Appendix A - Specifications [U3 Datasheet] | LabJack
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Appendix A - Specifications [U3 Datasheet]

Table A-1. Specifications at 25 degrees C and Vusb/Vext = 5.0V, except where noted.

Parameter Conditions Min Typical Max Units
USB Cable Length       5 meters
Supply Voltage   4 5 5.25 volts
Supply Current (1) Hardware V1.21+   50   mA
Operating Temperature   -40   85 °C
Clock Error -40 to 85 °C     1.5 %
Typ. Command Execution Time (2) USB high-high 0.6     ms
  USB other 4     ms
VS Outputs          
Typical Voltage (3) Self-Powered 4.75 5 5.25 volts
  Bus-Powered 4 5 5.25  
Maximum Currrent (3) Self-Powered   450   mA
  Bus-Powered   50   mA
(1) Typical current drawn by the U3 itself, not including any user connections.     
(2) Total typical time to execute a single Feedback function with no analog inputs. Measured by timing a Windows application that performs 1000 calls to the Feedback function. See Section 3.1 for more timing information.     
(3) These specifications are related to the power provided by the host/hub. Self- and bus-powered describes the host/hub, not the U3. Self-powered would apply to USB hubs with a power supply, all known desktop computer USB hosts, and some notebook computer USB hosts. An example of bus-powered would be a hub with no power supply, or many PDA ports. The current rating is the maximum current that should be sources through the U3 and out of the Vs terminals.     
Parameter Conditions Min Typical Max Units
Analog Inputs          
Typical input Range (4) Single-Ended, LV 0   2.44 volts
  Differential, LV -2.44   2.44 volts
  Special, LV 0   3.6 volts
  Single-Ended, HV -10.3   10.3 volts
  Special, HV -10.3   20.1 volts
Max AIN Voltage versus GND (5) Valid Readings, LV -0.3   3.6 volts
  Valid Readings, HV -12.8   20.1 volts
Max AIN Voltage versus GND (6) No Damage, FIO -10   10 volts
  No Damage, EIO -6   6 volts
  No Damage, HV -40   40 volts
Input Impedance (7) LV   40  
  HV   1.3  
Source Impedance (7) Long Settling Off, LV     10
  Long Settling On, LV     200
  Long Settling Off, HV     1
  Long Settling On, HV     1
Resolution All Ranges   12   bits
  Single-Ended, LV, 0-2.44   0.6   mV
  Differential, LV, ±2.44   1.2   mV
  Special, LV, 0-3.6   1.2   mV
  Single-Ended, HV, ±10   5.0   mV
  Special, HV, -10 to +20   10.0   mV
Integral Linearity Error     ±0.05   % FS
Differential Linearity Error     ±1   counts
Absolute Accuracy (8) Single-Ended %   ±0.13   % FS
  Single-Ended LV volts   ±3.2   mV
  Single-Ended HV volts   ±26.8   mV
  Differential %   ±0.25   % FS
  Differential LV volts   ±6.4   mV
  Differential HV volts   N/A    
  Special 0-3.6 %   ±0.25   % FS
  Special LV volts   ±6.4   mV
  Special HV volts   ±53.6   mV
Temperature Drift     15   ppm/°C
Noise (Peak-To-Peak) (9) Quick Sample Off   ±1   counts
  Quick Sample On   ±2   counts
Effective Resolution (RMS) (10) Quick Sample Off   >12   bits
Noise-Free Resolution (9) Quick Sample Off   11   bits
Command/Response Speed See Section 3.1        
Stream Performance See Section 3.2        
* LV specs refer to low voltage analog inputs which are available on the U3-LV and U3-HV.  HV specs refer to high voltage analog inputs which are available on the U3-HV only.
(4) Note that these are typical input ranges.  The actual minimum on the low voltage inputs might not go all the way to 0.0 as discussed in Section  These are with DAC1 disabled on hardware version < 1.30.
(5) This is the maximum voltage on any AIN pin compared to ground for valid measurements. Note that a differential channel has a minimum voltage of -2.44 volts, meaning that the positive channel can be 2.44 volts less than the negative channel, but no low-voltage AIN pin can go more than 0.3 volts below ground.     
(6) Maximum voltage, compared to ground, to avoid damage to the device. Protection level is the same whether the device is powered or not.     
(7) The low-voltage analog inputs essentially connect directly to a SAR ADC on the U3, presenting a capacitive load to the signal source. The high-voltage inputs connect first to a resistive level-shifter/divider. The key specification in both cases is the maximum source impedance. As long as the source impedance is not over this value, there will be no substantial errors due to impedance problems.     
