OM0065 ISSUE 5 PAGE 7 of 21
be selected if it is required to block the static background field and provide only an ac response.
The decision to use either a dc or ac response will depend upon several factors.
(a)
The need or otherwise to measure the static field.
(b)
The dynamic range of any associated measuring instrument.
If the analog signal is converted by a 16 bit A-D converter, this will have a resolution of
±
1 in
±
32768. This is just about sufficient to digitise a terrestrial field strength of 47
µ
T to the full
resolution of the magnetic field sensor. Many 16 bit converters do not guarantee full
monotonicity over 16 bits.
If only the alternating component of the magnetic field is of interest the static component can be
rejected by including the high pass filter. To do this, remove the two dome head screws in the
base of the power supply unit and remove the lid. This will reveal the filters. A diagram on the
inside of the lid of the power supply unit shows how the filters can be individually linked for dc or
ac response. (See Figure 4)
As supplied, the high pass filter is linked out and the filter section has a dc response.
11.3 ADDITIONAL FILTERS
The output from the
Mag
-03 sensor will contain noise over a wide frequency band, including
break-through from the operating frequency of the sensors at 15.625 kHz. To acheive the lowest
output noise level the bandwidth should be limited, using an external filter with a sharp cut-off, to
the frequency band of interest.
11.4 OVERLOAD
Exposure of the magnetic field sensor to a flux b120% and +200% or -120% and -400%
of the full scale range will cause the analog output to saturate. Beyond this the output will
become unpredictable, usually slewing to the opposite polarity. The magnetic field sensor will not
be harmed permanently due to overload but this situation should be avoided.
11.5 AC MEASUREMENT PRECAUTIONS
The measurement and roll-off characteristics of the magnetic field sensor ensure great precision
for measurements obtained within the specified bandwidth and freedom from errors at frequencies
above the -3dB point. It must be noted, however, that all fluxgate magnetic field sensors employ
some form of synchronous phase detector operating at a particular frequency. This gives rise to
modulation products in the presence of alternating flux close to the operating frequency. The
magnitude of the products will be equal to the magnitude of the detected flux (
±
3dB) for a
difference frequency of
±
1.5kHz. The operating frequency of the
Mag
-03 series of magnetic
sensors is 31.25kHz.
11.6 ELECTROMAGNETIC COMPATIBILITY
The
Mag
-03PSU contains no high frequency electronics likely to cause emissions which could
affect other apparatus. The constant voltage charger and lead-acid battery combined with
tantalum capacitors provide good filtering to minimise any emissions. The unit is also unlikely to
be affected by interference from other equipment in the normal operating environment. However,
Summary of Contents for MAG-03PSU
Page 12: ...OM0065 ISSUE 5 PAGE 12 of 21 DR1011 1 Mag 03 Mag 03PSU SIMPLIFIED SCHEMATIC FIGURE 1...
Page 13: ...OM0065 ISSUE 5 PAGE 13 of 21 DR0065 7 Mag 03PSU ASSEMBLY FIGURE 2...
Page 14: ...OM0065 ISSUE 5 PAGE 14 of 21 DR1013 2 PC36 SCHEMATIC FIGURE 3...
Page 15: ...OM0065 ISSUE 5 PAGE 15 of 21 DR1014 1 PC36 ASSEMBLY FILTER SELECTION FIGURE 4...
Page 16: ...OM0065 ISSUE 5 PAGE 16 of 21 DR1015 2 Mag 03PSU WIRING DETAILS FIGURE 5...
