how to reduce noise on my NI-USB 6008 DAQ measurements?

[sb_oxinst]

Hello.

I am getting all sorts of noise on my DAQ measurements. My range of measurements is 1-2V and the readings are fluctuating by +- 60mV. I am basically measuring the input current of a test station that I have at my company. I am taking the voltage across a 1ohm resistor and feeding into my NI-USB 6008 DAQ. I tried following this NI reference on noise reduction entitled "Field Wiring and Noise Considerations for Analog Signal" (https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/field-wiring-and-noise-consider...)  but haven't had any luck. I have a pretty short shielded cable now so it seems like I am missing some sort of filtering on my DAQ because when I run the same analog voltage signal into my digital multimeter, its read the signal just fine. This is a high precision meter so I am assuming it has some sort of A/D filter but that info is proprietary, heres the link to the meter I am using: http://us.flukecal.com/products/data-acquisition-and-test-equipment/bench-multimeters/8845a8846a-65-.... Any tips on what I could do to try and get an accurate mesurements. The DAQ minimum voltage range accuracy is 34.5mV, does that mean I will always be off by 34.5mV?

thanks for any help!

 

[RTSLVU]

Good luck with that I have a 6009 that is useless for any accurate voltage measurements for the exact same reason.

I could watch a signal on an oscilloscope and see it was rock solid DC but the 6009 reading would be constantly varying 40-80mV.

These things are basically "toys" (and rather expensive ones) for hobbyist and educational use.

Our 6009 has been relegated to DIO use only.

I have found Measurment Computing's USB DAQ  superior and have a better cost/performance ratio.

[RTSLVU]

I am sorry I forgot to add last night that instead of taking one measurement at a time you could try taking a lot of measurements very fast and averaging them. 

If your error is consistent you could then "calibrate" the measurement in your code by adding or subtracting a "fudge factor" to the average reading.

I tried this on our 6009 and it worked with somewhat limited success. 

[sb_oxinst]

Hi RTSLVU,

thanks for the reply.

I was hoping you wouldn't say that but it looks like averaging might be the only way to do this low cost NI DAQ.

I was thinking I could add a filter in labview that would match the meter's noise filtering but finding the meter's filtering technique would be proprietary info. I'll try averaging and let you know how it goes. 

thanks,

[SnowMule]

Current measurements are almost always going to be noisy. 

Do some hardware filtering before you bring that voltage into the DAQ; don't just read the tiny mV value from across the resistor directly by the DAQ.  The low input impedance of the 600x will add some measurement error to the number.

I use a current amplifier in most of my equipment (MAX4372); it's got a gain of 50 and is designed for doing current measurements.  Pumps a little more voltage into the analog input so the low resolution of the 600x makes less of a difference on the measurement.  Put the 1Ω resistor across the RS+ and RS- terminals on the amplifier, 5V, ground, and output to the DAQ.  Using differential measurements will clean some of that up too.  A small filter circuit can smooth out the current readings before the signal makes it into the input. 

[sb_oxinst]

Hi SnowMule.

Thanks for those suggestions. I'll see if it's worth a try!

[mahu]

I think you can use some current transformer or adapter like this http://www.tequipment.net/Extech380945.asp I used sometime for see the current in every phase of motor and worked great.

regards

[rpursley8]

I've had all sorts of problems trying to use any USB DAQ for sensitive measurements.  I finally got decent results using a USB hub with isolation (expensive) and an isolation transformer for the power supply of the powered USB devices.  Even for a USB-6211, putting a regular powered hub in between the PC and the device seemed to help some. There seems to be some ground loop issues.

I have since switched to a network based DAQ device (cDAQ-9181) and get much better results.

[sb_oxinst]

thanks for the responses everyone! I haven't got to work on this much because some higher prority item came up that I need to work on first. I'll get to it soon and let you guys know who is the winner lol.

[rpursley8]

To try to fix the problems associated with some of these USB devices, we developed an interface board to isolate the USB device from the PC/laptop that would power it.  A 5V-5V Isolated DC-DC Converter is used to isolate the power supply.  This limits the available current to around 400 mA.  A ADum1460 chip is used to isolate the data lines.

Attached is the following

1. Datasheet for the ADum1460 chip

2. Eagle Schematic

3. Eagle Board Layout

Initial tests show a cleaner acquired signal with little or no DC offset that often is present when the USB device is plugged directly into the PC/laptop.