PicoLog Self-Help Guide
AR346-1
© Copyright 2014 Pico Technology Ltd.
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It is possible to offset and scale a negative signal. There is a +2.5 V, 10 mA source
available on pin 17 of the PicoLog. To start with, you will need to add resistors to the
terminal board exactly as described in the PicoLog 1000 Small Terminal Board User’s
Guide (see Related Documents).
Here are the values to obtain a ±5 V range:
1.
Assuming you are using channel 1, add a 20 kΩ surface-mount resistor in
position R1 (cutting the track as well).
2.
Put another 20 kΩ resistor in position R2.
3.
Finally, to give the correct offset and final scaling you must add a 10 kΩ surface-
mount resistor in position R33 (On the back of the PCB, near the 25 connector)
These three resistors, will scale a +5 V to –5 V input to be seen at the logger as a
signal varying from 0 V to +2.5 V. The scaling function of PicoLog can convert
this reduced and offset range back to the original voltage reading by multiplying
the raw value by 4.02 and subtracting 5 V. To compensate for any residual
errors, the actual scaling offset and multiplier can be slightly varied to give exact
readings by doing a simple calibration, if required.
If required, the first four channels can be easily offset in this way. (Other
channels can too but the pull-up resistor has to be added externally to the PCB as
there are only four resistor positions available, R33, 34, 35 and 36.)
4.
Start PicoLog and set-up your sample rates as required. In PicoLog1216
measurements, click to Add the first channel. In the Name box, type Volts.
5.
Leave the Measurement as DC Volts and the Scan Time as it is.
6.
Now click Options. (You will be asked to save channel settings to continue.)
7.
Now click Scaling to go to the next dialog.
8.
In the Parameter scaling dialog, select Equation as the Scaling method and in the
text box, type:-
4.02*X-5
9.
Click OK to go back through the open boxes and channel 1 should now be scaled
from about +5 V to –5 V.
See the PicoLog manual for information on using PicoLog or click the Help button at any
stage in PicoLog.
See Related Documents for a link to the PicoLog 1000 Series User’s Guide.
When no input is connected, the DC voltage measured will float up to near +1.67 volts.
As long as the series resistance of the voltage you are feeding to the input of the PicoLog
terminal board is much lower than input impedance of the circuit above, the DC level is
pulled to the correct value upon connection.
15.3
Device-specific – ADC-20 and ADC-24
4–20 mA on the ADC-20 and ADC-24
All you have to do is to pass the 4–20 mA current output from the sensor through a
120 Ω resistor to generate a voltage varying from 480 mV to 2400 mV. The resistor is
best placed in parallel with the input terminals of the ADC-24 (from the channel input pin
to Analog Ground). The 0 V terminal of the +12 V to 24 V sensor supply is connected
also to the Analog Ground of the ADC-24 (and the sensor Com terminal if it is a 3-wire
sensor).