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Frequently Asked Support Questions (FASQ)

AIN (Analog Inputs)

A1. My analog input is stuck on some wrong value.

A2. My analog input is returning varying values but not the right value.

A3. My readings are too noisy.

A4. How do I measure temperature using a thermocouple, RTD, thermistor or other?

A5. My thermocouple readings are wrong.

  • Determine if the error is from the cold junction sensor, the A/D conversion, or the signal itself.

  • If the cold junction reading is off by a few degrees that error will show up in your calculated thermocouple temperature.

  • Remove the thermocouple, and instead jumper the analog input (or both analog inputs for differential) to GND. The thermocouple voltage is now 0.0 volts which is saying both ends of the thermocouple are at the same temperature and thus the thermocouple temperature you calculate will be equal to the cold junction temperature.

  • Remove the thermocouple from the system under test and instead just set it next to the LabJack so the thermocouple is measuring air temperature near the LabJack. If the terminals you are connecting to are at air temperature (e.g. CB37 terminals), the expected voltage is 0.0 volts. If the terminals you are connecting to are warmer than air temperature (e.g. T7 terminals), the expected voltage is a negative value dependent on the temperature difference.

  • See Thermocouple Complications.

  • See Thermocouple Troubleshooting Tips.

A6. I need to monitor a 4-20 mA current loop signal or measure some other current.

  • LabJack analog inputs measure voltage. The LJTick-CurrentShunt is an easy way to convert 4-20 mA to voltage.  See the Measuring Current App Note.

A7. Should I use differential?

A8. Problem with differential analog inputs.

A9. How do I read from a load cell?

A10. How do I measure pressure?

A11. Can I measure AC or DC?

  • Yes, both. When you ask for 1 sample from the LabJack you get the voltage at that moment.  If the signal is DC, you expect multiple samples over time to return the same value.  If the signal is AC, the samples are taken at different points of the waveform and thus will return different values.

  • If you want to acquire the waveform for an AC signal, you simply acquire multiple samples over time, which is often done using stream mode.  Then if wanted you can do math on the waveform to determine parameters such as max/min, RMS, or frequency.

  • On T-series devices you can use the AIN-EF system to acquire multiple samples over time, do some math, and return various values that describe the multiple samples (RMS, Peak-to-Peak, PeriodAverage, Max/Min).

  • See the Measuring 120/240 VAC App Note.

A12. What is the max data rate?

  • We have many details for our different device sample rates.  U12, U3, U6, UE9, T-Series.

  • For stream mode we usually specify the max sample rate. For the max scan rate, divide the max sample rate by the number of channels: SampleRate = ScanRate * NumChannels.

  • Extended channels on the Mux80 act just like normal channels.  The max sample rate of the LabJack is still the same, you just use a bigger number for NumChannels in the above equation.

DAC (Analog Outputs, D/A Converter)

A1. Does LabJack have any options for 4-20mA output?

A2. My U3 DAC output does not seem able to update faster than a few Hz?

  • The U3 DAC outputs are implemented using filtered PWM signals. There is a -3dB cutoff frequency at 16Hz that prevents faster update rates.

A3. Can I generate a waveform using the LabJack DAC outputs?

Digital I/O

A1. What logic levels do LabJack digital I/O use?

  • All LabJack digital inputs are TTL (~5V logic) compatible.

  • Most LabJack digital outputs output 3.3V for a logic high and 0V for logic low. There is one exception with the U12, which outputs 5V for a logic high.

A2. Can LabJack digital I/O work with 24VDC input signals?

  • Not directly as 24V is outside of the typical DI input range, but there are available interface options.

    • 24VDC PNP and driven/push-pull outputs can be measured by creating a voltage divider. One accessory we sell that could divide the signal is the LJTick-Divider-5.

    • 24VDC NPN signals switch between open and low, so they should not result in 24VDC on the LabJack digital I/O and can often be connected directly to the LabJack digital I/O. See our Open-Collector App Note.

A3. Can X signal be interfaced with the LabJack digital I/O?

  • Most likely, and we probably have an application note outlining our recommendations. See our Digital I/O App Notes.

A4. Do LabJack devices support encoder inputs?

  • Yes, most of our devices have dedicated quadrature input measurement features, frequency/period measurement features, and also simple counters. See the Timers/Counters section of our datasheets for UD devices (U3/U6/UE9) and the DIO Extended Features documentation for T-Series devices.

A5. I need to measure RPM or flow. What is the best option?

  • We recommend looking for sensors with pulse outputs. A 5V or 3.3V driven signal output would interface best with our digital inputs, but NPN outputs can also be interfaced easily. See the resources in A2 above. RPM and flow can easily be measured using our device frequency/period measurement timer features or a simple counter. See the resources linked in A4 above.

A6. I need to control voltages outside of my device digital output range or relays.

Host Interface

Under construction!


What software is available to talk to a LabJack device?

What is LabJack software licensed under?


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