AD and QEI

Questions on other types of hardware and getting it talking to the ARM CPU
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Joined: Fri Oct 19, 2012 5:11 am

AD and QEI

Post by YahooArchive »

>from the helpline
>1. When my device is at rest (QEI not moving and 0 load on the load cell), I'm
seeing a large amount of noise on the analog input the load cell is connected
to. I should mention that the load cell input is connected to this load cell
amplifier ( and the output from the
amp is connected to AD(3). I saw where you had mentioned using a cap on the AD
inputs to remove high frequency noise. Do you think this would be helpful with
my problem ??

I can't speak about the device you site, but if your analog section is small and
quiet you can get good readings on the A/Ds with just the ferrite bead isolation
to analog ground. Recently we were doing some measurement using 4-20mA pressure
transducers on some very long cables. Those cables even though mostly shielded,
were pretty good antennas for high frequency noise being coupled into the
converter. The symptom being random bit sets or drops in high order bits of the
converter. The cure for that was a 1K and 100pF cap, the signals we were
measuring were in 10s of Hz, pretty slow.

>2. The more confusing problem I'm seeing is that when I start the device
moving and QEIPOS begins to register counts, the input from AD(3) goes to 0 and
stays there!! When I stop, I start getting data on AD(3) again. I've
calibrated the load cell input using a static weight and the numbers are
consistent when I test by pulling on the load cell before and after moving the
device, but while I'm getting data on the QEI interface there is NO data on
AD(3). Any ideas???

What's a meter or a scope say? There's no reason for AD not to work when QEI is
counting. I see that device is a 5V device, the analog should not exceed 3.3V
into the SuperPRO, when it does I have seen some pretty strange readings (not
just full scale).

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