fpga

[Ron B]

I'd like to learn how to change decrease the scan rate of a 9205 module in CompactRIO FPGA.  I have 8 high output impedance sensors.  I know that I may end up adding a voltage follower to each channel, but I'm hoping to avoid that.  As it stands, there is a lot of "shadowing" from channel 0 onto all the other channels. 

I have the 9205 in Differential mode, with a 100Kohm resistor from each leg of each channel to ground.  I tried this with a while loop in FPGA, and I changed the minimum sample time in the 9205 setup (from the Labview project window), but that didn't make any difference.  I then added a delay in the while loop, but that did nothing.

Thanks,

Ron

[jareds]

Hi Ron,

What type of sensor are you connecting to?  What are the values you expect on the channel and what are you seeing?  If you want to slow the sampling rate down, you need to slow the loop rate down like you were doing.  What did you set the delay to in your loop?

[Ron B]

Hi Jared,

My sensors have +/- 10V output.  They are non-contact distance measurement devices.  Channel 2 shows only about 1/4 of the actual change and the offset is based on the reading of channel 1.   

I added a delay to the loop with the counts input to the delay set as a control.  While running, I changed the delay from 0 to 10, 100, 1000, 10000, 100000.  There was no effect on the readings.

I've read on the NI website about ghosting and I think that is exactly what I have.  Alternatively, do you know a source for a small-footprint array of isolation amps (8 amps) that can operate on 12VDC or 24VDC?  I can use something like 5B modules, but the package is getting a bit large for my application.

Thanks,

Ron

[jareds]

Hi Ron,

What is the impedence of these devices?  If it is higher than 1kOhm, then it is likely ghosting that you're seeing.  In order to eliminate this, you'll need to increase the time between reading each channel.  This means having multiple delays within one loop, so that each reading is separated from each other.  This article describes other ways of eliminating ghosting, as well.

I'm don't know of a source for a small-footprint array of isolation amps.  If you're looking for a smaller footprint than the 5B modules and you have an extra slot in your chassis, than you could look into using simultaneous sampling AI modules, which wouldn't have ghosting problems.