i am using the AD7606 in the following configuration:
V_drive = 3.3V
Range = GND -> +/- 5V Range
Ref_select = V_drive = 3.3V -> internal Reference
The 4 Channel version AD7606-4 worked fine on my pcb.
Then I replaced the chip with the 8-Channel version and made the following observations:
The voltages on the pins RegCap1, RefCapA and RefCapB are unstable.
RegCap1 switches between 2.5V and 3.3V.
RefCapA and RefCapB are switching between 4.5V and 5V.
The time between the switching ranges from 10ms to several minutes.
RefOut has been unstable in the past, but is now stable at 2.5V.
RegCap2 is stable at 2.5V.
The ADC returns a value of -300 for a grounded analog input.
If the voltages on RegCap1 and RefCapAB are wrong, the values from the ADC seem to have an offset.
Before I saw in the FAQ that Vdrive requires AVCC, both rails got powered up simultaneously.
Now there is an adjustabe delay from 10µs to 3s between the power up of Vdrive and AVCC.
Also, the Chip has been powered while the digital inputs were floating, drawing ~200mA more than usual.
Could that have damaged the Chip in any way to explain the observations or is my schematic or layout faulty?
Some Info that may help:
RegCap1, Range, Standby, Reset and Busy -lines on boot up: ad7606_RegCap1.jpg
Regards, and thanks for your time,