Output Noise - Time Electronics 1024 User Manual

Time Electronics
1024 DC Current Calibrator User Manual
2.3

Output Noise

a) Inherent Noise
The electrical noise on the output current consists of chopper intermodulation, thermal noise
and random variations. These are generated inside the 1024 and originate from the active
circuitry. At current levels of greater than about 1mA the noise fluctuations are those of the
accurate voltage reference circuitry.
At currents below a few hundred microamps it becomes difficult to differentiate between the
1024 inherent noise and picked-up noise.
b) Noise Pick-up
In addition to the inherent noise from the 1024 circuitry the output can contain fluctuations
which originate from outside sources.
As with voltage signals the effects of noise on current signals are usually measured as a
fraction of the signal (in % or PPM) and in general this fraction becomes larger as the signal
level is reduced.
Care must be taken in attempting to understand the cause and effect of noise pick-up on
precision current sources such as the 1024, and it is not proposed that this manual should
cover them in detail.
It is important when confronted with a problem of noise pick-up to stop and think logically
about the cause and effect, as 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. For example, 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 or DMM) 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.
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