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Analog input (RSE) from photodiode
ChemNerd
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06-09-2005 01:35 AM
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Hi,
I'm trying to input a voltage signal from a photodiode into a PCI 6229 card. I have one of the channels connected to the AI Ground and the other connected to ao channel 0. Using a voltage meter, I can determine that the potential between the two is about 3.5V. Using the test panels available in the measurement and automation application, I am told that my voltage input is maxed out at 10.6V. This doesn't make sense to me; I understand that the ground for an RSE measurement can float up outside of the +/- 10V input range, but wouldn't I get a measurement of 0V if that were the case ?(the hardware would not be able to distinguish between the input channel and the ground) Perhaps I am not understanding how this measurement works.
Also, does anyone know where I can find information on resistor sizes for differential measurement systems? I ultimately want to put a resistor between my channels and the AI ground (I have a high impedence device).
Any help would be much appreciated.
Thanks, Jason
I'm trying to input a voltage signal from a photodiode into a PCI 6229 card. I have one of the channels connected to the AI Ground and the other connected to ao channel 0. Using a voltage meter, I can determine that the potential between the two is about 3.5V. Using the test panels available in the measurement and automation application, I am told that my voltage input is maxed out at 10.6V. This doesn't make sense to me; I understand that the ground for an RSE measurement can float up outside of the +/- 10V input range, but wouldn't I get a measurement of 0V if that were the case ?(the hardware would not be able to distinguish between the input channel and the ground) Perhaps I am not understanding how this measurement works.
Also, does anyone know where I can find information on resistor sizes for differential measurement systems? I ultimately want to put a resistor between my channels and the AI ground (I have a high impedence device).
Any help would be much appreciated.
Thanks, Jason
Otis
NI Employee (retired)
06-10-2005 11:46 AM
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Hi Jason,
Sometimes getting the signal connections right can appear to be a bit of a tricky process, but once everything is accounted for it tends to be pretty easy.
If you want to learn more about how to properly reference all of your measurements then take a look at these documents.
Why Do My Analog Inputs Seem To Float?
Field Wiring and Noise Considerations for Analog Signals
From what you mentioned in your comments it looks like you are trying to read a differential measurement from a photodiode. If that's the case you may want to connect your photodiode to AI 0 and AI 8. Then take your differential measurement of the photodiode.
However, you mentioned that you were doing everything in in RSE. If that's the case, then for your RSE measurement you should connect the positive terminal to AI 0 (pin 68) and the negative terminal of the photodiode to AIGND (pin 67). Make sure that you have your board set for a RSE measurement too. The easiest way to do that is in the configuration panel when you create a task in Measurement and Automation Explorere (MAX) or in the Create Virtual Channel.vi when using LabVIEW.
What I think is happening in your case is that the board may not be configured properly so it is causing the board to saturate and that's why it looks like your measurements are railing.
You also mentioned that you wanted to know what a good size for your Bias Resistors would be. If you were able to look at the above links, then you should find that typical values for a Bias Resistor are in the range of 10 kOhms to 100 kOhms.
Best of luck getting your measurements back to normal!
Regards,
Sometimes getting the signal connections right can appear to be a bit of a tricky process, but once everything is accounted for it tends to be pretty easy.
If you want to learn more about how to properly reference all of your measurements then take a look at these documents.
Why Do My Analog Inputs Seem To Float?
Field Wiring and Noise Considerations for Analog Signals
From what you mentioned in your comments it looks like you are trying to read a differential measurement from a photodiode. If that's the case you may want to connect your photodiode to AI 0 and AI 8. Then take your differential measurement of the photodiode.
However, you mentioned that you were doing everything in in RSE. If that's the case, then for your RSE measurement you should connect the positive terminal to AI 0 (pin 68) and the negative terminal of the photodiode to AIGND (pin 67). Make sure that you have your board set for a RSE measurement too. The easiest way to do that is in the configuration panel when you create a task in Measurement and Automation Explorere (MAX) or in the Create Virtual Channel.vi when using LabVIEW.
What I think is happening in your case is that the board may not be configured properly so it is causing the board to saturate and that's why it looks like your measurements are railing.
You also mentioned that you wanted to know what a good size for your Bias Resistors would be. If you were able to look at the above links, then you should find that typical values for a Bias Resistor are in the range of 10 kOhms to 100 kOhms.
Best of luck getting your measurements back to normal!
Regards,
06-10-2005 05:56 PM
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It works now. Thanks for the help.
Jason
Jason
