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380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

Nerve Cuffs

Nerve cuff recordings have a frequency response up to 10 kHz and an amplitude in the low µV range.

The AI 402, x50 Ultra Low Noise Differential amplifier probe is designed for this application, with 10

kHz noise of less than 0.18 µV

rms

(1.1 µV

p-p

) in the 0.1-10 kHz bandwidth.

The nerve cuffs themselves are simply made by running fine bared stainless steel wires through a

small length of silicone tubing that is split longitudinally. Any bared wires that are in contact with the

outer surface of the cuff can be insulated with silicone adhesive.

Metal Microelectrodes

The resistance of metal microelectrodes may be several hundred kilohms or more. All of the low-

noise, AI 400 series active probes are suitable for recording from high-resistance microelectrodes.

Because the input capacitance of the AI 401 differential amplifier probe is approximately 5 pF, the

largest electrode resistance consistent with maintaining a 10 kHz bandwidth is about 3 MΩ.

Source Resistance Affects Noise and Bandwidth

If the source resistance (the electrode resistance) is high it attenuates signal amplitude, increases the

noise level, and filters out high frequency components of the signal.

1) The signal size is attenuated. The source resistance in series with the amplifier input

impedance acts as a voltage divider. The CyberAmp has an input impedance of 1 MΩ and if

the source resistance is 1 kΩ this reduces the signal by only about 0.1% (1kΩ/[1MΩ + 1kΩ]).

However, if the source resistance is 100 kΩ the signal would be reduced by 9%

(100kΩ/[1MΩ + 100kΩ]). If the source resistance is high, you should use one of the several

AI 400 series high-input-impedance probes between the electrode and the CyberAmp.

2) The noise level increases. With an active amplifier the only noise sources are: a) the thermal

noise of the electrode, b) the noise of the amplifier itself, and c) cross-talk from unwanted

signals.

Thermal noise, also called Johnson noise, is a noise voltage produced across the terminals of

all resistive elements (including electrodes) due to the random movement of charge carriers

within the element. Thermal noise increases with the resistance (R) and absolute temperature

(T) of the resistive element. To minimize the thermal noise contribution, the electrode

resistance must be minimized. This is usually accomplished by maximizing the surface area

of the electrode and ensuring good electrical contact.

The noise of the amplifier is negligible if the amplifier noise is less than the thermal noise of

the electrodes. All Axon Instruments amplifiers have very low noise levels (see Table 2 on

page 51). The amplifier with the lowest noise, the AI 402 x50 differential amplifier, boasts an

extraordinarily low noise level of just 0.18 µV

rms

(1.1 µV

p-p

) in the DC-10 kHz bandwidth.

This is approximately equivalent to the thermal noise of a 250 Ω resistor. Thus, if the

resistance of an electrode exceeds 250 Ω, the thermal noise of the electrode exceeds the noise

of the amplifier.

Cross-talk from unwanted signals in the subject can occur, although infrequently. For

example, if invasive EMG leads are subcutaneously routed near the heart, the ECG can

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Microsoft LPF-00004 Manuale dell'Operatore Pagina 65

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