PicoLog Self-Help Guide
AR346-1
© Copyright 2014 Pico Technology Ltd.
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The sample rate quoted is per channel, not per cycle round all the channels. There is a
single A to D converter with a multiplexer to give multiple channels. The fastest that an
ADC-24 will sample all 16 channels is 960 ms with 60 ms conversion time.
Thermistor temperature sensing with the ADC-20/24
As an example we will use the following thermistor:-
https://www.elfa.se/elfa3~se_sv/elfa/init.do?item=60-290-84&toc=19509
and we will assume measurement in the range of 60 to 110 °C. The centre of the range
is 85 °C so we will optimise for that value.
The thermistor above has a resistance of 8376 Ω at 85 °C so Ra needs to match that for
a centre-scale reading of volts (+1.25 V) at this temperature. Use an 8.2 kΩ resistor as
the nearest standard value. Do NOT use a variable resistor as you will never know where
you are for calibration.
The ADC-24 has an input resistance of 1 MΩ and this is in parallel with the thermistor.
Thus the thermistor resistance value is reduced to (8.376 x 1000)/(8.376 + 1000) kΩ =
8306.4 kΩ (@85 °C). Assuming you are using the +2.5 V sensor supply of the ADC-24,
this is fed to the 8.2 kΩ resistor and thermistor in series and the voltage at their junction
will be (2.5 x 8306.4) / (8306.4 + 8200) volts = 1.2581 V.
Doing the same calculations from 60 °C to 110 °C gives values:-
60C = 1.7997V
65C = 1.6987V
70C = 1.5931V
75C = 1.4828V
80C = 1.3707V
85C = 1.2581V
90C = 1.1473V
95C = 1.0419V
100C = 0.9415V
105C = 0.8464V
110C = 0.7579V
Assuming you have connected up the sensor, resistor and supply as described above,
you just need to set the scaling for the channels in question. Below is a series of steps
describing how to set-up channel 1 for temperature measurement from 60 °C to 110 °C.
1.
Start PicoLog and set-up your sample rates as required.
2.
When you get to setting up Channel 1 (ADC-24 Channels), highlight Channel 1
and click Edit.
3.
Type Temp Chan 1 as the Name, leave Conversion time as 60 ms and select
±2500 mV as the Voltage range.
4.
Leave Differential input enable unchecked.
5.
Now click Options. (You might be asked to save the present setup. Click OK.)
6.
In the Parameter Options dialog box tick Use parameter Formatting, put °C in the
Units box. Field Width = 4, Decimal places = 1, Minimum value = 50, Maximum
value = 120. Now click on Scaling to go to the next dialog.