and bias resistor networks on the 1303 or is the current passed to the 1102 to be converted there by installing resistors?  If the voltage is created by passing 4-20 mA current through 10 Mohm resistors,  wouldn't that create voltage that is very high? Also, if this is the case, do the pullup and bias resistors have to be the same (I investigated my setup and it seems that the pullup resistors are 10 Mohm, while the bias resistors are 10 ohm)?

[EEsRULEtheWORLD]

Hey,

The SCXI-1102 has a high input impedance.  Therefore, it cannot make a current measurement directly.  There is a need for external resistors to convert the current to a voltage that can be read by this modules.  This is all described in the SCXI-1102 manual in Appendix E.