Specifications

BARTINGTON INSTRUMENTS
Page 12 of 12 M2240 ISSUE 1
Appendix 1
Use of Alternative Power supplies
Bartington power supplies provide the most suitable methods for connecting to and operating your sensor.
If you have decided to use an alternative the information in this Appendix is important.
Caution: Failure to follow these instructions may result in incorrect sensor readings and in some
circumstances may cause irreparable damage to your sensor. Mag639 does not incorporate any
reverse supply polarity protection.
Power supplies should normally provide ±15V and, for the lowest noise applications, ripple in the output
should be in the mV region. The nominal current requirements are +60 mA and -35 mA with an additional
current in proportion to the measured field. The additional current is 1.4mA per 100μT per axis and will be
drawn from the positive or negative supply depending on the direction of the field.
The maximum output voltage swing from the sensor will always be less than the supply voltage. In the
temperature range -40°C to +70°C, and with an external load of 10kΩ, the maximum output voltage will be
less than each supply voltage by up to 3V. All parameters other than the output voltage range remain
unaffected for supply voltage changes in the range from ±12 to ±18V.
The current drain is independent of the power supply voltage and the unit will operate with input voltages
down to ±12V. As the output voltage swing is limited to 3V less than the supply voltage, for a supply of
±12V the output will operate normally with any output between +9V and -9V representing a field of 90% of
the full scale value in each direction. The scaling factor and linearity will remain at the normal value up to
this saturation point. The output will remain at the saturation level if the field is increased beyond this
point. Asymmetric supplies may be used provided that the minimum and maximum voltages are not
exceeded for either polarity.
Signal/power ground
The two signal/power ground conductors are connected to a common point within the sensor and the
power supply common (power 0V) should be connected to only one of them, which becomes the power
ground. The other conductor becomes the signal ground and is used as the 0V output from the sensor.
Each signal is then measured between the X, Y & Z output conductors and the signal ground output. In this
way, the signal ground carries no power supply currents.
The minimum current in the power ground conductor is approximately 35mA and, on long cables, this will
give rise to an appreciable potential difference between the power supply end and the sensor end of the
power ground conductor. The use of separate power and signal ground conductors will ensure that this
voltage is not included in the voltage measured between the signal outputs and the signal ground.