Microsoft LPF-00004 Manual del operador descargar pdf (Pagina 44)

Page 26 PRINCIPLES OF OPERATION

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

Pre-Filter vs. Post-Filter Gain

When low-level signals are recorded, it is essential that the first-stage amplification be sufficiently

high to minimize the noise contributed by succeeding stages. For example, in a microphone amplifier

the tiny output from the microphone is first coupled into an extremely low-noise transistor amplifier.

After this first-stage amplification, circuits with more modest noise characteristics are used for further

amplification and to introduce treble and base filtering.

If the only rule was to maximize the early gain, all of the gain would be implemented before the

offset, low-pass filter and notch filter stages. However, with some signals, too much gain in front of

the low-pass filter can introduce a different problem. This occurs when the signal is much smaller

than the wide-band noise.

For example, consider an 80 mV

p-p

signal in a 100 Hz bandwidth that is buried in 1 V

p-p

of wideband

noise. After filtering at 100 Hz, the noise is reduced to just 10 mV

p-p

. It is desired to amplify the

signal by x100 so that it occupies a substantial fraction of the ±10 V input range of the A/D converter.

If the x100 amplification is applied in front of the low-pass filter, the 1 V

p-p

wideband noise will be

amplified beyond the point of saturation. The data will be corrupted and, to make matters worse, after

filtering the clipped noise peaks might be indistinguishable from the signal.

A safer strategy is to apply the x100 gain after the low-pass filter. In this case, the signal plus the

remaining noise in the 100 Hz bandwidth will be within the linear range of the x100 post-filter

amplifier. It would also be safe to split the gain, using x10 in front of the filter and x10 afterwards.

AC Coupling and Autozeroing

AC coupling is used to remove DC offsets in the input signal. Signals below the -3 dB frequency of

the AC coupling circuit are rejected. Signals above this frequency are passed. For this reason AC

coupling circuits are more formally known as high-pass filters. As in most instruments with AC

coupling, the AC coupling circuit in the CyberAmp is first order. That is, the attenuation below the

-3 dB cutoff frequency increases at 20 dB/decade.

The CyberAmp 380 allows the user to select six AC coupling cutoff frequencies. Alternatively, the

inputs can be DC coupled.

It is important to appreciate that the AC coupling is independently applied to the positive and the

negative inputs of the CyberAmp. That is, it is possible to AC couple the positive input while DC

coupling the negative input.

When a signal is AC coupled, the DC component of the signal is eliminated and the low frequency

content is filtered out. This can cause significant distortion of the signal, as shown in Figure 10.

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Microsoft LPF-00004 Manual del operador Pagina 44