This happens almost daily: A customer contacts our support group to complain of noise on his measurement. Almost invariably a comparison is made between the output of his PC-based product and a connected DVM. The DVM shows a stable reading while the PC-connected measurement “is jumping all over the place.”
While there’s a slim chance that the PC-based instrument is to blame, that can only be true if the front end has been damaged in some manner. While not perfect, analog front ends on data acquisition and logger products are designed to inject only very small amounts of noise as a percentage of full scale measurement range.
What should you do if your setup exhibits a large amount of noise, so much so that it’s difficult to discern the true measured value? This series makes some suggestions that will help you identify and eliminate the problem. In this first part we’ll show you how to ensure that the instrument is functioning properly and is not the source of the noise. In subsequent posts we’ll dig progressively deeper into field wiring to expose some common problems that can manifest themselves.
So, is it the instrument, or not? Follow these steps to find out:
- Are you over reacting? Our software allows you to expand signal amplitude ranges to such a degree that a relatively small amount of noise can look overbearing. It’s a lot like cringing in horror as you look at a tick under a microscope. Read How To Put Signal Noise Into Perspective for guidance.
- Assuming that the noise you see is a significant percentage of the full scale range as determined by step (1), then your next move is to eliminate the instrument as the culprit. Disconnect all field wiring from it and simply short the analog inputs to common. I know. In the electronics world the word “short” implies something that’s very bad, but in this case it’s perfectly kosher. If your instrument has a single-ended configuration, short the (+) input to signal common. If it’s a differential configuration, connect the (+) input to the (-) and then connect those to signal common (if available, which it may not be for some differential configurations.) What you’re doing is establishing a firm zero volts on the front end of the channel so you can examine the instrument’s noise floor. In this configuration you should see no more than a few counts of noise around zero volts, dramatically and substantially less than you saw with a signal source connected, and roughly consistent with the manufacturer’s specification for the instrument’s noise figure. If you see anything else, then contact tech support for guidance because the chances are great that your instrument is defective.
Next time, we’ll delve into common field wiring issues that can and do produce more noise than a heavy metal concert.