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ADS1299 - Quick question about Vref

José Jair Alves Mendes Júnior
Intellectual 460 points

Hi!

According to this:

In bipolar mode and using internal reference (4.5V), if I XOR the data with 0x800000 (thus adding 4.5V) I am gonna have a scale ranging from 0x000000 (full negative) to 0xFFFFFF (full positive).

Since the image above says that I have a +Vref and a -Vref, does it mean that my LSB has a weight of actually 9/(2^23 - 1)??

So after XORing, to convert the output code to voltage I would have to multiply it by 9/(2^23 - 1).

And then I subtract the 4.5V I added (so I have negative values again).

Is this correct?

Thanks

  • 0
    TI__Mastermind 38745 points
    Hey JJ,

    In the scenario you are describing, you are only shifting rather than scaling the range of values. Therefore there is no change to the weight of an LSB. You are adding a number then subtracting it right after; algebraically this produces the same exact value as before (I assume you are doing this in order to make the negative voltages sign extend into your 32-bit processor word size). The reference does not need to be considered until you convert the codes into voltages.

    Brian Pisani