AC Coupling

A circuit for removing the constant DC voltage from an input signal so that just the AC portion can be processed.

Aliasing

The generation of unwanted additional frequencies in a wanted signal, caused by sampling the signal at too low a frequency.

A Brief Note on Filters for Aliasing (Please see In Depth Note 3 Also)
To judge how well a particular low pass filer response will act as an anti-aliasing filter, you need to know its stopband attenuation and its stop band ratio. To calculate the number of bits of protection that the filter will provide against aliasing, divide the stopband attenuation of the filter (in dB) by six. This number does not have to be equal to the number of bits of resolution of your sampling system, but it’s a good starting point. To calculate the lowest sampling ratio which you can safely use before aliases occur in your data, just add one to the stopband ratio of the filter. This criterion does not provide unconditional protection against all aliasing, but is sufficient in most cases. For instance, a filter giving 72dB rejection at two times the cutoff will provide 12 bits of protection and can be used with sample rates of three times cutoff (or higher).

Attenuation

The ratio between the output and input signal levels of a signal-handling device. Usually expressed in dB.

Bandpass

A filter in which a frequency region with a low attenuation lies between frequency regions in which attenuation is high. Simple band pass filters can be made by connecting a highpass filter and low pass filter in series.

Bandstop

Sometimes called a band-elimination filter. A filter in which a frequency region with a high attenuation lies between two frequency regions in which attenuation is low. Simple bandstop filters can be made by connecting a highpass filter and a lowpass filter in parallel in the appropriate way.

Bessel

A widely known class of filter responses with excellent overshoot properties.

Butterworth

A widely known class of general purpose filter responses.

Cutoff Frequency Span

The ratio between the lowest and highest cutoff frequencies that a particular model of a filter product can be set to.

Discrimination

The ratio of the frequencies (or frequency differences) which specify the extremes of the passband and stopband of a filter.

Dynamic Range

The ratio between an input signal and all the unwanted extra signal present at the same time. (Also In Depth Note 1)

Effective Slope

Measured in dB per octave. The slope of a straight line connecting the two points which specify the extremes of the pass band and the stopband of a lowpass or highpass filter. The higher the effective slope, the sharper is the filter.

Elliptic

A type of mathematical function which lends its name to a class of filters (sometimes called Cauer filters) which provide maximum effective slope for a given amount of ripple.

Gauss-Chebychev

A type of filter with quite good overshoot performance but more stopband attenuation than a Bessel filter.

IEPE Supply

A source of operating power for a popular type of signal transducer.

Order

The order of a filter is equal to the number of significant poles in the mathematical function describing its frequency response.

Overshoot

The amount by which the output of a filter exceeds the correct final value when fed with a sudden step change in the input.

Pulse Response

A filter feature which allows the amplitude of the filter response of a filter type to be altered to reduce or eliminate overshoot.

Rejection

The ratio between the gain of a filter in its stopband to the gain in its passband. Usually expressed in dB.

Ripple

The variation of the filter gain across the passband. Usually expressed in dB.

Sample

A measurement of the input signal at an instant in time.

Vector Error

Also called total error. The amount by which a filtered signal differs from a pure version of that signal simply delayed in time.

In Depth Filter Notes –a different series of notes - are available on the following list of subjects that may be of interest to the reader: NOTE 1 DYNAMIC RANGE; NOTE 2 WAVEFORM DISTORTION; NOTE 3 ALIASING; NOTE 4 SWITCHED CAPACITOR FILTERS; NOTE 5 SETTLING TIME; NOTE 6 RECONSTRUCTION. You can find these at our website in the RESOURCE LIBRARY.