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AD7606: wrong voltages on regulators

Question asked by Ole on Oct 27, 2014
Latest reply on Oct 29, 2014 by Ole

Hello,

 

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.

( AD7606 - powerup sequence )

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:

schematic: ad7606_schematic.jpg

ad7606_schematic.jpg

layout: ad7606_layout.jpg

ad7606_layout.jpg

RegCap1, Range, Standby, Reset and Busy -lines  on boot up: ad7606_RegCap1.jpg

ad7606_RegCap1.BMP

 

Regards, and thanks for your time,

 

Ole

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