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LJTick-Divider

The LJTick-Divider (LJTD) is a signal-conditioning module designed to take 2 single-ended higher voltage analog signals, and divide them down to 0-2.5 volt signals. The 4-pin design plugs into the standard AIN/AIN/GND/VS screw terminal block found on LabJacks such as the U3, U6, UE9, and T7. The use of large resistors and a precision op-amp buffer provides an input impedance of 1 megaohm. By adding or replacing resistors, many other configurations are possible, as discussed in the datasheet.



The stock versions available from LabJack convert 0-10 volt (UNI10V) or +/-10 volt (BIP10V) inputs to 0-2.5 volts. If the custom range is selected (for an extra charge), specify the desired range in the order comments, and expect a 1-5 day delay. See the datasheet for typical custom options and the names used to specify.

Details

Details

LJTick-Divider

The LJTick-Divider (LJTD) is a signal-conditioning module designed to divide 2 single-ended channels of higher voltage analog signals down to 0-2.5 volt signals. The stock builds are for 0-10 volt inputs (UNI10V) or ±10 volt inputs (BIP10V). The 4-pin design plugs into the standard AIN/AIN/GND/VS screw terminal block found on newer LabJacks such as the U3, U6, and UE9. The use of large resistors and a precision op-amp buffer provide an input impedance of 1 MΩ. By adding or replacing resistors, many other configurations are possible.

Prior to December 2007, all shipped LJTick-Dividers were the UNI10V configuration and were not specifically labeled. Starting December 2007, all shipped LJTick-Dividers have a label specifiying UNI10V, BIP10V, or other.


Figure 1: LJTick-Divider

Figure 2: LJTick-Divider With UE9
 

 

 

 

 

 

 

 

 

 

 

VINA/VINB: These screw terminals are for the 2 single-ended channels of input analog voltages. With the factory default configurations (UNI10V or BIP10V), the input to either of these terminals is typically 0-10 or ±10 volts, and produces 0-2.5 volts on the respective OUT pin.

GND: Same as LabJack ground. VINA/VINB must be referred to this ground.

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.

 
Figure 3: Schematic For Each Channel

The above figure is a schematic for one channel of the LJTD, showing the standard factory installed values for UNI10V. The input/output relationship is described by the below equations, assuming the op-amp is in the default unity gain configuration.

General Equations for Figure 3:
Vout = Vin*Rpar/(R1+R2)  +  Rpar*Vref/R4
Slope = Rpar/(R1+R2)
Offset = Rpar*Vref/R4
Rpar = Rparallel = 1 / ( (1/(R1+R2) + 1/R3 + 1/R4) )

The resistors R1+R2, R3, and R4, can be changed to provide other ranges as shown in the table below. The table shows the input voltage at the typical output voltage limits of 0.0 and 2.5 volts. It also shows the input voltage for an output voltage of 3.5 volts, as the internal buffer amplifier accepts a maximum input voltage of 3.5 volts when powered by VS=5.0 volts, and thus when the amp is configured for unity gain the maximum output voltage is 3.5 volts. The Slope and Offset columns go with the formula Vout = Slope*Vin + Offset. The labels in the Name column are used when ordering custom configurations.

The packages for resistors R1-R4 are 0805, while all other resistors and capacitors are 0603. The tolerance of the factory installed resistors is 0.1%, so a good option for the 180k resistor below would be digikey.com part number RG20P180KBCT.

U3: The LJTD is generally used with low-voltage channels on the U3-LV or U3-HV.  The nominal input range of a low-voltage channel is 0-2.44 volts, so the input range provided by the LJTD is from the "VIN (OUT=0)" column to a little less than the "VIN (OUT=2.5)" column.  For example, the UNI10V in this case will provide an input range of about 0 to 9.76 volts.  If you set the U3 analog input to the "special" range it takes an input of about 0-3.6 volts, so the input range provided by the LJTD is from the "VIN (OUT=0)" column to the "VIN (OUT=3.5)" column.  For example, the UNI10V in this case will provide an input range of about 0 to 14 volts.

