Hi
I am trying to connect a breathing belt to LabJack U3 with a LJTick-InAmp but I am struggling with the connections. I tried connecting the input wires to INA+ / - but I donot read any analog values. I only get random spikes on the input. Can somebody please advise how to make the connections properly and also what should be the positions of the dip switches, where and what value resistances should be connected? I also e mailed the Pneumotrace customer service and I got this message back. I donot have a background in electronics so it makes little sense to me,
The UFI model 1132 transducer is a piezo-electric sensor with a built-in charge reservoir capacitor. The output of the 1132 is approximately 20 mV for a normal breath, and so a gain of about 100 will be sufficient. The 1132 output can be connected single-ended and DC coupled, but the input impedance should be 1 MegOhm or more. This information should help you get your signal conditioning configured correctly. It sounds like maybe you have the amp setup for AC coupling?
Hoping to get some support soon.
Thanks
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The LJTIA is DC coupled. It passes DC and AC. The input impedance is 2 Gohms and the input bias current is typically 17nA. For amplifiers, bias current is usually a more important spec than input impedance.
You can't just connect 2 floating signals to INA+/-. See "Differential inputs must have a reference":
https://labjack.com/support/app-notes/differential-analog-inputs
The simple way to provide the reference is to add a resistor (100k is a typical value) from INA- (along with your negative signal wire) to GND. Since the manufacturer said single-ended is fine, you can just use a jumper wire (0 ohms) rather than a resistor if you want.
Table A1.1 is the signal range table for the LJTIA with Voffset=0.4V:
https://labjack.com/support/datasheets/accessories/ljtick-inamp/appendix-a
With your type of signal and connection your common-mode voltage is Vin/2, so use the last part of Table A1.1. For example, if you use Gain=x51 the input signal range will be about -7mV to +24mV and that span will produce an output voltage of +0.010V to +1.590V. The datasheet for your sensor says you could get 50mV or more so you might find the output is clipping and you need to drop the gain to x11.
The following describes a basic test so you can see the LJTIA in action:
https://labjack.com/support/datasheets/accessories/ljtick-inamp/appendix-e
If you push on the belt with your fingers or maybe a weight, can you get a steady 50 mV output? That would make testing easy as you could then use a DMM to measure the LJTIA input voltage (INA+ minus INA- = 50mV) and also the output voltage.
Note that the LJTIA works best with low voltage analog inputs (FIO/EIO) on the U3 & T4.
https://labjack.com/support/datasheets/u3/hardware-description/ain