« Close

Datasheets and User Guides

App Notes

Software & Driver

 

LJTick-CurrentShunt Datasheet

Stock: In Stock
Price: $39.00
LabJack LJTick-CurrentShunt 4-20mA measurement DAQ device accessory

The LJTick-CurrentShunt (LJTCS) is a signal-conditioning module designed to convert 2x 4-20 mA current loop signals into voltage signals that vary from 0.472-2.360 volts. The 4-pin design plugs into the standard Input/Input/GND/VS screw terminal block found on newer LabJacks such as the U3, U6, UE9, and T7. The major advantages of the LJTCS, compared to using a simple load resistor, are ease of use, high common-mode range, and lower voltage drop on the 4-20 mA signal.

Figure 1: LJTick-CurrentShunt

Figure 2: LJTick-CurrentShunt With U3

The pins shown on the right side of the LJTCS (Figure 1) connect to the LabJack. The VS/GND pins power the LJTCS, while the OUTA/OUTB pins send the output voltage signal to analog inputs on the LabJack.  Note that 4-20 mA corresponds to 0.472 to 2.36 volts, so on the U3-HV the low-voltage analog inputs (FIO/EIO) will provide the best results.

Following are descriptions of the LJTCS screw-terminal connections:

SGND: This terminal connects to LabJack ground (GND), with a 750 mA self-resetting thermal fuse in series. Often, the 4-20 mA sensor has its own power supply, and that supply ground needs to be connected to LabJack ground to provide a common reference. SGND allows that to be done, but protects from the risk of the external power supply dumping excess ground current through the LabJack.

VS: This is the same 5 volt output as the VS terminals on the LabJack itself. This is an output terminal, not an input. It can be used to provide 5 volt (nominal) power as needed.

INA+/INA- (or INB+/INB-): The 4-20 mA current loop should be connected to INA+ and INA- such that the current flows into INA+ and out of INA-. Each channel has a 5.9 Ω measurement resistor (0805, 0.10%, 50ppm), and the voltage across that resistor is amplified by x20. So 4 mA gives about 0.472 volts and 20 mA gives about 2.36 volts.

Relationship between output voltage and input current:

I = V / (20 * 5.9) = V / 118

mA = 8.475 * volts

In addition to the 5.9 Ω shunt resistor, there is a self-resetting thermal fuse in series. This has a typical resistance of 18 Ω (min=3.6 and max=50). This does not affect the measurement, but does affect how much voltage is dropped as the 4-20 mA signal passes through the LJTCS.

The common-mode input range of the LJTCS is -8 to +28 volts. That means that the voltage of each of the inputs must be within that range compared to LJTCS ground.

If the 4-20 mA signal does not have a common reference at all with the LJTCS, one needs to be made. One common way of doing this is by connecting ground from the sensor to SGND on the LJTCS. SGND has a fuse in series and then connects to normal GND. The fuse is a 750 mA self-resetting thermal fuse, and prevents other systems from using the LJTCS ground as their power ground.

The following figures show typical connections. Figures 3 & 4 show typical connections with a 3-wire sourcing type sensor. This type of sensor sources the 4-20 mA signal, which is then returned to sensor ground. The common-mode voltage of INA- is 0, while the common-mode voltage of INA+ is equal to the signal loading of the LJTCS (typically 0.48 volts @ 20 mA).

Figure 3: 3-Wire Sourcing Sensor

Figure 4: 3-Wire Sourcing Sensor w/ Ext. Supply

Figures 5 & 6 show typical connections with a 2-wire (loop-powered) sourcing type sensor. This type of sensor sources the 4-20 mA signal, which is then returned to sensor ground. The common-mode voltage of INA- is 0, while the common-mode voltage of INA+ is equal to the signal loading of the LJTCS.

Figure 5: 2-Wire Sourcing Sensor

Figure 6: 2-Wire Sourcing Sensor

Figures 7 & 8 show typical connections with sinking type sensors. This type of sensor sinks the 4-20 mA signal, and thus the common-mode voltage of INA+ is equal to the full supply voltage and the common-mode voltage of INA- is equal to the supply voltage minus the signal loading of the LJTCS.

Figure 7: 3-Wire Sinking Sensor

Figure 8: 2-Wire Sinking Sensor

 

Troubleshooting:

 

  • Note the discussion above about having a common reference.  When you are using external supplies (Figures 4, 6, 7 and 8), those supplies must be referred to LabJack GND.  If the supplies are not already referred (the DC output of an AC-DC supply is usually floating), make one connection from the common/negative of each power supply to SGND or GND on the LJTCS or LabJack.
  • Use a DMM to measure the voltage of IN+ and IN- versus GND.  Both must be in the range of -8 to +28 volts versus GND.
  • Use a DMM to measure the voltage difference of IN+ versus IN-.  The total input impedance of the LJTCS varies from 9 to 56 ohms, but the typical value is 24 ohms, and if you are putting 10 mA (for example) through 24 ohms you should measure a voltage difference of about 0.24 volts.
  • Use a DMM to measure the voltage of OUT/AIN versus GND.  The output voltage should be 20*5.9*I, so if the current is 10 mA (for example) the output voltage should be about 1.18 volts.
  • Put a DMM in series with the IN+ terminal to see what it reports for current.
  • Remove the LJTCS from the LabJack and remove all I/O connections, then use a DMM to measure the resistance from IN+ to IN-.  The total input impedance of the LJTCS varies from 9 to 56 ohms, but the typical value at room temperature is 24 ohms.
  • Rather than connecting your signal, do a basic test with a 1k resistor connecting VS to IN+ and a jumper from IN- to GND.  This should result in roughly 5 mA (VS/1000), and then you can do the above measurements with that known current.
  • Too much noise?  First step is to isolate the source of the noise to the LabJack, LJTick-CurrentShunt, or the signal itself.  Do the above test with a resistor, but rather than VS (which might be noisy) use a battery (great) or DAC output (good to great depending on device) with an appropriate resistor to get a current around 5 mA.

