AD7770 maximum range

Hello,

I'm using the Eval Board of AD7770.

The maximum amplitude I can measure between AIx+ and AIx- is +/- 1.65V (dual supply operation)

According to the AD7770 datasheet, he noted that the maximum amplitude is +/- 2.5V (according to table 11 of the datasheet).

A7770 datasheet extract :

How do I configure my eval board to get such an amplitude?

Thank you in advance and best regards,

Maxime Charrière

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  • Hi Lluis,

    I understand that well. However in your first answer you tell me that I can get +/- 2.5V differential.

    In your second answer you tell me that I can only get +/- 1.65V differential.

    There's something I don't understand... How to get +/- 2.5V differential then?

  • +1
    •  Analog Employees 
    on Oct 29, 2018 10:55 AM over 2 years ago in reply to ElectroMax

    Hello,

    Sorry i may have not expressed myself well here. Let me try with pictures.

    In my first e-mail i said: see figures 86, that's the way you can get +/-2.5V differential, i.e. if your signal is a true differential signal, that is AINx-=-AINx+.

    In my second e-mail, for more clarity, i extend to say that if your signal is not true differential, you will not be able to use +/-1.65V. If your signal is pseudo-differential for instance your range will be +/-1.65V in PGA=1.

    If you look at the figures below, i have noted what will be the maximum range in each case. On the left, figure 86, by having a +/-1.25V on each input of the true differential signal, the resultant is a +/-2.5V. In the first peak you have +1.25-(-1.25)=+2.5V, on the second peak -1.25V-(+1.25V)=-2.5V.

    However, on the right hand side figure 87, as the AINx- is tied to VCM (that I supposed for the example being VCM=0), AINx+ on its own cannot go beyond the rails, so for the first peak you will have +1.65V-0=+1.65V at maximum. And on the second peak, you will have -1.65V-0=-1.65V, at minimum. resulting in +/-1.65 pseudo differential range.

    Does that explain now how to get the 2.5V range? The answer is, if you are in PGA gain = 1, you can only have it if your signal is true-differential.

    Let me know please if further clarification is needed.

    Thank you!

    Lluis.

  • Thank you it's much clearer now!

    One last question: Where do we configure the AD7770 to true differential or pseudo differential?

    I don't see any information on that.

    Thank you very much!

  • 0
    •  Analog Employees 
    on Oct 29, 2018 12:59 PM over 2 years ago in reply to ElectroMax

    Hello,

    You don't need to tell the device if your input is true differential, pseudo differential or single -ended. It always takes the differential voltage AINx+ - AINx- as its input, no matter what you have tied up to AINx- pin. But you have to take into account the explained above when doing your design in order to know what ranges you can have.

    Regards,

    Lluis.