You need to pay attention to data acquisition circuits that use a input mux and a sample and hold circuit like that used by the BBB analog circuits. When the ADC samples the analog input, it charges a small capacitor (sample and hold) and then the ADC converts the voltage stored in the capacitor. When the capacitor is connected to the input circuit, it represents a short circuit and then the capacitor charges based on the time constand t = rc, where r is the source impedance. This is apparent if the channel before is fully 0 or full scale as the capacitor in the sample an hold could be 0 or 1v8 or somewhere in between and that can affect the accuracy of your measurements.
A better approach is to use an opamp circuit as a voltage divider as it will present as a low impedance to the sample and hold circuit and eliminate bleed through from adjacent circuits. Regards, John > On May 30, 2017, at 4:32 PM, Dennis Lee Bieber <[email protected]> wrote: > > On Tue, 30 May 2017 10:50:55 -0700 (PDT), philip k > <[email protected]> declaimed the following: > >> i'm attempting a build. >> >> I will be using 5 pressure sensors. here are the specs: >> >> Pressure Range: 0-200 psi >> Output: 0.5V – 4.5V linear voltage output. 0 psi outputs 0.5V, 100 psi >> outputs 2.5V, 200 psi outputs 4.5V > > I'm presuming this is the critical specification... > >> Working Temperature: -40—+125ºC;. >> Accuracy: ±1%FS; >> Thread: 1/8"-27 NPT. >> Overload Capacity: 2 times; >> Pressure Medium: The gas and liquid which is compatible with 316L stainless >> steel; > > Don't think knowing what size pipe thread it uses means much here... > >> Load Resistance: ?(supply power-6.5V/0.02A)?; > > This might be important... > >> Wiring: Red for IN+. Black for GND. Green for OUT. >> >> would i be correct by running a 600? resistor parallel to each sensor to >> bring the voltage down to ?1.8v and go from pin 7 or 8 to the five sensors, >> then run sensor1 signal and a 600ohm resistor in parallel to pins P9 35, >> sensor2 signal and a 600ohm resistor in parallel to P9 36, and so on? >> > > I doubt it. > > What you most likely need is a voltage divider system (use fixed width > font): > > 4.5V ------+ > | > ~ ?1 > | > + 1.8V -----> > | > ~ ?2 > | > GND ------+-----------> > > where ~ is resistor, values to be determined (do a Google on how to > determine a voltage divider -- I'm not going to do all the work <G>) > > {Unstudied: 4.5 / 1.8 = 2.5; say a 240Ohm for ?2, total needs to be 240*2.5 > => 600, so ?1 would be 600 - 240, or 360Ohm...} > > -- > Wulfraed Dennis Lee Bieber AF6VN > [email protected] HTTP://wlfraed.home.netcom.com/ > > -- > For more options, visit http://beagleboard.org/discuss > --- > You received this message because you are subscribed to the Google Groups > "BeagleBoard" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web visit > https://groups.google.com/d/msgid/beagleboard/1cvric9sgrbie713a2ghckmep0unioeiq8%404ax.com. > For more options, visit https://groups.google.com/d/optout. -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/6F98C44D-22F1-4E08-BC44-087F3DF48647%40gmail.com. For more options, visit https://groups.google.com/d/optout.
