USB 6211 noise problem

[Bill Wang]

Hi, I use USB 6211 to measure pressures, but I got too many noise. Please see attached figure. In order to test if that's the problem of the DAQ card. I tried to record a 5 V DC from a power supply, and I got -2 V noise without consistent frequeny. Power supply was confirmed no problem. I used DIFF mode. Could anyone help this out please? Thanks.

[AdnanZ]

Wow, that doesn't look like noise to me; you should mention your hardware setup and how you have wired your signals. Have a look at Table 1 in this article:

Field Wiring and Noise Considerations for Analog Signals

If you are using DIFF mode for a floating signal source, be sure to have some pull down resistors in there.

[Bill Wang]

Sorry about that.

Hardware setup: Continuours sample, sample number: 100, sample rate 1k.

I wired AI2 as + and AI10 as - .

What's  pull down resistor? Thanks.

[AdnanZ]

What happens if you use some other differential channel? Also, have you verified with a DMM that you are getting a stable signal? Have a look at Table1 in the link that I mentioned above. I maybe using the wrong terminology; the resistors are used to provide return paths to ground for bias currents IF you are using a floating signal source. They are not needed for a grounded signal source.

[Bill Wang]

Thanks, Adan.

I tried several other differential channels. They work the sam way. I've also verified with a DMM to get a stable signal.

Since I'm using floating signal source, I think I need this pull down resistor. 

For figure 5 in table 1 you mentioned above, can I put the pull down resistor between AI GND and AI -2 if I use AI -2 as negative signal input ?

Thank you very much.

Bill

[AdnanZ]

Well, just try the RSE mode then. You don't need resistors and can use channels AI 2 and AIGND (which is row 2 column 1 in the Table).

[Bill Wang]

Thanks for your suggestion. I recorded constant power supply again, and got signal like this. I don't think it's acceptable. Could you help please? Thanks, Bill

[EEsRULEtheWORLD]

Hi, Bill.

I have some questions for you:

1)  Is your power supply a grounded signal source (Does it plug into an outlet?)?  If this is the case, connecting the signal as RSE will not be a good option for you.  You are going to want to connect it either Differentially or Single-Ended-Nonreferenced (NRSE).

2)  What signal are you expecting to get?  A constant 9.91VDC signal (as shown in the attached screenshot) or 5VDC (as stated in the first post)?

3)  What kind of pressure sensors are you using?  Do they require to be powered?

[Bill Wang]

Hi Sara,

Thanks for your reply.

1) My power supply plugs into a APC UPS.

2) Actually I am recording two changing pressures from two pressure transducers. One is Omega, and the other one is PDCR. These two pressure transducers require to be powered by 10VDC.

The data I recorded from both the power supply and two pressure transducers have huge noise.

I'm sorry that I didn't make it clear.

Thank you very much for your help.

Bill

[Bill Wang]

RSE and NRSE model data were even worse. I guess Diff model is the best for me, but it still has too much noise.