Microsoft LPF-00004 Manual do Operador Página 46

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Page 28 PRINCIPLES OF OPERATION
C
YBER
A
MP
380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.
AUTOZERO
AUTOZER
O
DC WITH
0.1 Hz
1 Hz
10 Hz
DC
Figure 11. The top trace shows an ECG signal that is sitting on a 5 mV offset resulting from electrode junction
potentials. The pulses are distorted at AC coupling frequencies from 10 Hz down to 0.1 Hz. In the bottom
trace, a zero command is issued to the CyberAmp at the time indicated by the arrow. The DC component is
immediately removed, but the pulses are unaffected.
Autozeroing works best when the time of occurrence of signals is known, so that the zero command
can be issued in the baseline period preceding the signal.
Time Constant
The AC coupling frequency is inversely related to the time constant (τ) of the decay of a square wave
(see Figure ). The relationship is:
τ = 1/2πf
-3
The time constants for the six AC coupling frequencies in the CyberAmp 380 are:
f
-3
(Hz) τ (ms)
100 1.6
30 5.3
10 16
3 53
1 160
0.1 1,600
In one time constant, the signal decays to approximately 37% of its initial value and in 2.3 time
constants decays to 10% of its initial value.
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