confused: digital filters vs. analog filters,hard ware vs. software

[fenny]

I'm a little bit confused by the filters we can use in testing and data processing.
most resources talk about "digital filters" and "analog filters". I think they are referrring to the hard ware (analog ciruit or digital circuit),I think we still have filters in LABVIEW software which we can use after the data already acquired. those filters have the same mathematical function as the hard-ware filters. I'm wondering if they can also be digital and analog? PLease help me to clear these concepts if my understanding is wrong.

[chilly_charly]

Since software always deal with numerical (digital) data, any filter is a digital filter. Even when the data represent an analog function (data are generated by an Analog to Digital Converter)

[LocalDSP]

An analog filter is a filter that is used on your continuous analog signal, that is before the signal is eventually sampled by an analog-to-digital converter (ADC). These filters use resistances, capacitances and some times inductances to perform the filtering.

A digital filter works on sampled data, that is data that are for example generated by an A/D converter. The filtering is implemented as 'maths' (typically additions and multiplications) on numbers (your sampled data).

Note that a digital filter can be implemented in 'hardware', like inside a DSP chip or an FPGA, or in 'software' like in your PC using LabVIEW. But in realilty the calculation is always made in a hardware chip (DSP or your host PC processor). So the key word for digital filter is 's
ampled data' or 'discrete data values' while analog filters work on 'time continuous analog signals'.

Refer for example to the 'LabVIEW Analysis Concept' manual for more information on Digitial Filters. Start>>Programs>>National Instruments>>LabVIEW>>Search the LabVIEW Bookshelf

[fenny]

thanks.

[Dennis_Knutson]

You also need to be aware of when to use hardware or software filtering. You may need to use a low-pass filter prior to acquisition to remove aliasing effects. Frequency content higher than your sampling rate will be aliased as lower frequency and give erroneous measruements. No amount of software filtering will be able to remove these. For more information on this, refer to the KB articale here