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ADS1247/48 maximum Gain

Igor Jevtovic
Prodigy 100 points
Other Parts Discussed in Thread: ADS1247, ADS1248

Hi,


can somebody please help.

According to ADS1247/48  documentation:

AVDD max is 5,5V, Vref min is 0.5V and for, for example Iidac = 1mA, Vin_max = 0,39048V for RTD at 850°C .

That means that maximum Gain cannot be 32 or 128 which are the value used in  documentation of ADS1247/48 application examples.

Because than, Vcm must be smaller or equal than negative voltage?!

How is the Gain of 32, 64 128 possible if we don't wont to exceed Vcm limits from above? i.e. what Iiadc, Vref, AVDD set up can permit the gain of 128?

Thank you.

Kind regards

IJ

  • 0
    TI__Guru* 98195 points

    Igor,


    The equation for the input common mode range comes from the operating range of the PGA. First I'll show Figure 53 from the datasheet and how it is implemented.



    When PGA=1, the only thing that limits the measurement is the input ranges of A1 and A2 (amplifiers from the figure). In this case, the input can be within AVSS+0.1V to AVDD-0.1V.

    However, when you use the PGA in gain, the problem comes in the output range of the PGA. The input common-mode voltage of the PGA matches the output common-mode voltage of the PGA and the input must be in gain without either output going beyond AVSS+0.1V or AVDD-0.1V.

    In a previous post, there is a discussion about the common-mode input limitations. In particular, there is a set of powerpoint slides showing calculations and examples. I'm posting the link below.

    e2e.ti.com/.../1383202

    On another note, in your example VIN max is 0.39, with a reference of 0.5V so you shouldn't be using the PGA in gain at all. Even if you were to use the internal reference of 2.048V the maximum gain that you can use is PGA=4 without over ranging the input beyond the capability of ADC. By PGA=8, the gain pushes the input to 3.12V, which is outside the input range of the ADC.

    In PGA=32, the max range is +/-64mV and in PGA=128, the max range is +/-16mV.

    One last thing, if you are working on a design with the ADS1247/ADS1248, there is a reference design that you can read through as an example here:

    www.ti.com/.../slau520a.pdf


    Joseph Wu

  • 0 in reply to Joseph Wu
    Prodigy 100 points

    Hallo Joseph,

    Thank you for your fast response and for your time.

    Your answer helped me to understand this particular topic better.

    Kind regards,

    Igor J.