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VREFLOx in Launchpad LAUNCHXL-F28377S (PZP package, as far as I know).

Jigal Aharonovich
Prodigy 20 points

Hi all, 

And apologies in advanced if this has already been answered.

 I am building a PID controller based on this launchpad for demo purposes.

I would like to use the 12-bit faster sampling option on launchpad and have the ability to increase resolution by changing VREFLOx externally.

(it would start at 0 but would possibly increase to attain higher resolution once locked).

Now, in the launchpad, only VREFLOB is possible to change, although it is hard-wired to GND.

So questions:

1. In the datasheet, it is mentioned that VREFLOx max is VSSA. Can we set it to non-zero value above VSSA?  And if not, can we raise VSSA?

2. Is it possible to disconnect VREFLOB in the launchpad (assuming I have the tool for doing this) and attach it to some external circuitry?

Jigal

  • 0
    TI__Guru 61235 points

    Hello Jigal,

    Jigal Aharonovich said:

    1. In the datasheet, it is mentioned that VREFLOx max is VSSA. Can we set it to non-zero value above VSSA?  And if not, can we raise VSSA?

    No.  VREFLO should be connected to VSSA.  Also, you cannot really raise VSSA.  Remember voltages are always with respect to some reference.  VSSA is the reference against which VDDA is measured.  VDDA must be between 3.14 and 3.47 volts with respect to VSSA.  You also should not have a differential between VSSA and VSS.

    You cannot reduce VREFHI below 2.4 volts anyway (with respect to VREFLO), so you can only make the conversion range a little bit smaller than nominal anyway: 

    People often think they can reduce the full-scale range of an ADC down to increase the effective resolution of their measured signal.  This typically doesn't do what they want however, at least not for microcontroller ADCs.  The last few LSBs are often determined by the noise floor in the ADC.  Reducing the conversion range just increases the number of LSBs that are noise.

    Jigal Aharonovich said:

    2. Is it possible to disconnect VREFLOB in the launchpad (assuming I have the tool for doing this) and attach it to some external circuitry?

    You can perform whatever surgery you want on the board.  Given the restrictions I outlined in #1 above, I'm not sure I see the point however.

    Regards,

    David

  • 0 in reply to David M. Alter
    Prodigy 20 points
    Thanx for the quick and (very!) informative reply.

    I believe the important points you have mentioned in the reply should be given in the documentation, both regarding the resolution and the upper bound of VREFLO, either in the datasheet or in the TRM.

    Thanx again,
    Jigal