Microsoft LPF-00004 Manual del operador descargar pdf (Pagina 47)

PRINCIPLES OF OPERATION Page 29

YBER

380, COPYRIGHT MARCH 2002, AXON INSTRUMENTS, INC.

Saturation

The AC coupling circuit is the first circuit in the CyberAmp. If a large step is applied to AC-coupled

inputs, the AC coupling capacitors reject the step voltage with a time constant determined by the AC

coupling frequency. If the CyberAmp amplifiers are set to high gain, the output might be saturated for

a considerable time. For example, if the gain is x100, the AC coupling is 1 Hz, and the step size is

1 volt, the CyberAmp output will be saturated until the voltage at the output of the AC coupling

capacitor falls to 100 mV from its initial peak of 1 V. This will take about 2.3 time constants.

Because the time constant is 160 ms, the output will be saturated for at least 370 ms. For the next

several time constants, the output will settle towards zero.

Offset Control

Each channel in the CyberAmp 380 includes a 16-bit digital-to-analog converter (DAC) that is used

for adding an offset to the signal. Offsets are added for two purposes.

First, the DAC is used to remove internal offsets that are generated in the operational amplifiers

within the CyberAmp. This is done automatically by the CyberAmp and does not require an explicit

command.

Second, the DAC allows the user to add a DC level to the signal. A DC level might be added to the

signal to: (1) move the signal up or down on the display screen of the data acquisition system,

(2) shift the output of a pressure transducer so that it is zero at a fixed pressure reference, (3) balance

the output of a strain gauge, or (4) remove the DC content of the signal so that it can be amplified

without causing saturation. In this last example, the user may either adjust the offset manually or use

the Autozero facility described above in the AC Coupling and Autozeroing section.

The range of the DAC output is from -3.2768 to +3.2767 volts. Because the 16-bit DAC has 65,536

possible values, its output can be set in exact multiples of 100 µV. User control is restricted to the

±3 V range. The additional fraction of a volt is used for the internal offset adjustment.

Even though the same DAC is used to correct for the internal amplifier offsets and the DC level set by

the user, the user need not keep track of the internal offset correction. For example, if a DC level of

2 V is requested and the internal offset is 12 mV, the DAC output is actually 2.012 V.

The DAC output is added into the signal pathway after the pre-filter amplifier. This means that the

effect of the offset shift, when referred to the input, depends on the pre-filter gain. To illustrate this

statement, imagine that the input signal is a 10 mV sine wave superimposed on a +200 mV baseline.

If the pre-filter gain is x10, the output of the pre-filter amplifier is a 100 mV sine wave superimposed

on a +2 V level. To zero out the DC level, the DAC output must be set to -2 V. The input-referred

DAC output is -200 mV.

The CyberAmp automatically takes into account the pre-filter gain so that all offset commands issued

by the user are interpreted as input referred. Because the DC offset is added after the pre-filter

amplifier the input-referred range and resolution depend on the pre-filter gain.

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Microsoft LPF-00004 Manual del operador Pagina 47