1303 Ground Pin

[jonhelpmejon]

I am using a scxi 1102 module with the 1303 connector block.  I have all 10 Mohm bias and pull up resistors in place.  I'm measuring small floating voltages and they are only accurate when I connect the negative terminal input to chassis ground.  Do I have to connect all 32 negative terminals to chassis ground or is there a single pin that will connect all of  them to chassis ground.  Thanks.

[johnsold]

Each of the 1102/1303 inputs is separate.  You will need to connect all 32.

It seems strange that you would need to ground a floating source.  What is the nature of the source?  Are the sources connected to each other?

Lynn

[jonhelpmejon]

Lynn,

Thanks for your reply,

No the sources are not connected.  The nature of the source is a 0 to 100 mv output from a telemetry receiver.  I'm curious as to why you think its strange that I would need to ground a floating source.  Most of  the documentation I have come across say you need to either at the source or at the daq but not both.  Any input would be appreciated as I am not an expert.  Thanks

[johnsold]

Unless battery operated, your telemetry receivers may have a connection to ground, even if both output terminals are not grounded.

Grounding can be a complicated and confusing subject, partly because of poorly defined terminology and semantics. What is important is that there is exactly one complete path for current to flow from the source to the measurement terminals.  If the path is not complete, the voltage at the measurement terminals will be wrong or undefined.  If multiple paths exist, then voltages, currents, or impedances along the extra paths may introduce errors or even damage the equipment.  Consider a battery operated multimeter connected to a AA battery.  No ground is required for this configuration to make a measurement.

In an 1102/1303 the inputs are internally constrained to be within the specified common mode voltage range referred to Chassis Ground.  A completely floating source (such as a battery or a thermocouple) could have ANY voltage with respect to Chassis Ground.  The bias resistors in the 1303 will assure that this connection is made.  Because the bias resistors normally take care of the common mode issue, I am not sure why you would need to ground your source.  Is it possible that you have a common mode voltage between your sources and the SCXI system which exceeds the 10 V rating? Possibly pick up from a high voltage AC power line nearby?

Lynn

[jonhelpmejon]

Lynn,

Thanks again for your response,

I've messed around with the system and the receivers are definatley floating as they are isolated and running on battery power.  I've removed all other recievers and am only working with the one now.  The outputs measured with a fluke give me 140 mV (Correct Voltage Expected), when meausured with the negative input attached to the SCXI chassis, I get 140 mV (Correct Voltage Expected).  If I measure withouth the negative connection to chassis, I get 82 mV (Not The Correct Voltage).

Starting to wonder if my resistor pack may be cooked.

I'll have to keep playing with it.  Thanks again for your good advice.

[johnsold]

Interesting.

What do you get if you measure the voltage with the Fluke while the receiver is connected to the SCXI (in both configurations)?

I have seen bad connections between the 1303 input screw terminals and the resistor networks.  With the SCXI chassis power off and the receivers disconnected, try measuring the resistance from the "-"input to ground.  Unfortunately there is no good way to get at the +5 V to measure the pull up resistors.  If you have a thin enough probe, you may be able to get at the +5 V at the common terminal on the pull up resistor packs.

Lynn

[jonhelpmejon]

Thanks again Lynn,

I will try and let you know what I find.  I got a call and have to go into the field for the next few days.