NRSE with LabVIEW, BNC-2110, E-series card

[adrian 14]

I am trying to decide what input mode configuration I need i.e. RSE, NRSE or DIFF. My source (a photomultiplier, whose HV supply is grounded, is attached to a preamplifier (HV supply provides power to the preamp and grounding), which in turn is attached to a BNC-2110 connector block and finally to a computer which is grounded to the building ground so I believe I need either NRSE or DIFF modes (i.e. I have a ground-referenced signal source).

My first question concerns the DIFF mode. I have only one BNC coaxial signal cable but I would like to use the differential signal connection as I understand it may reduce noise (a problem I've been having - lots of large spikes upto 10V) and increase common-mode noise rejection. How can I achieve this with just the one cable connected to a BNC-2110 connector block? The conditions listed on page 4-11 of the 6034/6035E user manual state that the input signal requires a separate ground-reference point or return signal.

My second question concerns the NRSE signal connection mode. If I decide to use the NRSE mode instead, what should the setting be on the BNC-2110 connector block - Floating Signal (FS) or Grounded Signal (GS) (the same question applies to the DIFF mode)? On page 4 of the BNC-2110 Desktop Adapter manual/installation guide it says that the ground-referenced source position should be used for ground-reference signals. In the floating source position the negative input of the PGIA is internally tied to analog input ground (AIGND), which according to page 3-2 of the 6034/6035E user manual is the RSE configuration. For NRSE, the negative input of the PGIA must be connected to analog input sense (AISENSE). How is this achieved?

I see that it is possible to configure the E-series card via the NI-DAQ 'Measurement and Automation Wizard'. I notice that after selecting the NRSE mode in 'properties => configuring device => AI' and selecting the BNC-2110 'Accessory' option, that the default settings for 'coupling and input config' in the LabVIEW AI Read One Scan.vi do not change, instead remaining 'no change' and 'O' respectively. Why is this? Do I need to change the default settings in LabVIEW to reflect the changes I made to the DAQ card configuration in 'Measurement and Automation Wizard'?

Where should the source position switch be on the BNC-2110 connector block (FS or GS)?

I would greatly appreciate any help.

Thanks,

Adrian

[Filipe A.]

Adrian;

Page 4 of the 2110 User Manual explains how the connection in between the BNC adapter and the DAQ board is held. But, the connection you see in between the negative input of the 2110 and the AIGnd is done always by a BIAS resistor (to avoid the instrumentation amplifier bias current to create a offset voltage at your measurement). That connection is only held when you have a floating signal source connected to the BNC adapter. The DAQ board input mode is always kept as Differential Mode.
So, the switch at the BNC adapter which selects the input type will only insert or not a BIAS resistor at the DAQ board input channel, but on that side, the input mode must be kept as differential mode always.
In summary, if you have a signal which is ground referenc
ed, you should turn the switch to the GS position and if you have a floating signal the switch must be flap to the FS position.
Hope this helps.
Filipe

[jeremy_rutman]

Just to make sure: what one should do is ignore the diagram on p6 of the bnc2110 diagram, and switch

to GS mode but NOT connect the negative input terminal (the bnc shell) to ground.  Right?

[Laura F.]

Hello Jeremy,

Can you be more specific about your setup?  If you have a grounded source, you set the switch to gs and connect your BNC cable with the positive lead and negative shield from your source.  The BNC 2110 will disconnect the switch to AI ground internally so that no ground loops occur.  You do not have to disconnect the shield.  Your DAQ device needs to be configured in software as differential, and the positive lead will connect to AI+ and the negative shield will connect to AI-.  I think that the figure should not be disregarded. 

Let me know if you have questions.

Thanks,

Laura

[jeremy_rutman]

My setup consists of a sample of conductive material into which four leads
are placed for the purpose of carrying out a 'four-point conductivity measurement'.
If one were to call said leads "lead 1" , "lead 2", etc. then I am applying
a voltage V between lead 1 and lead 4.  (Lets say I connect ground of the applied voltage to AI ground,
ie I connect lead4 to AI ground.)
I am interested in the voltage drops between lead1 and 2, 2and3, 3and4.
One might call said voltage drops V12, V23, V34.
In a proper differential measurement one could simply connect a voltage probe between
lead 1 and 2 to measure V12, another between 2 and 3 to measure V23, and so on.
According to the famous figure of p6 (attached) I can't do this with the BNC2110 in FS mode, since it
connects the negative side of each voltage probe to ground thru a 5kOhm resistor,
severely affecting the current drawn by the sample, changing the voltages V12, V23, V34,  and
totally bunging up the measurement. 
I could consider, as an experimentalists gedanken-experiment,
doing a single-ended measurement instead; I would then  switch the box to GS, connect all my voltage
probes' negative sides to ground, and measure the voltages V1 V2 V3 and find the differences
by subtraction (and losing the extra noise rejection of differential measurement).
It would appear that this is what I am forced to do ; a true differential measurement
is an impossiblity with the BNC2110.
What is your learned opinion?

[Laura F.]

Hello Jeremy,

When using differential mode with a floating source, we recommend that you use bias resistors for a few reasons.  There is an extremely good Field Wiring and Noise Considerations for Analog Signals tutorial that explains these considerations, so I will refer you to it, rather than try to explain it myself. 

However, I understand that there may be situations where you would not want to use bias resistors, so for those situations, I would recommend a different accessory, such as the SCB-68 or BNC-2111 (preferably the SCB-68 for very easy connectivity). 

Or, you can set your accessory to ground referenced and then connect the positive and negative leads to the center and shield of the BNC because you connect to the amplifier in differential mode always with this accessory.  The ground connection would then be non-existent because the figure assumes that you will have a ground connection at the source and the bias resistor will not be used.  The subtraction in software would work as well.

Hope this helps,

Laura