Specifications:

Parameter Conditions Min Typical Max Units
General
Supply Voltage 3.6 5 12 Volts
Supply Current 0.5 mA
Operating temperature -40 85 °C
Signal Specs
Gain (1) 20 V/V
Sense Resistor 5.9 Ohm (Ω)
Gain Accuracy 0.5 %
Offset Accuracy 2 mV
Total Input Impedance (2) 9 24 56 Ohm (Ω)
Signal Loading (2) @ 20 mA 0.48 Volts
Common-Mode Range (3) VS = 5 V -8 to +28 Volts
Typical Output For 4-20mA 0.472 2.36 Volts
Typical Output Range 0.02 VS - 0.2 Volts
Typical Input Range Vout = 0.02 - 4.8 V 0.17 40.7 mA
-3dB Bandwidth 1200 Hz

(1) The 4-20 mA signal passes through a sense resistor, and the voltage across that resistor is amplified to produce the output voltage signal. Thus the response of the LJTick-Current is 118 mV/mA.
(2) The total input impedance is the sum of the sense resistor and the thermal fuse resistance, and is typically 21-27 ohms at room temperature. The voltage dropped across this total impedance by the 4-20 mA signal is the loading.
(3) The voltage from IN+/IN- to GND must stay within the common-mode limits.

Dimensions:

Declaration of Conformity

Manufacturers Name: LabJack Corporation
Manufacturers Address: 3232 S Vance St STE 200, Lakewood, CO 80227 USA
Declares that the product
Product Name: LabJack Tick Current Shunt
Model Number: LJTCS
conforms to the following Product Specifications:
EMC Directive: 89/336/EEC
EN 55011 Class A
EN 61326-1: General Requirements

8 comments

I have a sensor that needs 24 VDC +,-10%, it puts out a 4-20 ma signal. in the specs for the LJTick-currentshunt it says the maximum supply voltage is 12V so does that mean I can't uses this device to read my sensor? 

No, that is not what it means.  The Supply Voltage is the voltage powering the LJTCS via the VS pin.  Normally you clamp this in the screw-terminals on a U3/U6/UE9 so it is getting 5 volts.

The applicable spec for the signal is the Common-Mode Range and note #3.

Your sensor will get power from a 24 volt external supply that you provide and you will connect it as shown in Figure 4 or Figure 6 (or less likely Figure 7).

I have a U6 pro can i buy 2 of these LJTCS's and hook up four sensors to the labjack and then see them in lab view? I plan to make 2 loops. Each loop will have its own power supply powering 2 sensors on each loop. Should this work?

Sounds fine.  Ideally, though, you would use as few of external power supplies as possible, so can you just use 1 power supply for all 4 sensors?  I suggest you email [email protected] with the part numbers of your sensors and power supplies so we can take a look.  Also, if possible make a drawing in Google Drawings and share it with [email protected] so we can help further.

Does this have two inputs? The main page does explicit say this, so I am not sure.  Looking at the picture, it does seem to have input A and B.

Yes, the LJTCS is designed to handle 2x 4-20mA signals.

Signal loading is listed as 0.048V.  Should that say 0.480V?  That is, 20mA x 24Ω (or over 1V based on your maximum total input impedance of 56Ω).

I don't fully understand the benefit of the active amplification.  If the total input impedance was 5.9Ω (shunt resistor only), the power dropped would be a max of less than 2.4mW.  With no active amplification (118Ω shunt), the power dropped would be up to 47mW.  With your specs (total input impedance), the power dropped is up to 22.4mW.  So the cost and loss of precision incurred by the amplifier goes into saving about 25mW on the sensed line, and does not account for the power of the amplifier itself.  (Incidentally, the picture does not show an amplifier, but the text implies it is there.)  As for voltage drop, the calculation is similar - it saves a 1.24V drop, based on the max total input impedance.

So it seems that it would only be useful if there is reason to believe that a thermal fuse provides protection on a 4-20mA circuit (and without knowing its current cutoff, I don't know how much protection it provides).

The 0.048 was a typo.  Thanks.  The correct 0.48 value was actually earlier in the text.  At about 0.5 volts, the load voltage is substantially lower than the 2.4 volts you would get with a simple 120 ohm shunt resistor, so that is a small advantage of the LJTCS over a simple resistor.

The LJTCS uses the AD8202.  The key advantage to using this, compared to just a precision 120 ohm resistor, is that the LJTCS can handle a common-mode voltage of -8V to +28V.  This matters with the sinking configurations in Figures 7 & 8.

The reason for the fuse is to survive a situation where someone mistakenly connects a voltage source rather than a current source. Voltage sources are very common, and many people don't have a good understanding of the difference between a voltage source and current source, so such a mistake is quite common.  Because the shunt on the LJTCS is so small, 5.9 ohms, a voltage source such as 12 volts would try to source over 2 amps through the shunt.