Time Electronics 1024 DC Current Calibrator - Manual (Page 8)

8
1024 Technical Manual 8
It is important when confronted with a problem of noise pick-up to stop and think logically
about the cause and effect - a good deal of time can be wasted by indiscriminate screening
and earthing!
This is particularly important when dealing with current signals since many of the effects are
the exact opposite from those for voltage signals e.g. earthing one side of a voltage signal
may reduce the noise pick-up but earthing the same side of a current signal may increase
the pick-up. Before looking at possible sources of noise pick-up it is also important to
consider the way in which the presence of noise is being detected.
In many cases it will appear as an incorrect readout. Special care must be taken if additional
equipment (such as an oscilloscope of DVM) is connected for the detection of noise - there
is a possibility that this can introduce further noise.
Two of the more common causes of noise pick-up are discussed below. The actual level of
pick-up cannot be predicted and will depend on a number of factors.
(1) 50-60 Hz Mains Supply: This is probably the largest single source of noise pick-up. The
noise from this source can be divided roughly into two parts.
a) 50-60 Hz Signal pick-up - is generally caused by the close proximity of
unscreened mains cables or mains powered equipment. The effects of this type
of noise can usually be considerably reduced by correct screening and earthing.
Important Note:
When the 1024 is operated from the mains, special attention should be paid to the
possibility of noise pick-up occurring. Battery operation is recommended for
applications requiring the lowest noise pick-up condition.
b) Transient Signal pick-up - is generally caused by heavy load switching on the
mains from equipment such as electric motors, electric ovens, etc. Its effect on the
1024 is to cause a transient variation in the output which can last longer than the
actual duration of the noise. This is because the 1024 has been transiently over-
loaded and needs time to recover. The noise is often in the form of a burst of radio
frequency energy which in addition to being carried in mains cables is also radiated
into the atmosphere. It is often difficult to detect its presence due to its transient
nature and short duration (down to a few 10's of microseconds). The effects of this
sort of noise on radio and television reception are well known and a transistor
radio is often a useful tool in locating the source of the noise.
This type of noise often covers a very wide spectrum of frequencies and the most
effective solution is to locate its source and provide suitable filtering - such as
conventional capacitor suppression on arcing contacts.
(2) Common Mode Noise: Additional noise and variation of the output current can be
caused by large common mode voltages. These occur when the 1024 is used to calibrate
any input which is above ground potential or has an a.c. component with respect to ground.
When powered by the internal battery the 1024 has inherently a very high d.c. common
mode rejection, but it is not recommended under normal circumstances that 100V d.c.
common mode be exceeded. The a.c. common mode rejection is determined by the
capacitive unbalance to ground of the output terminals and associated connections.