Microsoft LPF-00004 Manuel de l'opérateur télécharger pdf (Page 45)

PRINCIPLES OF OPERATION Page 27

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

500 ms

0.1 Hz

1 Hz

10 Hz

INPUT

Figure 10. A 1 Hz square wave is AC coupled at three different frequencies. The distortion is progressively

reduced as the AC coupling frequency is reduced from 10 Hz to 0.1 Hz. In all cases the DC content of the

signal is removed. The problem with using the lowest AC coupling frequencies is that slow shifts in the

baseline may not be rejected. Furthermore, transient shifts in the baseline might take a long time to recover.

Since the distortion is less at lower AC coupling frequencies, it is tempting to suggest that the AC

coupling should always be set to very low frequencies, such as 0.1 Hz. However, this is sometimes

unacceptable because some unwanted shifts in the baseline are relatively rapid and need to be quickly

eliminated.

For very slow signals, even the lowest AC coupling frequency offered by the CyberAmp (0.1 Hz)

causes significant distortion of the signal. For these signals, an alternative technique known as

"Autozeroing" can be used. In this technique, the inputs of the CyberAmp are DC coupled and when

the zero command is issued, a sample of the signal is taken as a baseline value. This DC sample value

is then continuously subtracted from the signal by the CyberAmp until the next zero command is

issued.

The efficacy of the technique is illustrated in Figure 11.

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Microsoft LPF-00004 Manuel de l'opérateur Page 45