LJTick-CurrentShunt Datasheet | LabJack
 
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LJTick-CurrentShunt Datasheet

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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 AnalogInput/AnalogInput/GND/VS screw terminal block found on newer LabJacks such as the U3, U6, UE9, T4, 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, and thus on the U3-HV and T4 the low-voltage analog inputs (FIO/EIO) will provide the better results than the high-voltage analog inputs (AIN0-AIN3).

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. The fuse is the MICROSMD005F-2 with a hold current of 50 mA and trip current of 150 mA. It has a typical resistance of 18 Ω (min=3.6 and max=50). It 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.

Custom Ranges:

The current on each channel flows into IN+, then through a self-resetting thermal fuse (F2/F3, MICROSMD005F-2), then through the 5.9 ohm 0805 0.1% sense resistor (R2/R5), and then back out IN-.  The voltage across the sensor resistor is multiplied by 20 to provide the output signal.  Some considerations if modifying R2/R5 to provide a custom range:

  • If you plan to measure over 50 mA, you will need to change or short out the fuse.  Note that without the fuse if a low impedance voltage source is connected directly across IN+ and IN- there is high risk of damage.
  • The gain is provided by an AD8202.  R3/R6 is the location for an 0805 feedback resistor (used to increase gain) and is normally not installed which means the gain is x20.
  • The AD8202 gain can be reduced using a resistor from A1/A2 to GND.  The LJTCS has a 1000 pF capacitor C2/C4 from A1/A2 to GND, so that can be replaced with an 0603 resistor to reduce gain.
  • The typical output voltage of the AD8202 is about 0.02 to VS-0.2, so 0.02-4.80 volts if VS = 5.0.
  • The max recommended current due to the size of the PCB traces is 1 amp.  This is a conservative limit and 1 amp is definitely no problem.
  • The burden voltage your signal must drive is the sum of the voltage drop across the fuse (varies from 3.6 to 50 ohms), the sensor resistor, and the PCB traces (typically less than 0.02 ohms).

 

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

14 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.

 

Mo Asgari's picture

Hi

I have a LabJack 4 and going to connect four 2 wire AD590 Temperature Transducer to it. AD590 sensors are voltage input (4-30VDC)/current output (1 μA/K). I have few questions regarding that:

1- As far as the current value for each 1 K( kelvin) changes is only 1μA, can I still use "labjack LJTick-CurrentShunt" to convert my current output to voltage as LJTick-CurrentShunt can only work in 4-20 mA?

If yes, do I need to order four of LJTick-CurrentShunt to connect my four sensors?

2-As i mnetioned my sensors voltage input should be in rabge of 4-30VDC, can I connect such sensors to the low-voltage analog inputs (FIO/EIO) to get better results comapred to using the high-voltage analog inputs (AIN0-AIN3).

3-I am going to use a common power supply voltage input for powering those four sensros using Figure 6: 2-Wire Sourcing Sensor. Is it working in my case?

Regards,

Mo Asgari

LabJack Support's picture

  1. You could use the LJTick-CurrentShunt with your sensors. The minimum current input we specify for the CurrentShunt is 170uA, so the minimum temperature you could theoretically measure would be around 170K, which is well outside of the valid temperature range of the sensor. Each CurrentShunt could work with two current sensors, so you would only need two CurrentShunt devices for four sensors.
  2. As long as you are using the LJTick-CurrentShunt you could use the low voltage inputs to get better resolution for your readings.
  3. Seems like the sourcing configuration in figure 6 will work for your sensors. Please mind that the common mode range of the CurrentShunt is -8V to 28V; I would recommend using a supply voltage of 28V or less.
Mo Asgari's picture

Hi labjack Team

Thanks a lot for your helpfull comments on my case in using Currrent -based analug input AD590 sensors via LJTick-CurrentShunt. Just two questions left:

1) As you mentioned I can use the LJTick-CurrentShunt with my AD590 temparature sensors. As you stated the minimum current input you specify for the CurrentShunt is 170uA. My question is that even in that low micro-A current, the typical value for the shunt would be 240 Ω which results in 40.8 mV to the FIO inputs. If yes, what is the resolution/accuracy of volatage output per each 1K degree? Are those low voltage FIO inputs are snesitive enough to pick up input voltage even uV?

2) As you after scripting in Kipling enivorment and desfine all the inputs and connections, i need to covert voltage to ⁰C and see the temparaure in LJLogM using coversion equation between voltage and ⁰C. Would you please let me know which coversion equition I need to use to see my results/outputs in ⁰C for each sensor?

Regards,

Mo Asgari

LabJack Support's picture

  1. The CurrentShunt output is 118mV/mA. Converting to uA, that would be 118uV/uA and subsequently 118uV/K for your sensor. The T4 LV AIN have a resolution of 600uV, so you could only resolve within about 5 degrees K. A T7 would be better suited to measure in the uV range. See our analog input specifications: https://labjack.com/support/datasheets/t-series/appendix-a-3
  2. As stated above, the CurrentShunt output is 118mV/mA, which is 118uV/K for your sensor, which is also the same as 1K/118uV.
Mo Asgari's picture

Hi Labjack team

Thnaks for your responce.

1)Based on our comunication few days ago I already purchased labjack 4 and CurrentShunt for my sensor. But, reky on your last comment, you said "The T4 LV AIN have a resolution of 600uV, so i can only resolve within about 5 degrees K. A T7 would be better suited to measure in the uV range". I am going to measure temparaure in range of 10-50 ⁰C (283.15-323.15 ⁰K) with +/-0.5 ⁰C(273.65 ⁰K) accuracy.  Do, you think if I can reach to that accuracy based o the available resulution labJack 4? or I am forced to order labjack 7 and refound my lab jack 4? 

2)Is the any difffrence between Labjack 7 and lab jack 7-Pro interms of giving better resultion?

 

3) Previous time I asked you this question but you didnt responce me. Would you please reply to this as well: "As you after scripting in Kipling enivorment and desfine all the inputs and connections, i need to covert voltage to ⁰C and see the temparaure in LJLogM using coversion equation between voltage and ⁰C. Would you please let me know which coversion equition I need to use to see my results/outputs in ⁰C for each sensor?"

Regards,

Mo

LabJack Support's picture

You would not be able to get the resolution you want, the resolution of the T4 analog inputs is 600uV at best. One option would be to use a simple, large shunt resistor instead of the LJTick-CurrentShunt to get a higher voltage output:

https://labjack.com/support/app-notes/measuring-current#ShuntOther

Other than that, the T7 would give you better resolution and the T7-Pro has settings capable of higher resolution than the T7 (resolution indexes 9-12) as described on our resolution page:

https://labjack.com/support/datasheets/t-series/appendix-a-3-2-2-t7-nois...

I described the conversion for the CurrentShunt in my last post and if you end up using a different shunt resistor you will need to calculate your scaling according to the resistance used.