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,
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…
See figure 86. You can get +/-2.5V differential without hitting the rails. That should be the default configuration in your eval board.
Thank you for your answer.
Unfortunately, this is not the case... I get an overvoltage or undervoltage error (reg 0x4C to reg 0x53) when I exceed +/- 1.65 (so 3.3V /2).
The only modification I made on the Eval board is the main supply bridge in position "B".
Don't you have any idea what my mistake might be?
The evaluation board is configured by default in dual-supply, i.e. AVDD = +1.65V and AVSS = -1.65V. So, if either AINx+ or AIN- goes above +1.65V, an overvoltage event will trigger. If either AINx+ or AINx- goes below -1.65V, an undervoltage event will trigger. In any case, if your signal is differential, note this is outside range, as when AINx+=1.65V then AINx-=-1.65V, so 3.3V differential. If your signal is not differential, so you cannot take advantage of the whole range in PGAgain=1.
Not sure if you came across this A.N, but it explains all the diagnostic features, let me refer to it for further explanation.
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?
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 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!
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.