The following tables cover most common situations with the LJTIA, but for other cases there is an online tool available at analog.com:

http://www.analog.com/designtools/en/diamond/#difL=-0.015&difR=0.3&difSl=0.3&gain=11&l=2.5&pr=AD623&r=2.5&sl=2.5&tab=1&ty=1&vn=0&vp=5&vr=0.4

"+Vs" = VS (typically 5.0 volts), "-Vs" = 0, "Vref" = Voffset (0.4 or 1.25 volts), “Vcm” = Vcm, and “Vdiff” = Vin.  Our definitions of Vin, Vcm, and Voffset, can be found on the main page of the LJTIA datasheet.

The output amplifiers on the LJTIA have a range of about 0.01 volts to 4.5 volts (VS-0.5 actually).

The "Low" column on the left in the tables below is the minimum Vin.  Usually this is the Vin that results in the minimum Vout of 0.01 volts, but sometimes there are other limiting factors, so the "Low" column on the right shows the Vout corresponding to each Vin.

The "High 2.5V" and "High 4.5V" columns on the left are listing the upper limit of Vin that results in a Vout of 2.5 or 4.5 volts, although sometimes there other limiting factors and those Vout values cannot be achieved.  The columns on the right list the actual Vout that results from the Vin values on the left.

Example:  Assume Voffset of the LJTIA is set to 0.4V and gain of the LJTIA is set to x11.  Assume the Vin signals come from a bridge with 5 volt excitation, so Vcm is 2.5 volts.  According to the latest tool from analog.com, the signal voltage can go from about -0.018V to +0.309V.  Beyond those limits generates an error.  Note that the table below shows similar values of -0.035 to +0.327, which are not exactly the same likely because they come from an older version of the calculator.

Table A1.1. Voffset = 0.4V

Table A1.2. Voffset = 1.25V

The following tables use typical noise measurements with the LabJack U3 and UE9 to determine the noise-free and effective resolutions that can be expected with the LJTick-InAmp (LJTIA). The LJTIA was connected to an analog input on the LabJack and had IN+ shorted to IN- shorted to GND.

The counts of peak-to-peak noise were determined by collecting 128 points from the analog input and subtracting the minimum binary value from the maximum binary value. For the U3 these are based on 12-bit values, while for the UE9 these are based on 24-bit values.

The noise-free resolution is based on the peak-to-peak noise counts, and corresponds to the resolution where no variation would be seen.

The RMS noise counts is the standard deviation of the 128 collected binary values, and the effective resolution values are based on this RMS value. The effective resolution can be thought of as a specification met by most points, while the noise-free specifications are met by all points.

The “@LJ Inputs” values are in terms of the LabJack U3/UE9 analog input, which is the LJTIA output. Those values are divided by the LJTIA gain to determine the “@LJTIA Inputs” values, which are the resolutions that apply to the signal input to the LJTIA. For instance, a single-ended channel on the LabJack U3 with an LJTIA gain of 201 has a noise-free resolution of about 9 μV and an effective resolution of about 1.8 μV.

LabJack U3:

Table B.1. LJTIA Gain = 1-201 with a U3.

LabJack UE9 & UE9-Pro (LJTIA Gain = 1 & 11):

All "counts" data in the following UE9 tables are from 24-bit values. To equate to counts at a particular resolution (Res) use the formula counts/(2^(24-Res)). For instance, with the UE9 set to 12-bit resolution and the 0-5 volt range, there are 8192 counts of noise when looking at 24-bit values. To equate this to 12-bit data, we take 8192/(2^12), which equals 2 counts of noise when looking at 12-bit values.

Table B.2. LJTIA Gain = 1, Resolution 0-18 with a UE9

Table B.3. LJTIA Gain = 11, Resolution 0-18 with a UE9

LabJack UE9 & UE9-Pro (LJTIA Gain = 51 & 201):

All "counts" data in the following UE9 tables are from 24-bit values. To equate to counts at a particular resolution (Res) use the formula counts/(2^(24-Res)). For instance, with the UE9 set to 12-bit resolution and the 0-5 volt range, there are 8192 counts of noise when looking at 24-bit values. To equate this to 12-bit data, we take 8192/(2^12), which equals 2 counts of noise when looking at 12-bit values.

