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2.6.1 - Channel Numbers [U3 Datasheet]

The LabJack U3 has up to 16 external analog inputs, plus a few internal channels. The low-level functions specify a positive and negative channel for each analog input conversion. With the LabJackUD driver, the IOType LJ_ioGET_AIN is used for single-ended channels only, and thus the negative channel is internally set to 31. There is an additional IOType called LJ_ioGET_AIN_DIFF that allows the user to specify the positive and negative channel.

Table 2.6.1-1. Positive Channel Numbers

Positive Channel #







Temp Sensor




DI, Timers, Counters

Positive channel 31 puts the internal Vreg (~3.3 volts) on the positive input of the ADC. See Section 2.6.4 for information about the internal temperature sensor.

Table 2.6.1-2. Negative Channel Numbers

Negative Channel #








31 or 199


32 (UD only)

Special 0-3.6 or -10/+20 (HV AIN Only)

If the negative channel is set to anything besides 31/199, the U3 does a differential conversion and returns a pseudobipolar value. If the negative channel is set to 31/199, the U3 does a single-ended conversion and returns a unipolar value. Channel 30 puts the internal voltage reference Vref (~2.44 volts) on the negative input of the ADC.

Channel 32 is a special negative channel supported by the LabJack UD driver. When used, the driver will actually pass 30 as the negative channel to the U3, and when the result is returned the driver adds Vref to the value. For a low-voltage analog input this results in a full span on the positive channel of about 0 to 4.88 volts (versus ground), but since the voltage on any analog input cannot exceed 3.6 volts, only 75% of the converter’s range is used and the span is about 0 to 3.6 volts. For a high-voltage analog input, channel 32 (special range) results in a span of about -10 to +20 volts.

In the U3 examples that accompany the Exodriver, u3.c also supports channel 32 in calls to eAIN().

Channel 32 is also supported in LabJackPython

For the four high-voltage channels on the U3-HV, the special channel negative channel also puts Vref on the negative. This results in an overall range of about -10 to +20 volts on the positive input.

Channels 193 to 241 are special analog input channel numbers that read digital inputs, timers or counters.  See Section 3.2.1 for more information.

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