Difference between my Circuit and Picture ? Using Ground as Input ? | LabJack
 

Difference between my Circuit and Picture ? Using Ground as Input ?

6 posts / 0 new
Last post
boezkaya
boezkaya's picture
Difference between my Circuit and Picture ? Using Ground as Input ?

Hello LJ-Forum,

so i want to replicate a workbench to measure electrical signals in plants. The Original  Circuit (WorkbenchElectricalSignals.PNG ) is in the FIle Attachments. I use a LJ U3 and a LJ Tick-In Buffer instead of an NI USB6008 and LJ-EI1040. Do you think my Idea 1 is right and applicable ? Do i need to install resistors in parallel like in the original ? Is Idea 2 better for Signal to Noise Ratio ?

Thank you very much in Advance,

Benjamin

LabJack Support
LabJack Support's picture
The In Buff doesn't provide

The In Buff doesn't provide any signal amplification and does not provide a method for converting a differential analog input to a single ended measurement like you are doing with your original design so you'll need to do some experimentation to see if this works first.

The U6 would likely be a better fit for your application given your requirement of taking differential analog input readings.

boezkaya
boezkaya's picture
The In Buff doesn't provide

The In Buff doesn't provide any signal amplification and does not provide a method for converting a differential analog input to a single ended measurement like you are doing with your original design so you'll need to do some experimentation to see if this works first.

The U6 would likely be a better fit for your application given your requirement of taking differential analog input readings.

 Hello LabJack Support,

i successfully installed an optional Gain on my InBuff. So you tell me that Idea 1 (simmilar to the Original) is not possible with the U3 right ? Maybe i can try to get the "Ground" Electrode into In A instead of measuring another Leaf

Thanks,

Benjamin

LabJack Support
LabJack Support's picture
It might be possible to have

It might be possible to have the positive lead go to InA with the negative going to InB and then do the differential input calculation in software.  You'll still need a resistor from InB to GND to ensure the signal is read correctly.  If you can do this then you'll just need a second InBuff to measure a second leaf.

boezkaya
boezkaya's picture
You'll still need a resistor

You'll still need a resistor from InB to GND to ensure the signal is read correctly.  If you can do this then you'll just need a second InBuff to measure a second leaf.

That is a good idea and also cheaper than buying a U6. Would a 10kOhm Resistor be sufficient ? 

LabJack Support
LabJack Support's picture
Looking again at the USB6008

Looking again at the USB6008+EI1040 drawing, it is actually doing 2 single-ended measurements.  It defines the base of the plant as GND and then takes a single-ended measurement of the voltage at 2 different spots higher on the stem.

So, your "Idea_1.png" is perfect. That is doing the same thing as the EI1040 drawing.

The LJTIB has lower bias currents than the EI1040, so ignoring noise & gain the LJTIB should give you the same or a better measurement. Don't be surprised if you get different results than the EI1040, and if you do I suspect the LJTIB is more right then the EI1040.

One question is which has better noise performance ... the EI-1040 or your LJTIB with custom gain.  Remove your signals and jumper both LJTIB inputs to GND so you can get baseline readings of your noise level with a perfect signal.