Microsoft LPF-00004 Manuale dell'Operatore Scaricare il pdf (Pagina 58)

Page 40 PRINCIPLES OF OPERATION

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380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

The input resistance of the CyberAmp 380 main unit is 1 MΩ. In general, electrodes can be directly

connected to the CyberAmp either if the difference between the resistance of the positive and negative

electrodes is a few kΩ or less or if there is no common-mode signal.

If there is a large common-mode signal and a source imbalance of more than a few kΩ, a high input

resistance amplifier probe should be used. Several AI 400 series probes are available that have input

resistances of 10 gigohms (10

Ω) or more. These probes are distinguished on the basis of noise,

cost and size.

Peak-to-Peak and RMS Noise Measurements

Noise is a crucially important parameter in instruments designed for the measurement of low-level

signals.

Engineers prefer to quote noise specifications as root-mean-square (rms) values, whereas users prefer

noise quoted in peak-to-peak values. The user prefers a peak-to-peak value because this corresponds

to what is seen on the oscilloscope screen or data acquisition monitor.

Engineers prefer to quote rms values because these can be easily, consistently and definitively

measured. In statistical terms it is the standard deviation of the noise. True rms meters and

measurement software are commonly available and the measured values are consistent.

On the other hand, peak-to-peak measurements are badly defined and there are no instruments or

measurement software for their determination. Depending on the interpreter, estimates of the peak-to-

peak value of gaussian noise range from four to eight times the rms value. Observers may focus on

the "extremes" of the noise excursions (hence the x8 factor), on the "reasonable" excursions (x6

factor), or on the "bulk" of the noise (x4 factor).

At Axon Instruments we have developed software that simultaneously measures the rms and the peak-

to-peak noise. The peak-to-peak noise is calculated as the threshold levels that would encompass a

certain percentage of all of the acquired data. We have found that the following rules of thumb hold

true for gaussian noise:

Percentage of data encompassed

Peak-to-peak thresholds

95% 3.5-4 times rms value

99% 5-6 times rms value

99.9% 7-8 times rms value

These empirical measurements are confirmed by analysis of the gaussian probability distribution

function.

In this manual and in the specifications, noise measurements are quoted in both rms and peak-to-peak

values. Because there is no accepted definition of peak-to-peak values, we have chosen to calculate

the peak-to-peak values by multiplying the measured rms values by 6.

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Microsoft LPF-00004 Manuale dell'Operatore Pagina 58