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LJTick-Resistance

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Price: $25.00
LabJack LJTick-Resistance-1k Resistor Divider Accessory Compatible with LabJack USB, Ethernet, WiFi DAQ Devices

The LJTick-Resistance (LJTR) makes it easy for LabJack devices to measure resistive sensors. This tick gives users a regulated 2.5V signal that can be used as a excitation voltage and a known resistance value (1k, 10k, 100k, or 1M) to ground that forms a voltage divider with the unknown resistor.  Measuring the voltage output of the voltage divider allows the calculation of the unknown resistance value.

The LJTR connects to an analog input block on a U3, U6, UE9, or T-series LabJack DAQ device.  Thus it uses 2 analog inputs and measures 2 resistances.

 

Figure 1: LJTick-Resistance

Figure 2: LJTick-Resistance with U3-LV

 

Common Applications

The LJTR is designed to measure the value of an unknown resistance.  Some common examples are:

  • Resistors
  • Thermistors
  • RTDs (PT100, PT1000, etc.)

 

Screw Terminal Descriptions

Vref: A 2.5 volt reference voltage output. Internally this reference is used for level shifting, but very little current is used, leaving substantial current available to the user if a very accurate 2.5 volt reference is needed.

GND: Same as LabJack ground (GND).

VINA/VINB: These are the LJTick-Resistance input terminals.

 

LJTick-Resistance Hardware Block Diagram

Figure 3: LJTick-Resistance Hardware Block Diagram

Note: The part labeled as the "Precision Resistor" is the 1k, 10k, 100k, or 1M precision resistor that varies between the LJTick-Resistance variants.

 

LJTick-Resistance Schematic

Figure 4: LJTick-Resistance Schematic

Note: The part labeled as the "Precision Resistor" is the 1k, 10k, 100k, or 1M precision resistor that varies between the LJTick-Resistance variants.

 

Equations

Typical wiring is to connect Vref to one side of the unknown resistance, and the other side of the unknown resistance to VIN.  This forms a voltage divider with the precision resistor to ground which we will call Rfixed.

The Vref on the LJTR is exceptionally accurate.  By specification it is more accurate than an analog input using the +/-10V range on even the U6-Pro and T7-Pro, so AIN feedback is generally not used but rather it is best to simply assume a constant of 2.500V.

Vout = Vref*Rfixed/(Runknown+Rfixed)

Runknown = ((Vref-Vout)*Rfixed)/Vout

Example 1:  For a PT100 you expect that it will be 100.0 ohms at 0 degC and the resistance will change about 0.385 ohms/degC (most common coefficient).  Say room temperature is 22 degC and putting your fingers on the sensor warms it up to 25 degC.  At 22 deg C you expect 108.47 ohms and at 25 degC you expect 109.63 ohms.  The voltages you expect are:

22C => 2.255361V

25C => 2.253003V

Example 2:  For a PT1000 you expect that it will be 1000.0 ohms at 0 degC and the resistance will change about 3.85 ohms/degC (most common coefficient).  Say room temperature is 22 degC and putting your fingers on the sensor warms it up to 25 degC.  At 22 deg C you expect 1084.7 ohms and at 25 degC you expect 1096.3 ohms.  The voltages you expect are:

22C => 1.199213V

25C => 1.192577V

Note that on the T7 there you can use the AIN-EF system to do the resistance and/or RTD temperature math, but still the above is useful for understanding and troubleshooting.

See more information see forum topic #4257, which includes discussion of error due to lead-wire resistance and discussion of 3-wire and 4-wire resistance measurements.

 

Low-Pass Filter

All variations of the LJTR have C1=100pF installed.  This combines with Runknown to create a low-pass filter with a -3dB frequency as follows:

f = 1/(2*Pi*C1*(Ru+R1+R2))


Table 1. Cutoff frequency for C1=100pF with various source resistances.

