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TM4C1230H6PM: Reading a 5v signal with a 3.3v microcontroller

Andrew Harres
Prodigy 20 points
Part Number: TM4C1230H6PM

I'm designing a data acquisition board for my school's formula sae car which has 5v analog sensors. Right now I have a simple voltage divider consisting of a 33k and a 66k to bring the signal down to 0-3.3v. I was wondering if that was sufficient or if I should add a filtering capacitor. Or would it be reasonable to use a 5v adc with logic level converters to communicate with the tiva? Any other relevant input would be greatly appreciated as I'm a computer science student and a bit out of my league.

  • 0
    TI__Guru 72500 points
    Hi Andrew,
    That is a good question, and there are lots of considerations. Using a voltage divider is usually sufficient. However, you want to make sure you don't have too high of a source impedance to the ADC. What happens when the ADC does a conversion is that an internal switch opens and it charges a small sample capacitor. The bigger the resistor, the longer it takes to charge or discharge that internal capacitor. You can compensate by increasing the sample time if the signal you are sampling is not changing too quickly. Or you can add an external capacitor that is much larger than the internal capacitor. When the sample is taken, the big external capacitor charge shares with the internal capacitor. This method also filters out higher frequencies which may be good or bad. The easiest solution is to use a low impedance source. Here is a good application note that talks about this subject, although they reference our Hercules family instead of the Tiva family:
    www.ti.com/.../spna118b.pdf

    So much for theory. My practical advise is to use 3.3K and 6.6K if the analog sensor can provide the necessary current (0.5mA). If not, use the 33K and 66K with a 1uF capacitor.
  • 0 in reply to Bob Crosby
    Prodigy 20 points
    Thank you, Bob. That was very helpful. Sorry if this is the wrong place to ask, but do you have any advice or recommend any literature for someone outside the field with little formal knowledge diving head-first in hardware design? I have the book called "High Speed Digital Design: a Handbook of Black Magic" which is a bit over my head. I also reference the TI hardware design checklist often. Besides that, I mostly search the internet for specific information.