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LJTick-InAmp

Signal-conditioning module provides two instrumentation amplifiers ideal for low-level signals such as bridge circuits (e.g. strain gauges) and thermocouples. Each amplifier converts a differential input to single-ended. The LJTIA has switches to choose from 5 gain settings (x1, x11, x51, x201, or custom) and two output voltage offsets. 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.


Details

Details

Datasheet Revision 1.070, Aug 7, 2007.

The LJTick-InAmp (LJTIA) is a signal-conditioning module that provides two instrumentation amplifiers ideal for low-level signals such as bridge circuits (strain gauges) and thermocouples. The LJTIA has 5 gain settings per channel and two selectable output voltage offsets (Voffset). The 4-pin design plugs into the standard AIN/AIN/GND/VS screw-terminal block found on LabJacks such as the U3 and UE9.

The pictures below show the LJTIA plugged into the U3 on the left and plugged into the UE9 on the right.


Figure 1: LJTick-InAmp (LJTIA)

Figure 2: LJTIA With U3

Figure 3: LJTIA With UE9

 

 

 

 

 

 

 

 

The block of 4 screw-terminals at the left edge of the LJTIA (Figure 1 above) provides a positive and negative input for each differential channel. Towards the LabJack side of the LJTIA is a pair of screw-terminals that provide a ground connection (GND) and a +2.50 volt reference (VREF). The reference is capable of sourcing enough current (see Specifications) to function as the excitation voltage for most common bridge circuits.

In between the blocks of screw-terminals is a 10-position DIP switch used to specify gain and offset.

 
Switch # Name Description
1 BxR32 Custom gain determined by R32 Applies to channel B only. All off equals a gain of 1.
2 Bx11 Gain of 11
3 Bx52 Gain of 51
4 Bx201 Gain of 201
5 0.4V Output offset of +0.4 volts. Voffset applies to both channels. Switch # 5 or 6 should always be on, but not both.
6 1.25V Output offset of +1.25 volts.
7 AxR17 Custom gain determined by R17 Applies to channel A only. All off equals a gain of 1.
8 Ax11 Gain of 11
9 Ax51 Gain of 51
10 Ax201 Gain of 201

Table 1: DIP Switch Descriptions

Each channel has a switch (numbers 1 & 7) that has been left without factory-installed gain resistors:  R17 for channel A and R32 for channel B.  Resistors can be installed by the end-user to provide custom gains according to G=1+(100k/R). For example, a resistance of 100 ohms would provide the maximum allowable gain of 1001.  Also, multiple switches can be closed at the same time to get a few other gains (x61, x211, x251, and x261), as the gain settings resistors (10k, 2k, and 500) wind up in parallel. The packages for resistors R17 & R32 are 0805, while all other resistors and capacitors are 0603. The tolerance of the factory installed resistors is 0.1% & 25 ppm/degC, so consider the RG20P series from digikey.com (100ohm = RG20P100BCT).

Extending from the back of the LJTick-InAmp are four pins. The first two pins provide +5 volt power and ground from the LabJack. The other two pins are the instrumentation amplifier outputs and connect to analog inputs on the LabJack. The four pins plug directly into the 5.0 mm spaced screw-terminals on the LabJack U3, UE9, or other future devices as shown in Figure 4.


Figure 4: LJTick-InAmp lined up to UE9

 

 

 

 

 

 

 

 

 

 

Each channel on the LJTIA has an AD623 instrumentation amplifier (in-amp) from Analog Devices. The allowable signal range (Vin) is determined by a combination of Gain, Voffset, Vcm, and Vout. See the Signal Range Tables in Appendix A.

Voffset: This is an offset voltage added to the in-amp output. If DIP switch #5 is on, the offset is +0.4 volts, and if DIP switch #6 is on, the offset is +1.25 volts. The same offset applies to both channels of the LJTick-InAmp. One offset must always be selected (0 volts is not an option), but both offsets should never be enabled at the same time. The +0.4 volt offset is generally used with signals that are mostly unipolar, while the +1.25 volt offset is generally used with bipolar signals.

Vcm: This is the common mode voltage of the differential inputs. For an in-amp, that is defined as the average of the common mode voltage of each input. For instance, if the negative input is grounded, and single-ended signal is connected to the positive input, Vcm is equal to Vin/2. Another common situation is when using a wheatstone bridge where VREF=2.5 is providing the excitation. In this case, each input is at about 1.25 volts compared to ground, and thus Vcm is about 1.25 volts.

Vin: This is the voltage difference between IN+ and IN-. In the following Signal Range Tables, the “Low” column is the minimum Vin where Vout is 10 mV or higher, the “High 2.5V” column is the maximum Vin where Vout is 2.5 volts or less, and the “High 4.5V” column is the maximum Vin where Vout is 4.5 volts or less.

Vout: This is the single-ended (referred to ground) voltage output from the in-amp. Because of the power supply to the in-amp, the full output swing is about 0.01 volts to 4.5 volts. The “Low” and “High” columns in the Signal Range Tables give the output at the respective Vin.

 

Specifications

Parameter Conditions Min Typical Max Units
General          
Supply Voltage   3.6 5 5.5 volts
Supply Current (1) No Loads   1.5   mA
Operating Temperature    -40   85 °C
Signal Specs          
Gain Accuracy     0.35   %
Offset Accuracy G = 1    0.5    %
  G = 11    0.5    %
  G = 51    2.5    %
  G = 201    10    %
Input Bias Current (3)     17   nA
 Input Impedance      2  
Each Input vs. GND (2) Normal Operation     -0.3 to +5.3 volts
Each Input vs. GND (3) No Damage     -10 to +15 volts
Typical Output Range Load ≥ 10 kΩ 0.01   VS - 0.5  
-3 dB Bandwidth x1   18   kHz
  x11   18   kHz
  x51   18   kHz
  x201   10   kHz
Vref          
Output Voltage   2.495 2.50 2.505 volts
Initial Accuracy     0.2   %
Current Output (1) For rated V accuracy 0   25 mA

(1) Higher currents will not cause damage, but the reference voltage will start to sag. The reference output can handle a continuous short-circuit to ground and has a short-circuit current of about 45 mA typically.

(2) This is the limit of the voltage on any input terminal versus ground.  See Appendix A for actual limits in different situations.

(3) The current in/out of the input terminals is nanoamps from -0.3 to +5.3 volts.  Beyond that range it increases up to 10mA at -10 or +15 volts.

Dimensions


  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)