U6/T7: The LJTD is used with the +/-10 or +/-1 volt range on the U6.  With the +/-10 volt range the full 0-3.5 volt output of the LJTD can be measured, but only 3.5/20 = 17.5% of the U6 input range is used.  With the +/-1 volt range, the 0 and 1 volt output columns above apply, and 50% of the U6 input range is used.

UE9: The LJTD is used withe the 0-2.5 or 0-5 volt range on the UE9.  The 0 and 2.5 volt output columns above use 100% of the 0-2.5 volt UE9 input range, or the 0 and 3.5 volt columns use 70% of the 0-5 volt UE9 input range.

 

Resistance Measurement with the LJTick-Divider-UNI2V (LJTD-UNI2V)

The LJTD-UNI2V-100K has R1=R2=1.8k (thus R1+R2=3.6k) and R3=100k.
The LJTD-UNI2V-1K has R1=R2=47 (thus R1+R2=94) and R3=1k.
The LJTD-UNI2V-10K has R1=R2=270 (thus R1+R2=540) and R3=10k.
The LJTD-UNI2V-1M has R1=R2=22k (thus R1+R2=44k) and R3=1M.
All LJTD-UNI2V variations have C1=100pF.

Typical use is to connect Vref to one side of an unknown resistance (Ru), and connect the other side of Ru to VIN on the LJTD.  For best results on the U6/T7 you would also connect Vref to some analog input to measure the actual value.  Then use the following equations to determine Ru:

Simplified Equations for LJTD-UNI2V (or any UNI LJTD):
Vout = Vref*R3/(Ru+R1+R2+R3)
Ru = (Vref-Vout)*R3/Vout - (R1+R2)

Example:  Vref is jumpered directly to VIN (Ru=0).  Vref is measured as 2.50V.  You should get Vout = 2.5*100000/(0+1800+1800+100000) = 2.413 volts.

Example:  Vref is connected through an unknown resistor Ru.  Vref is measured as 2.50V and Vout is measured as 1.2279V.  The means Ru = (2.5-1.2279)*100000/1.2279  - (1800+1800) = 100000 ohms.

All variations of the LJTD-UNI2V also have C1=100pF installed (other LJTD variations have no C1 installed).  This combines with Ru+R1+R2 to create a low-pass filter.

-3dB Frequency:
f = 1/(2*Pi*C1*(Ru+R1+R2))

The reason for this 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 helps eliminate much of this noise.

 

Specifications:

(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.

(2) The input impedance and bias current is dominated by the input resistors not the buffer amplifier. The input bias current of the internal buffer amplifier is less than ±200 pA across the voltage range, which is an important number as far as sizing the input resistors to not create excessive offset.

 

  Product Comparison

U12 U3-LV U3-HV U6 U6-Pro UE9 UE9-Pro T7 T7-Pro
AIN Voltage ±10V 0-2.4V ±10V [1] ±10V ±10V ±5V ±5V ±10V ±10V
Analog Inputs 8 16 16 14 14 14 14 14 14
Effective Resolution [2] 12 bit 12 bit 12 bit 16 bit 22 bit 12 bit 20 bit 16 bit 22 bit
Digital I/O 20 20 16 20 20 23 23 23 23
Logic Level 5V 3.3V 3.3V 3.3V 3.3V 3.3V 3.3V 3.3V 3.3V
Analog Outputs 2 2 2 2 2 2 2 2 2
Counters 1 Up to 2 Up to 2 Up to 2 Up to 2 Up to 2 Up to 2 Up to 10 Up to 10
USB Yes Yes Yes Yes Yes Yes Yes Yes Yes
LJTick Compatible No Yes Yes Yes Yes Yes Yes Yes Yes
Internal Temp Sensor No Yes Yes Yes Yes Yes
Yes
Yes Yes
Thermocouple Ready [3] No No No Yes Yes No Yes Yes Yes
Ethernet No No No No No Yes Yes Yes Yes
Scripting No No No No No No No Yes Yes
Wireless No No No No No No No No Yes
Real-time Clock No No No No No No No No Yes
[1] Can be configured for -10V to +20V range.
[2] According to actual measured data, see related Noise and Resolution (App Note)
[3] If not thermocouple ready, can purchase an amplifier like the LJTick-InAmp, see related Thermocouples (App Note)