Acquiring Differential vs RSE

[gstanczak]

I just bought a used B&K 1660 triple power supply and I'm trying to decide if its bad or not.  One of the outputs is powering a small light bulb, and I am trying to look at the voltage in the Measurement&Automation test panel in conjunction with a cDAQ module NI-9205.  When the channel is in RSE mode (pos. to ACH0 and neg. to COM) the voltage reading follows really well the digital display on the power supply.  But in differential mode (neg. moved to ACH08), the voltage fluctuates several volts, peaking at what the voltage is set to on the power supply.  (Please see attached image) A 1.5 volt battery provides a steady reading in differential mode, which really leads me to believe the power supply is bad.  The same fluctuation appears with all three of the power supply outputs.

I am about ready to return the item, but I first would like somebody else's opinion, and if it is bad, how it appears to work in RSE mode.  I know this is not strictly labview, but I didn't know who else to turn to.

Thanks,

Greg

[MarcoMauri]

What's connected on COM in differential measurement?

[gstanczak]

Marco, I don't have anything connected to COM in differential mode.  Two of the outputs on the power supply have a pos, neg, and ground.  One of the outputs has only a pos and neg, and this is the output I am testing.  So in diff mode I am only have ACH0 and ACH08 connected, and in RSE mode I only have ACH0 and COM connected.

Thanks,

Greg

[MarcoMauri]

Hi!

I'd suggest to cable your module as shown in Figure 6 (Connecting a Device to the NI 9205 Using Differential Connections) in the operating instructions, i.e. grounding the COM pin.

Maybe this can solve the issue...

Regards,

Marco

[gstanczak]

Great, I will try this.

Greg

[tbob]

When using RSE mode, you are grounding the COM line through the ground in the DAQ device.  When in differential, everything is floating, which is why you have a varying voltage reading.  Grounding the COM side will work.  But why use differential at all if one side is grounded?  Differential is for reading voltage drops where there is no ground connection, like reading the voltage across a shunt resistor, or reading the voltage out of a differential op-amp.  Just ground the COM line and all of your power supply negative leads together, and use RSE.  Unless I'm missing something important?

[gstanczak]

tbob, at this point I am more just experimenting with how things work, and what things work.  I do realize that when its time to acquire for real, RSE mode is sufficient to acquire signals in the range of whole volts and higher.  But, shouldn't the power supply generate a steady potential between the red and black terminals, and shouldn't the A/D device detect that steady voltage in DIFF mode when the red is hooked to the ACH0 and black to ACH08, with no other connections or groundings?  Isn't by nature DIFF supposed to be insensitive to grounding?

 

I apologize for my ignorance.  Just trying to better understand all of this.

Greg

[johnsold]

Greg,

The issue with using differential mode is not that it will not measure the signal correctly, but that the real devices inside the differential input circuitry do draw a small input current.  That current is called the bias current and must flow to ground.  If you read the fine print in the applications notes for many differential input devices you will find a requirement to have a DC path to ground.  Try your differential measurements again but connect a resistor to ground from each input. To minimize errors the resistors should have the same value and should be large enough that they do not draw too much current from the source being measured.  1 MΩ is probably a suitable value, although you could use much smaller values since you are measuring a power supply.

Lynn 

[gstanczak]

Fantastic Lynn!  My understanding is then, the differential input circuitry needs a ground to accurately measure the potential between two other points.  If that's accurate, your first couple sentences were absolutely key for me.

Thanks for all the posts, they were all helpful.

Greg

[johnsold]

Greg,

That is correct.  This is what tbob was saying also, just in different words.

If you have a passive voltmeter with two leads, there is no ground reference.  With electronic devices you always have current flow to the power supply and the measurement is always referenced to that ground internally in some way.  Some circuits make the reference to ground very obvious and explicit, while in others it may be quire obscure.

Lynn