Microsoft LPF-00004 Manual del operador descargar pdf (Pagina 43)

PRINCIPLES OF OPERATION Page 25

CYBERAMP 380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

Typically, as the order of a filter increases, the attenuation in the pass band decreases, and the slope of

the voltage attenuation in the pass band becomes flatter.

For most data, there is little to be gained by increasing the order of a Bessel filter above four. If the

input noise spectrum is constant with frequency (i.e., white), a fourth-order Bessel filter reduces the

noise in the stop band almost as much as an eighth-order filter. On the other hand, if the input noise

spectrum is rising with frequency, such as occurs in voltage-clamp current records, an eighth-order

filter reduces the noise a few percent more than a fourth-order filter.

Sampling Rate

If time varying data is to be digitized by a computer, sufficient samples per time must be taken so that

transients and pulses are adequately sampled. The "sampling theorem" states that the minimum

sampling rate is twice the signal bandwidth. That is, if the filter -3 dB frequency is 1 kHz, the

sampling theorem requires a minimum sampling rate of 2 kHz. In practice, a significantly higher

sampling rate must be used because it is impractical to implement the reconstruction filters required to

reconstruct time domain data acquired at the minimum sampling rate. It is usual to employ a sampling

rate of five or more times the -3 dB cutoff frequency of the low-pass filter.

If the data will be transformed into the frequency domain, as in a power spectrum analysis,

Butterworth or Elliptic filters are more suitable than the Bessel filter. The cutoff near the -3 dB

frequency is sharper in these filters than in a Bessel filter, and thus they better prevent the

phenomenon known as aliasing. Aliasing causes noise in the spectrum of the analog signal above the

-3 dB frequency to appear in the digitized record as noise below the -3 dB frequency. The Bessel

filter can still be used for frequency domain analysis if the data are sampled at a rate higher than

would otherwise be necessary. As a rule of thumb, the data should be sampled at approximately 4

times the -3 dB frequency if a Bessel filter is used, and at 2.5 to 3 times this frequency if a

Butterworth filter is used.

10-90% Rise Time

The 10-90% rise time (t

10-90

) is reduced by increasing the -3 dB cutoff frequency. As a rule of thumb:

10-90

≈ 0.3/f

-3

For example, if the -3 dB cutoff frequency (f

-3

) is 1 kHz, t

10-90

is approximately equal to 300 µs.

As a rule of thumb, when a signal with t

10-90

= t

is passed through a filter with t

10-90

= t

, the rise time

of the filtered signal is approximately:

ttt

rsf

1 2 ... 38 39 40 41 42 43 44 45 46 47 48 ... 156 157

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Microsoft LPF-00004 Manual del operador Pagina 43

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