Table B.4. LJTIA Gain = 51, Resolution 0-18 with a UE9

Table B.5. LJTIA Gain = 201, Resolution 0-18 with a UE9

Appendix C - Thermocouples

Thermocouples are not particularly accurate or easy to use, but they are a very common way to measure temperature. Some applications with extreme temperatures or specific mechanical requirements might require thermocouples, but whenever possible consider instead using a silicon type temperature sensor such as the EI-1022 (-40 to +100 degrees C) or the very accurate and waterproof EI-1034 (-17 to +150 degrees C). Both of those probes are available from LabJack, and provide a high-level linear voltage output that connects directly to analog inputs.

There are three main issues when making measurements with a thermocouple:

1. Cold Junction Effects: The voltage generated at the LJTIA by the thermocouple is related to the difference in temperature of the end of the thermocouple and the LJTIA. In order to know the temperature at the end of the thermocouple, the temperature of the LJTIA must also be known. Most LabJacks have an internal temperature sensor, but even then for improved performance an external sensor (e.g. EI-1034 or LM34CAZ) might be needed. Once the cold junction temperature is known, it is easily handled in software.
2. Non-Linear Output: The output of a thermocouple is non-linear. NIST (nist.gov) provides tables and equations to convert a thermocouple voltage to a temperature. The LabJack UD driver provides a convenient function that uses the NIST equation to handle the conversion, and DAQFactory has built-in conversion functions of its own.
3. Small Output Voltage: The small output voltage of a thermocouple makes it difficult to get good temperature resolution. The LJTIA is used to amplify the thermocouple voltage before it is sent to an analog input.

The following diagram shows the typical connection of a thermocouple to the LJTIA:

The thermocouple is connected to IN+ (positive lead) and IN- (negative lead), and IN- is also connected to GND through a 10 kΩ resistor (meaning that Vcm=Vin/2).

Care must be taken when placing the negative thermocouple lead and resistor lead into the same screw-terminal, to make sure that both are solidly contacted. Sometimes the thermocouple wire is thicker than the resistor wire, making it easy for the resistor to lose contact. In such a case the LJTIA output will likely be stuck near 0 volts, Voffset, or 5 volts.

In most situations, a short can simply be used for the GND connection instead of a 10 kΩ resistor, but note that if a short is used the negative lead of the thermocouple is grounded at the LJTIA. This is not recommended when using multiple probes and when the probes could be contacting grounded metal somewhere in the system. The 10 kΩ resistor maintains some isolation between the negative thermocouple lead and ground, and yet provides a path for the LJTIA bias currents.

It is possible to install the 10 kΩ resistors (Digikey #P10KGCT) on the LJTIA PCB. For channel A, install the resistor on R8. For channel B, install the resistor on R23. Obviously this reduces the input impedance of the negative input of the LJTIA to 10 kΩ, which might not be desirable for some signals besides thermocouples.

The output offset of the LJTIA can be set to 0.4 volts or 1.25 volts. For the best accuracy, the actual system offset should be measured. If the end of the thermocouple is at the same temperature as the cold junction, the thermocouple voltage should be zero, so place the end of the thermocouple near the LJTIA and note the voltage measured by the analog input (should be near 0.4 volts or 1.25 volts). This is the actual system offset, and should be subtracted from further analog input readings before dividing by the gain to get the thermocouple voltage.

The UD driver for Windows has a convenient function that takes in thermocouple type, thermocouple voltage, and cold junction temperature, and returns the thermocouple temperature. The following pseudocode demonstrates a measurement:

//Read the amplified and offset thermocouple voltage from an analog input. 
eGet (lngHandle, LJ_ioGET_AIN, tcChannel, &valueAIN, 0); 
 
//Get the internal temperature reading (channel 133 on the UE9 or channel 30 on the U3). 
eGet (lngHandle, LJ_ioGET_AIN, internalTempChannel, &cjTempK, 0); 
 
//To get the thermocouple voltage we subtract the offset from the AIN voltage 
//and divide by the LJTIA gain. 
tcVolts = (valueAIN - offsetVoltage)/51; 
 
//Convert TC voltage to temperature. 
TCVoltsToTemp (LJ_ttK, tcVolts, cjTempK, &TCTempK); 

Go to labjack.com for thermocouple examples in C, VB, LabVIEW, and more. For DAQFactory examples go to daqexpress.com.

The following table shows the minimum and maximum allowable temperatures (from NIST) for some common thermocouple types. The table also shows the thermocouple voltages generated at those limits. Note that these extremes might not be allowed continuously, and thermocouple manufacturers will often provide a reduced range that is recommended for continuous use.

