how to measure low-level signal(less than 1V)

[iriszhaoyu]

Hello Everyone,

I use a Keithley DAQ(KUSB-3108) to collect the low voltage,but the data fluctuated all the time.

I can't deal with it.Can somebody please help me ?thanks

[mross]

QUOTE (iriszhaoyu @ Mar 20 2009, 05:11 AM)

Hello Everyone,

I use a Keithley DAQ(KUSB-3108) to collect the low voltage,but the data fluctuated all the time.

I can't deal with it.Can somebody please help me ?thanks

You have entered into one of the experiences that cannot be avoided if you are going to do data acquisition, and you will need to learn your way around it - there is not necessarily an easy solution, or the easy solution may elude you for a while.

Start out with the Field Wiring white paper from NI:

http://zone.ni.com/devzone/cda/tut/p/id/3344

And this one "Five Tips to Reduce Measurement Noise":

http://zone.ni.com/devzone/cda/pub/p/id/262

Keithley will mail you an excellent reference if you call and ask them (When I got mine they did not have any way to get it online.): "Low Level Measurements." It is not for sale you have to ask them for it, it was free. I also have another refernce they publish called "Switching Handbook."

You have not given any information that allow someone to help you sort out the problem. no circuit, no specifics of the transducer or signal conditioning, nada. You may get more information if you yourself provide more.

Absent any ideas what you are actually doing, I can only offer general comments. Use differential measurement systems exclusively. Only use single ended measurement if you have a very good reason; such as you cannot get an appropriate differential transducer, or you cannot afford to buy equipment with enough channels for differential measurement. You can easily waste in time, the cost of better equipment and more channels of DAQ if you are too "cost conscious." In an academic environment it is sometimes possible to borrow equipment temporarily.

It is possible that the building in which you operate has a very noisy ground circuit. I had to drive a copper clad ground rod into the earth beneath a lab floor once to get a clean enough ground for DAQ, an "instrument ground." It was an industrial environment and there was a great deal of transient load and unbalanced loads on that grid. You must beware of safety issues when you have a separate ground from the building grid and act accordingly. Even this can be difficult if the earth is partiularly dry. I have seen building grounds require extensive laying of cable and the application of salt to improve earth conductivity.

You may have unintentionally created a ground loop in your circuitry - you must understand how to properly shield wires, you should try to use a single point ground as much as possible. These problems are often cause by inattention to detail in wiring. Sometimes circuits "just grow" to become troublesome; then you might need to rethink the whole system up to and including the building grid.

Basic good wiring practices, a proper ground circuit, and appropriate other circuit design are necessary and will help avoid these troubles in the future..

Or you may simply have a bit of frayed shielding touching the wrong surface.

Good luck,

Mike

[Neville D]

Mike's covered most of the basics. But check the gain on your board. "Low cost" USB devices usually have low gain and are not ideal for use with thermocouples (signals in uV) or other tiny signals. Using a DAQ device with isolated inputs will also help, but again this will be expensive.

For future reference read this.

Neville.