(8) Absolute error includes INL, DNL, and all other sources of internal error at 25 C and VS=5.0V. To equate the percentage to voltage, multiply the full voltage span by the percentage.  For a single-ended low voltage input using the normal range the span is about 2.4 volts, so 2.4 * 0.0013 gives ±0.003 volts. For a single-ended high voltage input using the normal range the span is about 20 volts, so 20 * 0.0013 gives ±0.026 volts. Differential readings are not calibrated on high voltage channels.
(9) Measurements taken with AIN connected to a 2.048 reference (REF191 from Analog Devices) or GND. All "counts" data are aligned as 12-bit values. Noise-free data is determined by taking 128 readings and subtracting the minimum value from the maximum value.     
(10) Effective (RMS) data is determined from the standard deviation of 128 readings. In other words, this data represents most readings, whereas noise-free data represents all readings.     
Parameter Conditions Min Typical Max Units
Analog Outputs (DAC)          
Nominal Output Range (11) No Load 0.04   4.95 volts
  @ ±2.5 mA 0.225   4.775 volts
Resolution     10   bits
Absolute Accuracy 5% to 95% FS   ±5   % FS
Integral Linearity Error     ±1   counts
Differential Linearity Error     ±1   counts
Max Output Current (12) @ 2.0V   30   mA
Error Due To Loading (12) @ 100 µA   0.1   %
  @ 1 mA   1   %
Source Impedance (12)     50   Ω
Short Circuit Current (12,13) 5V to GND   50   mA
Cutoff Frequency (14) -3 dB   16   Hz
Time Constant (14)     10   ms
Digital I/O, Timers, Counters          
Low Level Input Voltage   -0.3   0.8 volts
Hight Level Input Voltage   2   5.8 volts
Maximum Input Voltage (15) FIO -10   10 volts
  EIO/CIO -6   6 volts
Output Low Voltage (16) No Load   0   volts
--- FIO Sinking 1 mA   0.55   volts
--- EIO/CIO Sinking 1 mA   0.18   volts
--- EIO/CIO Sinking 5 mA   0.9   volts
Output High Voltage (16) No Load   3.3   volts
--- FIO Sourcing 1 mA   2.75   volts
--- EIO/CIO Sourcing 1 mA   3.12   volts
--- EIO/CIO Sourcing 5 mA   2.4   volts
Short Circuit Current (16) FIO   6   mA
  EIO/CIO   18   mA
Input Impedance Pull-up to 3.3V   100  
Series Impedance (16) FIO   550   Ω
  EIO/CIO   180   Ω
Counter Input Frequency (17) Hardware V1.21+     8 MHz
Input Timer Total Edge Rate (18) No Stream, V1.21+     30000 edges/s
  While Streaming     7000 edges/s
(11) Maximum and minimum analog output voltage is limited by the supply voltages (Vs and GND). The specifications assume Vs is 5.0 volts. Also, the ability of the DAC output buffer to driver voltages close to the power rails, decreases with increasing output current, but in most applications the output is not sinking/sourcing much current as the output voltage approaches GND.     
(12) If the output is set to 3.5 volts and sourcing 30 mA, there will be about 2.0 volts at the DAC pin due to the 50 ohms of series impedance. Each DAC output is driven by a channel on an AD8544 op-amp, powered by VS & GND, and then goes through protection circuitry that includes 50 ohms of series impedance. The max output current is determined by 3 main factors: short circuit current, ability of AD8544 to sink/source near power rails (Figure 22 of AD8544 datasheet), and the 50 ohms of series impedance.
(13) Continuous short circuit will not cause damage.     
(14) The DAC outputs are creating by filtering PWM signals, and the 2nd order 16 Hz output filter works great for the default PWM frequency of 732 Hz, but with lower frequency timer clocks the DAC outputs will be noisier.  See Section 2.7 for more details.  Time constant is the time it take for the output to settle 63% of the way towards a new value.
(15) Maximum voltage to avoid damage to the device. Protection works whether the device is powered or not, but continuous voltages over 5.8 volts or less than -0.3 volts are not recommended when the U3 is unpowered, as the voltage will attempt to supply operating power to the U3 possible causing poor start-up behavior.     
(16) These specifications provide the answer to the question: "How much current can the digital I/O sink or source?". For instance, if EIO0 is configured as output-high and shorted to ground, the current sourced by EIO0 into ground will be about 18 mA (3.3/180). If connected to a load that draws 5 mA, EIO0 can provide that current but the voltage will droop to about 2.4 volts instead of the nominal 3.3 volts. If connected to a 180 ohm load to ground, the resulting voltage and current will be about 1.65 volts @ 9 mA.     
(17) Hardware counters. 0 to 3.3 volt square wave. Limit 2 MHz with older hardware versions.     
(18) To avoid missing edges, keep the total number of applicable edges on all applicable timers below this limit. See Section 2.9 for more information. Limit 10000 with older hardware versions.     


