0-4 mA current measurement and more!

[sadeghi]

1.Is there a particular reason why the current range on some SCXI-devices (e.g. SCXI 1102)are specified as 4-20mA rather than 0-20mA? Would it not be possible using the maximum gain and a high resolution daq device to measure sub-micro amp signals?
(If I'm not mistaken, with a gain of 10,000 and 249ohm resistor and an 18bit daq on a +/- 10V range I should be able to get a resolution of 0.03nA, of course the input bias current of the SCXI amplifier will be limiting at this resolution and absolute accuracy must be considered for smallest detectable signal)

2. For microvolt signals, is it possible to combine the maximum gain of the SCXI, 100 for SCXI 1102 and +/-0.1V range of the M series for a total amplification of 10000x in two stages as I had assumed above?

3. Are there any provisions planned for the SCXI 1102 modules where the analog filter would be jumper or program selectable for better flexibility?

4. In the catalogue I saw that the filter setting for the M-628x cards are programmable but I could not find what the selectable range is for the cut-off frequency.

5. Does NI offer a fast digital solution to account for inter-channel delays to avoid phase shifts in real time multi-channel analysis?

This brings me to a final point. I'm struggling to see the advantage of SCXI for conditioning and multiplexing. As far as I can see there are no cost advantage as a single 1102 SCXI module is only slightly cheaper than an M-series card, the amplfier characteristics (bias and offset currents) are better in the M series than SCXI 1102 modules, giving better absolute accuracy, one can scan with less inter channel delay with multiple M card than using SCXI multiplexing, and they both offer anti-aliasing filters (although not very flexible)!


I guess I had more questions than I thought. Please feel free to comment on any part.
Many Thanks,
Sam Sadeghi

[Otis]

Sam,

I think the following might clear up a bit of the confusion.

The way the SCXI 1102 takes measurements is it inputs the current across a resistor. The voltage drop across there is then read, the signal from there is conditioned, multiplexed, and sent to the DAQ board as a voltage measurement.

M-Series DAQ boards do not have the ability to measure current directly.

The signal amplified in a cascaded system would have to be a voltage signal

The amplification is based upon the range set for the channel, and it is handled by the drivers (eg if the range is from +/- 100mV then the amplification is set to 100).

I hope that clears up a little of the confusion.

As far as ways of having no interchannel delay you will want to look at S-Series Devices, or consider connecting several Multifunction DAQ cards with a RTSI cable.

[sadeghi]

Thanks for the reply,
I'm now clear on the gain settings.

However as I understand, current can just as well be read by adding a shunt resistor and reading a voltage through the M-series cards just as it would have to be done with the SCXI.

[sadeghi]

Oh and I forgot, I am looking for a real time way to account for phase shift after the signal has been digitized, this is to reduce cost in my high channel count application by avoindoing "S-Series Devices, or consider connecting several Multifunction DAQ cards with a RTSI cable". I know there are simple to more complicated algorithms that can do this, and was wondering if NI has any solutions (e.g. labview programs) which do this already.

[Brent B.]

The last part of the Simultaneous Sampling DAQ Architectures whitepaper discusses software correction for inter-channel delays.
http://zone.ni.com/devzone/conceptd.nsf/webmain/8C329C461DD8612F86256F2C004F41C9

Good luck.