Time Electronics 1024 DC Current Calibrator - Manual (Page 12)

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1024 Technical Manual 12
b) The 1024’s output can be converted to voltage by using precision resistors on the 1024
output. High performance resistors are required with accuracies of better than 0.01%, 1ohm,
10 ohms, 100 ohms, 1K ohm and 10K ohms are usually adequate. The low values must
have excellent stability at currents up to 100mA if errors are not to be introduced due to the
temperature rise in the resistor under load.
Power dissipation is 1 watt in the 10 ohm at 100 mA.
The voltage across the precision resistors can be measured by normal voltage measuring
techniques provided adequate precautions against noise pick-up are taken, these are more
likely to occur with the higher resistor values.
Method a) is to be preferred since the problems of noise pick-up, conversion and dissipation
errors, and thermal emf’s are much less.
It is also recommended that the 1024 be operated in its battery powered mode for all
calibration procedures. This is to reduce the possibility of noise pick-up which can give
erroneous results.
A total of 8 multiturn trimmers are provided, 3 for zero adjustment and 5 for range full scale
calibration. The sequence of adjustment is important since some of the trimmers affect more
than one range.
The calibration procedure is as follows:
Ensure the internal battery pack is fully recharged.
Remove the front panel and locate the 7 trimmers, VR1-VR5 are located on the front panel
P.C.B. The other 2 trimmers are located on the side of the encapsulated circuit block and
marked ‘CAL’ and ‘ZERO’.
Switch on the 1024 and allow the circuits to stabilise for a few minutes. Ensure that no
draughts or direct heating (e.g. sunlight) affect the 1024’s circuitry.
Calibrate at a stable temperature of between 20-24 °C if possible.
4.2.1. Calibration Procedure
For serial numbers 1218K6 and later.
1) Set the digit switch to zero and select 100uA range. Adjust output to zero (+/- 0.5uA)
with ‘ZERO TRIMMER’ on module.
2) Select 10uA range and adjust output to zero (+/- 0.3nA) with VR1.
3) Set digit switch to 99999 and select 100uA range. Adjust output to 100uA +/- 5nA with
‘CAL TRIMMER’ on module.
4) Select 10uA range and set output to 10uA +/- 1nA with VR2.
5) Select 1mA range and set output to 1mA +/- 50nA with VR5.
6) Select 10mA range and set output to 10mA +/- 500nA with VR4.
7) Select 100mA range and set output to 100mA +/- 5uA with VR3.
Note: If method (b) is being used for calibration 7) above, a highly stable 10 ohm resistor is
required since 1 watt is dissipated in the resistor. A 1 ohm may be used as an alternative if
a suitable 10 ohm is not available. 0.1 watt will be dissipated in a 1 ohm resistor.
4.2.2. Linearity
The linearity of the output is not adjustable and is determined by the precision resistors
mounted on the digit switch. These have been selected and adjusted in the factory and
should not require further alteration.