if i use a digital to analog converter....the signal will be alternating.....where can i find a bridge rectifier that supports the 3.3 V digital output

The DACs on the U3 output ~0-5 volt voltages, which are DC if you are not changing the output.  I am really not sure what you are asking about.


ok.....let me clarify.....i am planning to utilize the digital Outputs along with the Analog Outputs, since the control system i am designing requires analog DC signals. Since the 2 dedicated analog outputs area already DC. then i have to focus on the digital signal output. The digital output is ~0-3.3V on high.i plan to convert that signal to analog. the Digital to anaog converter i found is,C1,C1155,C1005,C1156,P85545#descriptionSection. i am new to electrical control systems, however, i think this converts to alternating analog signals..or probably i am mistaken???

thank you for the information  on the LJTick........The analog signals that i require should be DC, since I am planning to use the signal to operate a transistor switch. So could I utilize the LJ tick-DAC system with the current U3-LV labjack to convert the signals from the Flexible 16 I/O digital to a direct (not alternating) output analog signal.........Regards....THank you for your help thus far.....

The U3 has 2 DACs built-in.  If you need more you can add LJTick-DACs to the U3.

I am still confused about what you are doing, though.  Usually when someone is controlling a transistor they just use a digital output and set it high or low to turn the transistor on and off.  Are you trying to do something different?


Ok......i wasn't aware that digital signals could be used to turn on and off a transistor...Overview of the system i am designing. I have a 12V Dc Solenoid valve i wish to operate. I am going to operate the valve on an independent power supply. I am planning to use the transistor switch to turn on and turn off (open and close) the valve. When i read some info on transistor switches...i was of the impression that voltage or current analog signals can be used to saturate the transistor, since a resistor is used to acquire the base current and voltage...I am new to these systems, so maybe i am on the wrong track. That is why i was concerned about the DAC's on the labjack. Since i will be operating more than two valves. I would utilize the digital outputs and the LJtick in order to convert the digital signals to a 3.3V analog signal in order to saturate the transistor switch......Could the digital output be used to turn the transistor switch on and off for this system??


THank you again

To control a transistor which is controlling a 12V load, as you describe, you will use simple digital I/O on the U3.  See Section of the U3 User's Guide.  I think one option you should definitely consider is using the LJTick-RelayDriver.  This is controlled by digital I/O, and can in turn control your 12V load up to 200 mA.  Other options are the RB12 board with appropriate modules, controlling discrete SSRs, or transistors.  Rather than a discrete transistor, though, I would consider using the ULN2003A chip from TI. the relay configuration can be used to operate the transistor. I will be using multiple transistors as switches to contr4ol multiple 12V  solenoid valves. Can the Vs on the labjack support all of them???i am planning to use a valve for each digital ouput. Or will the LJ Tick Relay be recommended for such a setup.

It depends how much current you need from VS.  From Appendix A you can see that you can typically draw 450 or 50 mA.

In this case, though, it does not sound like you are really drawing any current from VS.  Rather you are going to use a digital output to control a transistor which in turn controls some other power supply you want to control.  So you will just draw a few mA from each digital output which turns on each transistor.

yes that is the my use the transistors as a switch to turn on a 12V supply/..... ok..............thank you for help....i will make further queries when necessary

which is the best program would u recommend to use to operate the labjack for a control program and data acquisition?

Consider LJLogUD, LJStreamUD, or DAQFactory.  See the "software options" section on the U3 support homepage:

Ok Thank you very much

I am trying to calculate the accuracy of my measurements made with a LabJack device. An important factor in this is any difference between my reference voltage on the signal source and the internal reference of the LabJack. I think I looked everywhere where there is a chance to find a specification for this value but I can't find anything. All I find is values like 2.44V in the specification (not specifically referring to the A/D reference, only to the input range) and then in some description it says things like 'normally about 0-2.44V' and at other places it says something about 2.4V and I found a user comment talking about 2.47V. 

