Thanks for your answer.
Anyway, I've just found out the problem in the meanwhile I was reading your post.
Look at the schematic: I forgot to connect the GND to AGND!
That's why it didn't work!
Marco
Added after 2 hours 47 minutes:
Ok, now I can successfully read and write the registers of AD7730.
I performed the internal calibration, wrote the right settings on filter and mode registers and I'm starting to acquire data.
I built a simple bridge with four 1% 470 ohm resistors so with a +5V supply voltage I get about 3 mV on the ADC input.
Anyway, I always read 0x000000 from data register.
My oscilloscope tells me that the configuration seems ok: the RDY pin goes down at 25 Hz, according to the filter settings. And the readback of all register is coherent with my settings.
I tried to change the settings, for example CHOP/NO CHOP, output rate, unipolar/bipolar, input range, with or without internal calibration, but the reading is always 0.
Again, I removed the bridge, so the input pins are floating. I expect a lot of noise. Instead, I get 0....
How to investigate on this?
Thanks again
Marco
Added after 2 hours 21 minutes:
Some progresses...
I set the wrong value in the filter register. Now its value is 0x800010, that is 50 Hz output and CHOP mode.
With my resistor bridge I get a count of 0x7C11 (16 bit, bipolar, range 80 mV) that is the value of the differential voltage applied at the ADC (about 2,8 mV).
Now the strange thing: if I reverse my bridge, I apply the positive voltage to the negative input and viceversa so I expect a count of about 0x83EF. Instead I read again 0x7C11.
I miserunderstood the datasheet?