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0.0025 voltage fluctuation

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I'm just getting started using an SCXI-1102 to (hopefully) read some voltages. The problem is that there is a constant "float" of 0.0025 V on EVERY channel - meaning that a shorted channel will read 0.0001V, with an occasional 0.0026V thrown in there. Likewise, hooking up a 1.5V battery gives me 1.5437V, with the occasional 1.5462V thrown in there. Also does this for an open channel (nominal value of 5V).

Seems like I should be able to get a constant voltage, especially for a short and open channel. Am I asking too much? How can I fix this? Thanks

Frank
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Frank,

I think I've seen this answer to your question before, but I'll reiterate it. It seems that you are using a 12-bit MIO DAQ board (such as the 6070E) in conjuction with your SCXI-1102. If your MIO board is set to bipolar +/- 10V, then your resolution is 0.00244 volts per bit. Therefore, a 1-bit change on your A/D is .00244 volts, which is what you're seeing. A 1-bit noise factor is not uncommon.

Also, the discrete voltage values of the D/A conversion don't always have an absolute 0 value in bipolar mode. Therefore, a short will give you a reading very close to zero, but not exactly zero. There is a good discussion of this in the beginning of the Measurements section of the NI catalog.

As far as the 1.5V battery, batteries usually read high when not
under a load, so it's not surprising that you read 1.5437 volts, with a 1-bit change of 0.00244 volts (your 1.5462 reading).

I'm not sure what you mean by an open channel reading 5 volts, but keep in mind that any inputs which don't have a signal attached float because the input impedance of your MIO board is very high. For example, if you connected your battery to one of the inputs, several of your inputs may also "track" this voltage until another input of lower impedance is connected to it.

Mark
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