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PRINCIPLES OF OPERATION Page 35

YBER

380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

Figure 15. Change in the waveform of an Electrode Test response produced by lowpass filtering. Two Ag/AgCl

wires were immersed in saline and directly connected to the differential inputs of the CyberAmp. The upper trace

shows the Electrode Test response recorded with the lowpass filter set to 10 kHz. The lower trace shows the same

response recorded with the lowpass filter set to 12 Hz.

For low values of the electrode resistance (R

), the 1 V

p-p

square wave drives an approximately

1 µA

p-p

current into the electrode. Thus the electrode resistance can be approximately determined

from:

= 1 kΩ per mV, R

< 50 kΩ Equation (1)

This equation is accurate within 5% for all values of R

up to 50 kΩ.

The exact value of R

, whether large or small, can be calculated by analyzing the input circuit as a

voltage divider comprising R

and the 1 MΩ resistor. The voltage developed at the input is:

in p p

−

1 Ω

Equation (2)

This equation shows that the true response of V

is hyperbolic. As R

approaches infinity, V

asymptotically approaches 1 V

p-p

. That is, the open circuit condition corresponds to V

= 1 V

p-p

Graphical representations of V

according to Equations 1 and 2 are shown in Figure 16.

For practical purposes, the electrode resistance can be calculated from the output voltage of the

CyberAmp, V

, as described in Box 1.

1 2 ... 48 49 50 51 52 53 54 55 56 57 58 ... 156 157

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Microsoft LPF-00004 Manuale dell'Operatore Pagina 53