We have a fifteen-year old product design using the AD7705 which has shipped in the thousands. In the last 10 years, we have had three reports of the unit displaying the same constant value until the power is cycled. I have ruled out many potential causes of this, but am currently investigating the front-end, including the AD7705. To cut a long story short, the raw data from the ADC is post-calibrated by software, so I can only speculate on what raw data coming from the ADC would generate the constant value. However, if I do speculate, then it would translate into 39321 - a seemingly random number, until converted into hex: 0x9999 or decimal: 0B1001100110011001.
I was wondering if the undocumented Test Register referred to in the datasheet is, or could be documented somewhere. It would at least help me rule the AD7705 in or out.
The basic device configuration is: 500kHz resonator, 2.5V reference, gain of 2, 10Hz conversion rate, bipolar mode, unbuffered, input range approximately 0 to 1.25V.
Thanks in advance.