Appendix A - Specifications
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Export allSpecifications at 25 degrees C and Vusb/Vext = 5.0V, except where noted.
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LabJack
Measurement & Automation Simplified
Specifications at 25 degrees C and Vusb/Vext = 5.0V, except where noted.
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Comments
#1
Can we capture open collector signals with the digital inputs ports ? Are there pull-ups at the level of the digital inputs ?
#2
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.
#3
What is the input capasitance of the AINs? Many opamps accepts 100p.
#4
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.
#5
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.
#6
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.
#7
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.
#8
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.
#9
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
#10
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.
#11
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 http://www.linear.com/pc/productDetail.jsp?navId=H0,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???
#12
That DAC chip from Linear is just like the DACs already on the U3 and like you would get from the LJTick-DAC. What sort of output signal do you need? I gather you need a varying AC waveform of some sort? Is it periodic (repeating), what is the frequency, and what is the max/min voltage?
#13
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.....
#14
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?
#15
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
#16
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 2.8.1.4 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.
#17
ok.....so 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.
#18
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.
#19
yes that is the my intention...to use the transistors as a switch to turn on a 12V supply/..... ok..............thank you for help....i will make further queries when necessary
#20
which is the best program would u recommend to use to operate the labjack for a control program and data acquisition?
#21
Consider LJLogUD, LJStreamUD, or DAQFactory. See the "software options" section on the U3 support homepage:
http://labjack.com/support/u3
#22
Ok Thank you very much