OM1561 ISSUE 3 PAGE 6 of 11
selected if 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 a 16-bit A/D converter is used, it will have a resolution of
±
1 in
±
32768. This is just about sufficient
to digitise a terrestrial field strength of 47
µ
T to 2 or 3 nT. 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 selecting AC on the AC/DC switch.
12.3 ADDITIONAL FILTERS
The output from the
Mag
-03 sensor will contain break-through from the operating frequency of the
sensors at 15.625kHz. The
Mag
-03-OCTO-PSU attenuates this component but care should be
exercised when connecting to a sampling A/D converter. Avoid sampling rates which are close to this
frequency and ideally choose an A/D converter with an anti-aliasing low pass filter with a cut-off
frequency as low as possible for the application.
12.4 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.
12.5 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 an 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.
12.6 ELECTROMAGNETIC COMPATIBILITY
The
Mag
-03-OCTO-PSU contains no high frequency electronics likely to cause emissions which could
affect other apparatus and is heavily shielded. The constant voltage charger and 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.
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REPLACING THE INTERNAL BATTERY PACK