Runknown [ohms] -3dB Freq [Hz]
100 15,923,566.9
1,000 1,592,356.7
10,000 159,235.7
100,000 15,923.6
1,000,000 1,592.4
10,000,000 159.2
100,000,000 15.9
 

The reason for the filter is that this type of resistance measurement application often leads to an input wire with very high source impedance which is therefore very susceptible to noise.  For example, say wire A connects Vref to a 1M resistor, and wire B connects the other side of the resistor to VIN.  Wire A is driven strongly by the low-source impedance Vref, and is not particularly susceptible to noise.  Wire B, however, has 1M of source impedance and thus is weakly driving VIN, and is quite susceptible to noise.  Capacitor C1 can help eliminate much of this noise.

 

Choosing the 1k, 10k, 100k, or 1M Version

A rule of thumb is to choose the version that most closely matches your typical resistance value, but to really get into details here is a spreadsheet that lists output voltage versus resistance for the different versions.  Save a copy of this to your Google Drive or local machine if you want to edit.

For example, say you have a 10k thermistor and want to measure a 0.1 degree change around the area of 20 degrees C.  From L73 we expect a resistance of 12260 ohms at 20.0C, and from F73 we know that we will get a change of about -490 ohms/degree round 20C, so at 20.1 degrees we expect 12309 ohms.  We put those resistances in B46 & B47, and then look at the resulting Vout changes for the different LJTR variations in row 52.  We can see that the -10k variation gives the largest change at 2.467 mV and thus is a likely choice.  One other consideration, though, is that if we choose the -1k variation the voltages being measured are less than 1 volt so we can use the +/-1 volt analog input range on the U6/T7 and get better resolution than on the +/-10 volt range.

 

Specifications:

Parameter Conditions Min Typical Max Units
General
Vs, Supply Voltage (1) 2.8 5 5.5 Volts
Supply Current 1.2 mA
Operating Temperature -40 85 °C
VREF
Output Voltage 2.499 2.500 2.501 Volts
Initial Accuracy ±0.04 %
Maximum Output Current 10 mA
Op-Amp Buffer
Typical Input Voltage Range -0.1 VS - 1.5 Volts
Typical Output Voltage Range Load ≥ 100kΩ 0.001 VS - 0.001 Volts
Input Bias Current ±70 ±200 pA
Offset Voltage 1 5 μV
Rfixed
Accuracy ±0.05 %
Temperature Coefficient ±10 ppm

(1) The maximum input voltage to the buffer amplifier is VS-1.5, so for proper operation with signals up to 2.5 volts, VS must be greater than 4.0 volts.

 

For more specifications about the reference voltage IC and Op Amp used in the LJTick-Resistance look at the following datasheets:

 

2 comments

ezDAQ's picture

Feasible to Connect/measure Pt1000 for tempeature?

Can you please explain in detail for wiring  w/ pictures?

LabJack Support's picture

Yes, the LJTR-1k is recommended for RTDs.  In fact, it is our preferred method for RTDs:

https://labjack.com/support/app-notes/temperature-sensors

As of this writing, the documentation is not properly gathered for this.  The applicable information is still on the LJTick-Divider Datasheet where you want to see the section "Resistance Measurement with the ...":

https://labjack.com/support/datasheets/accessories/ljtick-divider

For wiring, simply connect one side of the RTD to Vref and the other side to VINA.

For the equations, R1+R2=0 on the LJTR, and if you have the LJTR-1000 then R3=1000, so the equations for the LJTick-Resistance-1k are:

Vout = Vref*1000/(Ru+1000)

Ru = ((Vref-Vout)*1000)/Vout

For a PT1000 you expect that it will be 1000.0 ohms at 0 degC and the resistance will change about 3.85 ohms/degC (most common coefficient).  Say room temperature is 22 degC and putting your fingers on the sensor warm it up to 25 degC.  At 22 deg C you expect 1084.7 ohms and at 25 degC you expect 1096.3 ohms.  If Vref is 2.50, the voltages you expect are:

22C => 1.199213V

25C => 1.192577V

Note that on the T7 there you can use the AIN-EF system to do the math, but the above is still useful for understanding and troubleshooting:

https://labjack.com/support/datasheets/t7/ain/extended-features/rtd