DAQ 6001 drift on zero

[Sim1]

Hi all,

I´m having a issue with my DAQ 6001. In my measurements I always have an offset of about 0.1 mV. I give you some detail:

• The terminal configuration is RSE.
• I even connected the two wires, in order to make a short circuit in which the voltage should be 0.
• I have one wire on positive pin, and the other one in ground pin.
• I tried to do the measurement with NI example, the results is the same.
• A bit value in my DAQ is 1,2 mV (20V/2^14), so the offset is about ten times lower!
• I don´t know if it´s important, but if the wires are not connected to something I have a sine wave centered on -0.17 V, I have no idea why!

If somebody know the reason of this offset, I would appreciate it 😄

By, and thank you!

[Bob_Schor]

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@Sim1 wrote:

Hi all,

I´m having a issue with my DAQ 6001. In my measurements I always have an offset of about 0.1 mV. I give you some detail:

• The terminal configuration is RSE.
• I even connected the two wires, in order to make a short circuit in which the voltage should be 0.
• I have one wire on positive pin, and the other one in ground pin.
• I tried to do the measurement with NI example, the results is the same.
• A bit value in my DAQ is 1,2 mV (20V/2^14), so the offset is about ten times lower!
• I don´t know if it´s important, but if the wires are not connected to something I have a sine wave centered on -0.17 V, I have no idea why!

---

I have some questions and some comments.

• It is not clear to me whether you are talking about Analog Input or Analog Output.  You talk about a voltage offset of 0.1mv, which makes me thing you mean Analog Output, but then talk about RSE, a term used with Analog Input.
• You note that the precision of your DAQ system is 1 bit = 1 mv (approximately).  The offset you are worried about is a tenth of that, so it represents a bit value of 0, or "0 volts", which is the accurate value, given the precision of the 6001.
• I don't know what you mean by "connected the two wires" -- connected them to what?  Do you mean you have one wire going from AI0 (say) to Analog In Ground?  (See my confusion between Analog In and Out?)
• I'll bet that the sinusoid you see when nothing is connected has a frequency of 60 Hz (or perhaps 50 Hz in some countries).  I wonder what this could be?  Try touching one of the inputs -- I bet the sinusoid amplitude increases a lot.  Now learn a little Practical Electrical Engineering, especially the Chapters "On Input and Output Impedance", "Proper Grounding", etc.
• The reason for the "mis-match" (where the output that is supposed to be 0 and, in fact, is 0 in the digital world, being a small fraction of a single bit) is that the design engineers used circuitry that works and gives the "right digital answer" over a reasonable temperature range.  They could have given you more precision (16 bits would still be 0.3mv/bit), but that costs more money ...

Bob Schor

[Sim1]

1) I´m reading a voltage, so voltage input.

2) you´re right, but I have all the bit values shifted of the amount of the offset. I mean: 0 bit= 0.1 mV, +1 bit= 1.3 mV, -1bit= -1.1. So it´s seems to have the same offset in all the values.

3) I have, one wire on ground analogic input and the other one on the positive pole. I connected these wires in order to get a short circuit and have 0V measurement.

4) I´m not an electrical engineer but I will take a look, thank you for the advice!

5) ok, if you say it´s normal, it´s just looked strange to me, having the same offset in all values I have!