How can increment a voltage input by a specific multiple?

[FWalker]

I am using LabVIEW Realtime 8.5 and cRIO-9012 with an analog input module to sense voltage from a pressure transducer.    The transducer increments about .8mV per PSI (0PSI at 1V, 5000PSI at 5V)  but in LabVIEW this voltage is constantly fluctuating between .9988V and 1.004V.   This results in my scaled pressure reading to show pressures of up to 12PSI when there should be no pressure at all.   Is .8mV too sensitive of a reading?   Is there a way to take the input voltage and only increment it in multiples of .8mV?

[Graziano]

Hi!
   First: what module for Analog Input do you have? This is quite important.... 
  
   0.8 mV is really too low for many analog inputs:  If you have a 9201 module, it is 12 bit resolution, +/- 10 Volt, it means that in 2^12 gaps you have 20 Volt, id est approx 4 mV each quantization step (of course, with a linear ADC scale).

   I think you'll have to do some signal conditioning!

   Have a nice day!

graziano

[Matthew_Kelton]

You didn't say which analog input module you are using.  Depending on which module you are using, you may not have the resolution you need to do the job.  For example, the 9201 and 9221 definitely don't have the resolution you are looking. for.

[FWalker]

I was afraid of that, I have the 9201.    OK as far as signal conditioning what are my options?

[ryan_d]

Hi FWalker,

Since many cDAQ modules have built in signal conditioning, there really is not a signal conditioning ‘product’ that National Instruments offers that is meant to work specifically with the 9201. Amplifying the signal externally, however, is still an option.

Two National Instruments cDAQ modules that have a higher resolution and some relevant built in signal processing and are capable of acquiring the 0.8 mV signal changes include the 9205 and the 9239.

Finally, keep in mind that since the signal you’re measuring requires a large quantization level, small noise may have a very noticeable effect. Noise and your restricted acquisition resolution are why you’re seeing values fluctuate so much when there is no pressure.

Please post back if you have any questions. Have a great day!

[FWalker]

 Thanks Ryan, after taking a closer look at my application I realized that I really don't need to be exactly within 1PSI at all times so it seems that I can get away with averaging out the signals to keep the values from jumping around.   I used the RMS ptbypt function with about 1000 samples, I'm all ears for a better method ;-).   There is a delay but with the current application it is not critical.    Now, soon I will need to be pretty accurate and that 9205 sounds like a winner.    Does it do 250KS/s per channel or does that figure divide by the channels?

[Graziano]

Hi!
   You can find in datasheet that 9205 has an aggregate (multiplexed) sampling rate of 250 kS/s, if you want to do 250 kS/s for each channel you have to search for simultaneous sampling feature (or find in datasheet 250 kS/s/ch ).

   Hope this helps.....

graziano

[FWalker]

Thanks Graziano, ryan_d and everyone else who provided input.