Can you please provide the actual specification on this value in a clear and unambiguous way? Thank you.

The simple answer to your question is that the internal Vref is specified by the chip manufacturer to be 2.38 to 2.50 volts.

I can't think of a situation where that is useful information, however.  The only way to use that would be to get back raw binary readings and only assume an accuracy of +/-2.5% (2.44 +/- 0.06).

Rather, we do a calibration of each device and provide calibrated voltages.  On a single-ended low-voltage channel on a U3 each device is calibrated to provide readings within +/-0.13% full-scale of the actual value, so +/-3 mV.  This includes error due to Vref and all other sources.

What sort of measurement or circuit do you have?  If you are trying to do a ratiometric measurement, such as a potentiometer excited by some Vex, I suggest you measure Vex and the circuit output, knowing that each of those measurements is +/-0.13% FS.

Thanks for the answer, that's clear and it helps. I have to deal with a variety of measurements and with dozens of channels (there are several LabJacks at work) and come up with error estimates on the results our setup is producing. For example one of the sensors is a relative humidity sensor with a full scale output of 1V. A reference voltage is not available from the chip (same problem in some other cases as well), so your suggestion about measuring this as well does not work. If I want to figure the error of the sensor reading, taking into account all the factors, I need to know more than just 0.13% FS. I actually need to know what FS is. So far I have been guessing that it is the maximum specified input voltage for the channel in question, but the absence of an actual clear statement to that effect and somewhat varying statements at different places of your documentation left me a bit uneasy about this.

While we are at the topic, there is no specification at all for the high voltage inputs. It looks like you are using 1% components around the amplifier circuit. If that is not calibrated I would expect an error on the order of about 2% on these channels.

I understand.  Full scale for a low-voltage channel is nominally 2.44 volts, but could be as high as 2.50 volts if you want the worst case possible error (0.0013*2.50 = 0.00325 V).  You can check the full scale of your U3 by connecting VS to an input and see what it reads.

The high-voltage channels use 0.1% components.  Not because we need the high accuracy (we calibrate), but rather for the good tempco.  You also have to consider offset and gain error from the op-amp, but this also is part of the calibration we do.  The accuracy specs above apply to high-voltage and low-voltage channels.

The nominal full-scale span on a high-voltage channel is 20.6 volts (for the normal single-ended range).  To figure out th e worst case we have to consider Vref could be 2.5% high, the 0.1% scaling components, and worst case errors from the op-amp.  Perhaps connect +12 and -12 volts to see what your limits are for each high-voltage channel.

What is the input capasitance of the AINs? Many opamps accepts 100p.


Low-voltage AIN:  While converting you go through ~5k of resistance then hit a 5pF sampling capacitor.  When not converting you just have parasitic capacitance which is not specified.

High-voltage AIN:  At all times you go through a 1M resistor and then to an op-amp input which is specified to have an input capacitance of 8pF.

Can we capture open collector signals with the digital inputs ports ? Are there pull-ups at the level of the digital inputs ?

Yes, there is a 100k pull-up to 3.3V. The lines are 5V tolerant, so you can add pull-ups to 5V if you need a faster rise time.   

What is the temperature stability of the U3 DAC?

I am using a U3-HV to measure 0-10VDC signals from transmitters, actuators, etc.  Are the FIO inputs able to read up to 10VDC or are only the AINs able to read up to 10V and the FIOs only go to 2.44?

Only AIN0-AIN3 have the +/-10V range.  See Section 2.6 and Section

I'd like to use the FIOs and EIOs of a U3-LV to measure some voltages of ca. 1V in a differntial manner, i.e., I want to calculate my voltage as the difference of FIO3 and FIO2, for example. In the specifications, I can see the maximum voltage to GND is 10 and 6V for the FIOs and EIOs, respectively. However, my voltage source is not grounded, I think. It is a fuel cell stack with a maximum voltage of 60V and I want to measure the single cell voltages that add up to this stack voltage. Can I just connect the bipolar plates' potentials to the FIOs and EIOs or do I need a voltage divider?


See the Differential Readings App Note.  You can't leave all the signals floating ... you are going to have to make a common reference.  Best case you connect the middle of your stack to GND, and thus the U3 will see +/-30 volts of common-mode, but that is still way to much.  So I think you will have to use voltage dividers.