Table C1. Minimum and maximum temperature and voltage ranges

The following tables show the measurement range for different thermocouple types with different gain and offset settings for the LJTick-InAmp. Also shown is the resulting temperature resolution.

For example, from the LJTIA Signal Range Tables the max input voltage (for an output of 2.5 volts or less) is about 5.9 mV with a gain of 201, an offset of 0.4 volts, and a common mode voltage of Vin/2. From the NIST K-type thermocouple table the temperature corresponding to 5.9 mV is about 144 degrees C. The basic resolution of a K-type thermocouple is roughly 37 uV/degC, so since this example has a gain of 201 the resulting resolution is about 7.5 mV/degC. From the Resolution Tables in Appendix B, the U3 has a noise-free single-ended resolution of about 1.2 mV, so the noise-free temperature resolution would be about 0.16 degrees C. The UE9 has a noise-free resolution of about 94 uV (0-2.5 volts, 17-bit), so the noise-free temperature resolution would be about 0.013 degrees C. The UE9-Pro has a noise-free resolution of about 29 uV (0-5 volts, 18-bit), so the noise-free temperature resolution would be about 0.004 degrees C. These calculations assume no noise from the thermocouple signal itself.

Table C2. Temperature measurement ranges and resolution for LJTIA Gain 1-201, Voffset 0.4-1.25

Appendix D - Bridge Circuits Overview

A bridge circuit generally outputs a small differential voltage. The LJTick-InAmp provides the amplification needed to acquire such a signal, converts the differential signal to single-ended, and provides a 2.5 volt reference voltage (VREF) that can be used for excitation.

A common example would be a Wheatstone bridge made of 350 ohm strain gauges. If VREF/GND is used to provide the excitation voltage for the bridge, it will draw about 2.5/350 = 7 mA, and the common mode voltage (Vcm) of the differential signal will be about 1.25 volts. The outputs would be connected to IN+ and IN-. If Voffset is set to 1.25 volts and the gain is set to 201, the allowable input range for the LJTIA is –0.00616 to +0.00622 volts (with 0-2.5 volt output).

Testing Basic Operation

Following is a procedure for testing basic operation of the LJTick-InAmp.

For T-series devices, use the Register Matrix tab to read the analog input voltages from the LabJack and to also control the analog output.

For the U3/U6/UE9, use the test panel in LJControlPanel to read the analog input voltages from the LabJack and to also control the analog output.

Start by removing all signals from the LJTIA, and jumper all 4 inputs (INA+/INA-/INB+/INB-) to GND.  Turn off all switches on the LJTIA, except #5, to give you a gain of x1 and offset of 0.4 volts.  The output voltage of the LJTIA is Vout = (Vin * Gain) + Voffset, so you should see that both channels measured by the LabJack measure about 0.4 volts.

Leave INA- & INB- jumpered to GND, but change INA+ & INB+ so they are both jumpered to DAC0.  Set DAC0 to 1.5 volts, and the output should read about 1.5*1 + 0.4 = 1.9 volts.

Turn on switches 2 and 8 (in addition to 5), to give a gain of x11 and offset of 0.4 volts.  Set DAC0 to 0.1 volts, and the output should read about 0.1*11 + 0.4 = 1.5 volts.

You can do similar to test the gains of x51 and x201, but the analog output on your LabJack might not go low enough.  The limit for Vin at G=x51 is 0.024 volts, and the limit for Vin at G=x201 is 0.0059 volts.  Set DAC0 & DAC1 to 0.0, use a DMM to measure the actual voltage output by both, and if either is low enough use that for testing.

Troubleshooting

Signals Not Referred to GND

You can't just connect 2 floating voltages to IN+ and IN- of an instrumentation amp.  See the Differential Readings app note.

Common-Mode Voltage Out of Range

This is a common issue when trying to measure individual cell voltages in a battery pack with multiple cells in series.  See the Differential Readings app note, and see the Signal Range Tables in Appendix A.

Source Impedance Too High

The LJTick-InAmp has high input impedance and low input bias currents, but some signal sources are particularly sensitive (e.g. some sources called "electrodes").  Such sources need an amp with very high input impedance, but usually more importantly ultra low input bias currents.

Don't forget about Voffset

Vout = (Vin * Gain) + Voffset.  If your Vout is 0.4 or 1.25 volts higher than you expect, perhaps you have not accounted for Voffset.