We have several types of data acquisition devices where I work. They're used primarily to measure low frequency voltages (<100HZ, +/-10V). One device has a built in filter that does an excellent job. I asked the manufacturer for details about the filter. They would not elaborate on the specific design of the filter, or where to purchase one. Here's the way they describe the filter in an email:
"the filter before the ADC is a 2nd order RC filter with 90dB at approximately 128kHz"
The user manual describes it simply as a 50/60 Hz filter.
To my surprise I'm having difficulty finding such a filter using Google. Can anyone tell me how to construct such a filter or where to purchase them? Ideally it would be a module with BNC connectors so I could simply attach it inline with the signal cable to our other data acquisition devices.
As a mechanical engineer I have little knowledge about filters. However I did try connecting a 2000uF capacitor across the test leads. To my surprise this actually helps a lot to remove noise. It is an electrolytic type which I understand is polarized and only to be used with positive signals. I haven't found any non-polarized types of capacitors with that much capacitance. But 20000uF seems like way too much. I want to find a more appropriate solution.
How do I design an appropriate filter for my application?