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5.
Measurement Configuration Recommendations
Analog measurement Parameters
Channel, Gain, Buffering
Internal to the module, all analog measurements are differential (just like a voltmeter has two leads to make a
differential measurement between them). The module allows specification of which input channel is positive
(Red lead) and which is negative (black lead). In addition to the four analog input terminals, the unit also
allows specification of the Ground and Vref terminals as an input channel. This allows all four channels to be
connected to signals of interest, if the measurements are single ended (referenced to ground). Unlike a
handheld voltmeter that is powered from an internal battery, the module is powered by the SDI-12
connection, and is therefore not floating. What this means to the user is that the inputs need to be referenced
to the same supply as the module. This is generally not an issue, since powered sensors are usually powered
by the module, or the same supply as the module. For floating sensors this means that a reference connection
must be made so the sensor will not float with respect to the SDI-12 AnalogPlus module. For a low level high
impedance sensor like a pyranometer, Vref is the recommended reference point. For a high level low
impedance sensor like 4-20 mA output, Ground would be the recommended reference point.
All inputs to the SDI-12 AnalogPlus module must be in the 0 to 5V range referenced to the module‟s ground.
The unit can measure voltages in this range when it is in the unbuffered mode. In the unbuffered mode, the
input impedance varies with the gain utilized. The table below lists the typical input impedance for the
different Gains and input ranges.
Gain
Input range
Input Impedance (typical)
1
0-5V, + 2.5V
4 M ohm
2
+1.25V
2 M ohm
4
+ 625 mV
1 M ohm
8
+ 312 mV
500 k ohm
16
+ 156 mV
250 k ohm
32
+ 78 mV
125 k ohm
64
+ 39 mV
60 k ohm
128
+ 19 mV
30 k ohm
For low impedance voltage output sensors, the loading due to the input impedance is insignificant. Even for a
1000 ohm output impedance sensor, the error is less than 0.03% FS on the gain 1 setting. For high
impedance sensors, a high impedance input buffer can be added to the input. This is done by specifying a
“Buffered” measurement. The high impedance input buffer accepts signals in the 50 mV to 3.5 V range. For
this reason it should not be used for sensors with a 0 to 5V output. It is also for this reason that Vref (2.5
Volts) is recommended as the reference point for a low output, high impedance, floating sensor such as a
pyranometer. In such a case, the negative output of the sensor would be connected to Vref and the positive
output would be connected to the analog input channel.
Averaging / Filtering
The SDI-12 analog module has selectable filtering of the input. All settings have 50 and 60 Hz rejection. For
most uses, the 15 Hz setting (averaging times less than 0.09) is adequate and is the default. The measurement
times can be increased to add more filtering. Specifying a time greater than 0.19 seconds results in the most
filtering and will increase the effective resolution by about one bit. (Noise floor is lower.)
