Figure 2-4 shows a single-ended connection used to measure the output voltage of a typical voltage-divider circuit. The voltage
divider circuit is a simple way to convert a varying resistance (thermistor, photoresistor, potentiometer, etc.) to a varying voltage.
With nothing connected to Va, the value of the unknown resistance, R2, can be calculated as:
R2 = Va*R1 / (Vs-Va)
where Vs is the supply voltage (+5V in Figure 2-4).
When Va is connected to AI0, as shown in Figure 2-4, the input bias current of the LabJack affects the voltage divider circuit, and if
the resistance of R1 and R2 is too large, this effect must be accounted for or eliminated. This is true for any signal with too high of
a source impedance.
All measuring devices have maximum analog input bias currents that very from picoamps to milliamps. The input bias current of
the LabJack U12’s analog inputs varies from +70 to -94 microamps (µA). This is similar to an input impedance of about 100 kΩ,
but because the current is nonzero at 0 volts, it is better to model the analog input as a current sink obeying the following rule:
Iin = 8.181*Va - 11.67 µA
Figure 2-4. Single-ended measurement with voltage divider circuit.
Because the input bias current is known, as a function of input voltage, the simple voltage divider equation can be modified as
follows to account for input bias current:
R2 = Va / [((Vs-Va)/R1) – (8.181µ * Va) + 11.67µ]
As an alternative to the equation above, Va can be buffered by a single-supply rail-to-rail operational amplifier, and the original
simple voltage divider equation can be used. This solution works for any single-ended signal which stays between 0 and +5 volts.
Some op-amp choices are:
TLV2462
LMC6482
MAX4166
Software
Readings from the analog inputs are returned by the functions EAnalogIn, AISample, AIBurst, and AIStreamRead.
EAnalogIn is a simplified (E is for easy) function that returns a single reading from 1 analog input channel. Execution time is up to
20 ms.
AISample returns a single reading of 1-4 channels, and takes up to 20 ms to execute, providing a maximum date rate of about 50
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