Thank you for the clarification. The way it is done right now is the incrementing stack voltages (cell 1, cells 1+2, cells 1+2+3, ...) are passed through voltage dividers to the U3 inputs as single ended and one end of the stack is connected to the U3 GND. The problem with this configuration is noise, which is increased by the multiplication factor of the dividers. Also, the accuracy of the resulting cells voltages is bad for some reason despite a calibration has been done. Is there a workaround? Thanks.

Sticking with voltage dividers, the likely route to improvement is higher resolution analog inputs which you would find on the U6 or T7 (or -Pro versions).  I don't see an inherent reason for accuracy problems, so we should be able to help you resolve that.

Without voltage dividers, the only way to do a direct differential measurement would be with channel-to-channel isolation.  You could look at 5B signal conditioning modules, but you are looking at $100+ per channel.

Hello, I am using the U3-LV for a final project and the one they gave me only has 2.47 volts at VS and there is no documentation on your site (that is easy to find anyway) that shows any way to adjust this output; so I was assuming this was supposed to be a fixed output of 5 volts.  Seeing that I get about half this expected voltage I was guessing that I have a defective unit, is this correct?

My professor has not replied yet and the project is due on Tuesday the 9th of June so a replacement is only possible at the time of demo from the professor.  However if there was some burried method of changing this VS output or a way of adjusting it please let me know as soon as possible.  Otherwise I will be writing code for a device that does not work properly and just hoping the code will function on a different device when it comes time for a grade.


I wish you had everything on a single page that allowed quick searching with ctrl-f because the page search only brings up comments from users and does not appear to search through actual code samples or documentations.

Thanks again.

Mike Joynt

To get the entire U3 User's Guide in a single page, go to the first page and click "Print all".  See that tip and others on the first page of the U3 User's Guide.  In my opinion, though, a better solution is to use the search box on any page of our site.  If you search "vs u3", the first hit is Section 2.4 which is the documentation for the VS terminals.  Our main search page had more good tips for how to search our site.

VS is the USB power supply, and per USB spec should be 4.75-5.25 volts.  If it is not, then either the USB port is not providing the proper power supply or your U3 is damaged and pulling too much current which is causing the supply to droop.

Is your DMM working right?  Measure some known voltage, such as a AA battery.

To test this further there should be no connections to the U3 except for the USB cable and the DMM.  Make sure the DMM leads are securely clamped in VS and GND terminals.  Just touching different areas of the screw terminals is not always valid.

Remove all connections, including USB, and use the DMM to measure the resistance from VS to GND.  This typically measures >100k.  If it is too low, and you want us to look at the unit, contact [email protected] for an RMA number.

Try different USB ports, USB hubs, and USB cables.

Josefina Meirovich's picture

Hello, I'm using de U3-HV for a final project and I need to plot 12 signals wich go form -0.1V up to +2.3 V. I'm having problems trying to measure below -0.03V Why could it be?
As I read, I uderstand that I can measure signals form -0.3V up to 2.44V (using any channel) without damaging the divice. Is this correct?
Thank you!

labjack support's picture

For a single-ended low-voltage AIN on the U3, the typical range is 0 to 2.44 volts.  With a differential input you can read down to about -0.3 volts:

So you can jumper EIO7 to GND, for example, and then do a differential reading of the other 11 low-voltage channels versus EIO7.  To get to 12 channels you will have to use 1 of your high voltage analog inputs since you have the -HV.

Josefina Meirovich's picture

Thank you very much for your answer!

Josefina Meirovich's picture

Hello again! As I wrote before, I'm working with a U3-HV. I need to plot 12 different signals form -0.1 to +2.3V. I'm using the 4 high-voltage analog inputs and 8 low-voltage channels versus EIO7 (which I jumper to GND). When I try to measure only one signal with AIN0, I jumper the other channels to GND, but when I plot I see the signal repeated In the low-voltage channels also (not in the AIN1, AIN2 and AIN3 plots), I don't know what may be wrong, do you have any clue?. Thank you in advance!

labjack support's picture

I'm not sure if you are describing a software problem or a signal problem.  I suggest you email a screenshot to [email protected] so we can help further:

MoonMonkey's picture

Hey there,

I just wanted to ask what the power-off charaterstics are when I pull out the USB connection? So in what is the power off timing in us/ms?


labjack support's picture

The time from power-loss to brown-out is ~600 µs.