Lake Shore Model 480 Fluxmeter User’s Manual
Advanced Operation
5-13
5.10.2 AC Measurement Mode
AC mode is a natural extension of the measurement capabilities of an integrating DC fluxmeter. With
the selection of only a few different parts the Model 480 is made ready to measure periodic AC fields.
These fields may be present as stray field around transformers or leakage from switching power
supplies. AC mode measures in RMS and only the AC part of the field change is represented in the
reading value; the steady state DC field is ignored. Peak hold can be used with AC mode to measure
the peak value of periodic fields or to capture fast transients as described in Paragraph 5.11.
AC measurements do not require drift compensation or integrator reset because modifications to the
integrator bring the reading to zero when no signal is present. Frequency response and noise
dominate the error present in AC mode. The Model 480 has a predictable frequency response which
is illustrated in Figure 5-1. Sense coils also have a frequency response which can limit accuracy.
AC measurements can be susceptible to noise because most fields are small and require a low
measurement range. It may be necessary to shield environmental noise during low field AC
measurements.
The RMS converter in the Model 480 requires a significant signal amplitude for proper operation. AC
measurements are specified with a minimum reading for each range because of the RMS converter.
If the input amplitude is below the specified minimum the AC annunciator will blink on the instrument
display. The minimum reading is different for AC peak operation because the RMS converter is
bypassed.
To select AC mode press the
AC/DC
key. AC operation is indicated by the letters
AC
on the normal
display to the right of the units indicator.
P-480-5-1.bmp
Figure 5-1. Model 480 AC Frequency Response