Hi, I am using the AD7730 and i have some problems with the Internal zero-scale calibration. I use the SPI interface to communicate between my uC and the AD converter.
I am able to wirte and read from the registers of the AD7730.
The converter is configured in bipolare mode, HIREF = 1( 5V ), Range : +-10mV, 16bit mode.
I am using two AD7730 parallel which are selected with two CHIP-Select.
After a calibration cycle the value of the corresponding Offset-Register changed by a very small value but there is still an offset(approx. 30150) to the expected value of 32768 when the input pins are shorted.
Has anyone any advices?
First I have to say thank you for the support.
In my last message I forgot to mention that the pins were shorted on a common-mode voltage of +2.5V.
But this was also not the problem.
After spending a lot of time I finally got it.
Befor a calibration I had to disable the CHOP-Mode of the AD converter.
If I do the calibration as follows it works and I am getting the expected value(32768).
1. Set the required SF value in the filter-register, and disable the CHOP-Mode.
2. Set the required value in the DAC-register.
3. Internal full-scale calibraton(+/- 80mV).
4. Wait for the /RDY flag in the status-register.
5. Internal zero-scale calibration(+/- 10mV).
6. Wait for the /RDY flag in the status-register.
The value I received is ~32720.
This is realy colse to the expected value.
why this value 32768 as correct value?, where have you seen it?