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Maximum value was 2.13V in AIN0 in U3-HV with LJTick-InAmp


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#1 jose

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Posted 12 May 2013 - 07:30 PM

Dear all, I used LJTick-InAmp to amplify an analog signal from a solar cell. I set offset to 0.4V and gain to 11. I used AIN0 input in my U3-HV, but maximum value I got was 2.13V where I should have bigger voltage value, around 5V. Does AIN0 allows bigger voltage? Thank you very much, Jose

#2 LabJack Support

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Posted 13 May 2013 - 10:58 AM

It depends on Vin and Vcm. See Appendix A of the LJTIA datasheet:

http://labjack.com/s...heet/appendix-a

So what is Vcm? See the main page of the LJTIA datasheet:

http://labjack.com/s...inamp/datasheet

#3 jose

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Posted 20 May 2013 - 06:27 PM

Thank you for the information. I recorded solar data during some days with and without LJTick-InAmp. 1. Using LJTick-InAmp voltage drops a lot before sunset every day, please see file attached. 2. If I do not use LJTick-InAmp and I connect directly the solar cell with a resistor in parallel I do not get this behavior but the problem is that I get some variable voltage around 0.03V when I should get 0V. Do you know if is it possible to solve any or both of this problems? I used the following function using python in Ubuntu AIN0_REGISTER = 0 #Note AIN0 = 0, AIN1 = 2, AIN2 = 4, ainValue0 = d.readRegister(AIN0_REGISTER) Thank you very much, Jose

#4 LabJack Support

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Posted 21 May 2013 - 07:17 AM

There was no attachment showing the drop at sunset. Do you have a wiring diagram of your system?

#5 jose

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Posted 21 May 2013 - 07:54 PM

Sorry, now attachment has been successfully uploaded. If I do not not use the LJTick-InAmp, I connect directly the positive of the solar cell to AIN0 (AIN1 or FIO4) and negative to GND with a 100 Ohms resistor in parallel. If I use LJTick-InAmp I connect positive of the solar cell to INA+ (or INB+) and negative to INA- (or INB-), then OUTA or OUTB to AIN0 (AIN1 or FIO4). And I also connect a wire from INA- (or INB-) to the GND in the middle (near VREF). I select switch 5 (offset of 0.4V) and gain of 11. Thank you very much for your help, Jose

Attached Thumbnails

  • SolarLabjackLJTickAmp.jpg


#6 LabJack Support

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Posted 22 May 2013 - 09:23 AM

I don't totally follow, so I made a diagram for you:

https://docs.google....dit?usp=sharing

From what I understand, you have a solar cell and use a 100 ohm resistor as your load. You are then trying to measure the voltage across the load.

Per my drawing, the negative side of the load connects to GND on the U3. This is the only ground connection you need. I assume the solar cell & load have no other connections to anything, including earth ground or any chassis grounds?

Per my drawing, the positive side of the load connects to FIO6 (low voltage analog input), INA+ (measured with FIO4), and AIN0 (high voltage analog input). You then also need a small jumper from INA- to GND.

Connect that way, then acquire AIN0, AIN4, and AIN6. Record as voltages without any extra math so we can see what the U3 is measuring.

#7 jose

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Posted 23 May 2013 - 03:48 AM

Thank you very much for your reply.
Yes, solar cell & load have no other connections.

Before I had the diagram attached, the positive side of the load connected to INA+ (measured with FIO4), and AIN0 (or AIN1) (high voltage analog input), and an small jumper from INA- to GND.
The negative side of the load connected to GND (near AIN0) on the U3 and INA- on the Amplifier.

I guess it is not possible to eliminate the value 0.03V I get during the night in AIN0, when it should be 0.

I will connect tomorrow your diagram and leave it for 4 days, these 4 days I will be out of work.

I guess I need to do the following to connect FIO6:
d.configIO(FIOAnalog = 127)
AIN6_REGISTER = 12 #Note AIN0 = 0, AIN1 = 2, AIN2 = 4, ...
ainValue6 = d.readRegister(AIN6_REGISTER)


I don't totally follow, so I made a diagram for you:

https://docs.google....dit?usp=sharing

From what I understand, you have a solar cell and use a 100 ohm resistor as your load. You are then trying to measure the voltage across the load.

Per my drawing, the negative side of the load connects to GND on the U3. This is the only ground connection you need. I assume the solar cell & load have no other connections to anything, including earth ground or any chassis grounds?

Per my drawing, the positive side of the load connects to FIO6 (low voltage analog input), INA+ (measured with FIO4), and AIN0 (high voltage analog input). You then also need a small jumper from INA- to GND.

Connect that way, then acquire AIN0, AIN4, and AIN6. Record as voltages without any extra math so we can see what the U3 is measuring.

Attached Thumbnails

  • GraphsAIN0FIO4.jpg
  • LabjackFIO4AIN0.jpg


#8 LabJack Support

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Posted 23 May 2013 - 10:51 AM

Yes, you want to read a Float32 from registers 0, 8, and 12 to collect AIN0, AIN4, and AIN6.

#9 eka

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Posted 25 May 2013 - 10:32 AM

For best linearity, voltage drop of LESS than 30 mV per Si photo cell is recommended. "Dark current" during night time can vary due to temperature. Empiracaly determine and subtract from reading. You probably should be using a load resistor of about 1 ohm for a solar cell that has 5 volt output at full sun at noon. You give no info on what "pyranometer is nor relative locations. Could be solar cell is shaded near sunset. eka


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