- 6 -
If a load R is grounded to a local loop
ground LG, the available common mode
voltage CMV is limited on the positive
side by the unit’s internal power supply
and on the negative side by the 600 mW
power dissipation limit of an output tran-
sistor. The diagram to the right shows
allowable CMV as a function of output
load resistance R. For example, with a
250 ohm load, CMV can range from
-13V to +17V. With a 500 ohm load,
CMV can range from -18V to +12V. The
unit will not work correctly if CMV limits
are exceed or load resistance is greater
than 600 ohms.
8. SETTING & CALIBRATION
Zero and span of the four output loops (
L1
,
L2
,
L3
,
L4
) are set be means
of precision 25-turn potentiometers marked
Z
(for zero) and
S
(for span)
on the front panel. The unit is factory jumpered and calibrated so that
4-20 mA in produces 4-20 mA out on all output channels.
To set or calibrate an output channel, apply the low input signal (4 mA, 0V
or 1V) which should produce the 4 mA low output. Measure the actual
output (e.g., 4.218 mA). Then apply the high input signal (20 mA, 5V or
10V) which should produce the 20 mA high output. Adjust the
S
poten-
tiometer until actual output current equals the just-measured low output
plus 16 mA (e.g., 4.218 mA + 16 mA = 20.218 mA). Then reapply the low
input which should produce a 4 mA output, and adjust the
Z
poten-
tiometer until the actual output current equals 4.000 mA.
Since electronic components may change as they age, annual recalibration
of the output channels is recommended using the
Z
and
S
potentiometers.
The input channel does not require calibration.
