Microsoft LPF-00004 Manuel de l'opérateur télécharger pdf (Page 52)

Page 34 PRINCIPLES OF OPERATION

YBER

380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

Electrode Test

The electrode resistance is measured for several reasons. First, it establishes the basic continuity of

the electrode circuit. Sometimes electrodes or electrode leads break, causing the measured resistance

of previously low resistance electrodes to go very high and consequently resulting in no incoming

data. Second, it verifies that the electrode is making acceptable contact, as in surface electrodes. For

example, to achieve low-noise recording levels the electrode resistance may have to be less than

5 kΩ. Third, the recording surface of the electrode can change. Insulation on the electrode can

develop cracks or other defects, leading to an increase in surface area and, in previously high

resistance electrodes, a decrease in resistance. Many researchers therefore keep a daily record of the

resistance of each electrode with respect to the reference electrode, or between electrodes in a

differential configuration.

When the Electrode Test is activated, a 1 V

p-p

(±0.5 V), 10 Hz square wave is connected to every

input via individual 1 MΩ resistors. (Note that these are the same 1 MΩ resistors that establish the

input resistance of the CyberAmp inputs.) The connections are shown in Figure 14.

The square wave current may not always produce a square wave response. Some electrodes may

polarize during the test, showing an apparent charging curve. This will not detract from a measure of

the ohmic resistance of the electrode if the response reaches an asymptotic peak value. Users may

prefer to inspect a simpler sinusoidal response, obtained by setting the lowpass filter to 12 Hz.

Figure 15 shows the simplified form of a highly filtered Electrode Test response.

INPUT

1 M

Ω

1 M

Ω

GND

TEST

NORMAL

10 Hz

1 V

p - p

GAIN

Figure 14. The connection of the Electrode Test circuit to the inputs of a CyberAmp channel. When switch S1 is

in the NORMAL position the 1 MΩ resistors at the input provide the standard 1 MΩ input resistance of the

CyberAmp. When S1 is in the TEST position, a 1 Vp-p square wave is applied to the 1 MΩ resistors. The negative

input is grounded in this example, so that the only voltage that is recorded is Vin on the positive input. The

magnitude of Vin is described by Equations 1 and 2 in the text.

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Microsoft LPF-00004 Manuel de l'opérateur Page 52