LJTick-InAmp 3 (LJTIA 3) Datasheet
Features
The LJTick-InAmp3 (LJTIA3) is a signal-conditioning module with two independent channels. Each channel uses an instrumentation amplifier to amplify, and offset the signal while rejecting noise. This is ideal for small-voltage signals such as bridge circuits (strain gauges) and thermocouples.
The LJTIA 3 is available with gains of 11x, 51x or 201x and has a fixed output voltage offset of 1.25V. Custom gains and offsets are possible by replacing resistors on the Tick.
The 4-pin design plugs into the standard VS/GND/AIN/AIN screw-terminal block found on LabJacks such as the T4 and T7.
The pictures below show the LJTIA 3 by itself on the left and a possible connection with a T4 on the right.
Figure 1: LJTick-InAmp 3
Figure 2: LJTick-InAmp 3 with a LabJack T4
The block of 4 screw-terminals at the left edge of the LJTIA 3 (Figure 1 above) provides a positive and negative input for each differential channel. Towards the LabJack side of the LJTIA 3 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.
Gain Options
The gain for each instrument amplifier is set by a gain resistor. The gain from InA+/InA- to OUTA is determined by RGA1 and the gain from InB+/InB- to OUTB is determined by RGB1. These resistors can be found towards the input side of the LJTIA 3 directly above their corresponding instrumentation amplifiers (See Figure 3). The gain resistor values can be calculated according to:
Equation 1: Gain Resistor Calculation
(For more info see the amplifier datasheet)
LabJack provides LJTIA 3s with one of three possible pre-configured gain options:
11x, where RGA1 = RGB1 = 10k
51x, where RGA1 = RGB1 = 2k
201x, where RGA1 = RGB1 = 500.
Additional gains (1 ≤ G ≤ 1000) can be achieved by replacing RGA1 and RGB1 with 0.1% tolerance 0603 package resistors with values according to Equation 1. NOTE: Unity 1x gain can be achieved by removing RGA1 or RGB1 completely.
Figure 3: Location of Gain Resistors on the LJTIA 3
The three default gain options of 11x, 51x, and 201x lead to the following transfer functions to calculate VOUT on either channel:
VOUTA=[(VINA+-VINA-) X 11] + 1.25V
Equation 2a: Transfer Function With 11x Gain (RGA1 = RGB1 = 10kΩ) .
VOUTA=[(VINA+-VINA-) X 51] + 1.25V
Equation 2b: Transfer Function With 51x Gain (RGA1 = RGB1 = 2000Ω) .
VOUTA=[(VINA+-VINA-) X 201] + 1.25V
Equation 2c: Transfer Function With 201x Gain (RGA1 = RGB1 = 500Ω) .
Pinout
Extending from the back of the LJTick-InAmp 3 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 U-Series, T4, T7 or other future devices as shown in Figure 4.
Figure 4: LJTick-InAmp 3 lined up to the LabJack T4
Signal Names
Each channel on the LJTIA 3 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 a fixed offset voltage of 1.25V added to the in-amp output.
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 a 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: Vout = (Vin * Gain) + Voffset. 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 | No Load | 1.5 | mA | ||
Operational | 6 | mA | |||
Operating Temperature | -40 | 85 | °C | ||
Signal Specs | |||||
Gain Accuracy (1) | 0.35 | 1.2 | % | ||
Offset Accuracy (1) | G = 11 | 0.5 | 1.2 | % | |
G = 51 | 2 | 5 | |||
G = 201 | 10 | 15 | % | ||
Input Signal Limits | -0.15 | Vs-1.5 | volts | ||
Output Signal Limits | Load >= 10k | 0.01 | Vs-0.5 | volts | |
Input Bias Current | 17 | nA | |||
Input Impedance | 2 | GΩ | |||
Each Input vs. GND | -0.3 | 5.3 | |||
Each Input vs. GND | -10 | 15 | |||
-3dB Bandwidth | G = 11 | 18 | kHz | ||
G = 201 | 10 | kHz | |||
Vref | |||||
Output Voltage | 2.495 | 2.50 | 2.505 | volts | |
Initial Accuracy | 0.2 | % | |||
Current Output | 20 | mA |
Dimensions
LabJack Tick-InAmp 3 Dimensions
Differences From LJTIA 2
The main difference from the previous LJTIA 2 is that the gain and offset are no longer set with dip switches. The offset is always at 1.25 V and the gain can be modified by swapping out gain resistors as described above.