2.1 AI0 – AI7
Hardware
The LabJack U12 has 8 screw terminals for analog input signals. These can be configured
individually and on-the-fly as 8 single-ended channels, 4 differential channels, or combinations
in between. Each input has a 12-bit resolution and an input bias current of ±90
µ
A.
•
Single-Ended: The input range for a single-ended measurement is ±10 volts.
•
Differential channels can make use of the low noise precision PGA to provide gains up
to 20, giving an effective resolution greater than 16-bits. In differential mode, the voltage
of each AI with respect to ground must be between ±10 volts, but the range of voltage
difference between the 2 AI is a function of gain (G) as follows:
G=1
±20
volts
G=2
±10
volts
G=4
±5
volts
G=5
±4
volts
G=8
±2.5
volts
G=10
±2 volts
G=16
±1.25 volts
G=20
±1 volt
The reason the range is ±20 volts at G=1 is that, for example, AI0 could be +10 volts and AI1
could be -10 volts giving a difference of +20 volts, or AI0 could be -10 volts and AI1 could be
+10 volts giving a difference of -20 volts.
Figure 2-2 shows a typical single-ended connection measuring the voltage of a battery. This
same measurement could also be performed with a differential connection to allow the use of
the PGA. In general, any single-ended measurement can be performed using a differential
channel by connecting the voltage to an even-numbered analog input, and grounding the
associated odd-numbered analog input (as shown by the dashed connection to AI1 in Figure 2-
2).
Figure 2-2. Single-ended measurement.
Figure 2-3 shows a typical differential connection measuring the voltage across a current shunt.
A differential connection is required when neither leg of the shunt is at ground potential. Make
sure that the voltage of both AI0 an AI1 with respect to ground is within ±10 volts. For instance,
if the source (Vs) shown in Figure 2-3 is 120 VAC, the difference between AI0 and AI1 might be
small, but the voltage from both AI0 and AI1 to ground will have a maximum value near 170
volts, and will seriously damage the LabJack.