Analog input card voltage source
Analog input card voltage source
(OP)
We have a linear pot that precisely measures position. The pot is fed from a precision 10VDC source, and is fed into a ±10V analog input card (14-bit resolution; only 13 bits are utlized since the signal is 0-10V).
My idea is to do away with the 10V source and instead use the pre-existing 24VDC power supply source, running the pot output to a 0-30V analog input card (14-bit). This card has two channels--the second channel, currently unused, would have the 24V line running in to its analog input directly. This way I could take the ratio of the pot output to the 24V signal and remove any fluctuations in line voltage.
This should result in a 60% increase in resolution, plus do away with the 10V power source.
The 10V and 24V supplies both have a 0.5% line regulation, and 0.7% and 0.5% load regulation respectively. Of course the load on the 24V supply will vary much more greatly.
Both cards are differential input, although that shouldn't really matter.
Is this a bad idea? What are some caveats I am not thinking of?
Thanks!
My idea is to do away with the 10V source and instead use the pre-existing 24VDC power supply source, running the pot output to a 0-30V analog input card (14-bit). This card has two channels--the second channel, currently unused, would have the 24V line running in to its analog input directly. This way I could take the ratio of the pot output to the 24V signal and remove any fluctuations in line voltage.
This should result in a 60% increase in resolution, plus do away with the 10V power source.
The 10V and 24V supplies both have a 0.5% line regulation, and 0.7% and 0.5% load regulation respectively. Of course the load on the 24V supply will vary much more greatly.
Both cards are differential input, although that shouldn't really matter.
Is this a bad idea? What are some caveats I am not thinking of?
Thanks!





RE: Analog input card voltage source
Another is that while the voltage can now be ratio'd that only works if you're taking the samples closely and then it works not at all for noise related issues which are very fast events. If you can supply some filtering that will carry noise on the 24V across both samples, then it should work fine. The filter would consist of an R between the 24V rail and a bulk capacitor to the rail return. Feed the pot from the the R/C capacitor. This will actually drop some of the 24V but since you're now ratioing it that won't matter.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Analog input card voltage source
The pot's recommended maximum input voltage is 42 VDC, and has a resistance of 10kΩ.
The analog input card has a first order low pass filter at 5kHz. Is that low enough? Also what if we did software filtering instead of hardware--the sampling rate does not need to be super fast.
RE: Analog input card voltage source
5kHz on the inputs suffers from the same issue. You can have an instantaneous noise spike that occurs on only the pot reading or only the reference reading and there is no way to resolve it after-the-fact, but it will change the result of your measurements in a manner having no relationship to pot position - something I image is not desired. You have to keep the reference voltage stable during the short time between adjacent readings.
How fast are you going to be reading the these two inputs?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Analog input card voltage source
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RE: Analog input card voltage source
For the same reason adding an RC filter is out as well, unless it has industrial, probably DIN-rail, packaging, which will considerably increase the price.
I'm not 100% sure on the input scan rate, but it is between 15-50ms.
RE: Analog input card voltage source
RE: Analog input card voltage source
At this point I'm thinking this idea will not work. Thanks everyone for walking me through this--I kinda figured there were some aspects I wasn't considering, and I appreciate the feedback. Good stuff!
RE: Analog input card voltage source
Keith Cress
kcress - http://www.flaminsystems.com