Time Electronics 1024 DC Current Calibrator - Manual (Page 7)

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1024 Technical Manual 7
2.2. Operating Procedures and Precautions
2.2.1. Normal Operation
Operation of the 1024 is self-explanatory from the front panel controls and specifications. It
is important to understand the 1024’s voltage capability and voltage limit indicator to ensure
that your requirement is within the limit. It is also important to read section the section on
output noise if your requirement is for current levels of less than a few tens of microamps.
When the 1024 is operating from batteries and required to supply currents in excess 10
10mA it is important to check the battery condition with the load connected since the
battery’s condition may be adequate for small output currents but not large ones.
Measurement of an unknown current:
1) Select ‘NULL’ position on the front panel function switch.
2) Select ‘NORMAL' output.
3) Adjust ‘SENS’ for minimum sensitivity (fully anticlockwise).
4) Select range consistent with current to be measured.
5) Connect the unknown current to the terminals in the same polarity i.e. unknown current
flows into the + Ve terminal.
6) Adjust digit switch and sensitivity control for null balance on meter as required.
7) When balance at required sensitivity is reached the unknown current is equal to the digit
switch setting.
2.2.2. Overload conditions
The 1024 can withstand indefinitely either short or open circuit on the output terminals.
Overload conditions can easily result if any attempt is made to drive current into the 1024 by
applying a voltage to the output terminals. In most cases this will cause the output fuse (See
Section 2.5) to blow but can still cause damage to the output circuitry.
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 (see Section 2.3b)
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.
A good deal of care must be taken in attempting to understand the cause an 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.