AD4695 no activity on SDO, always high-Z

Hi, 

We are currently developing a board using 3 AD4695BCPZ ADCs connected to the same SPI bus. We are experiencing an issue where no data is appearing on any of the SDO pins of the ADCs - all 3 SDO pins are connected onto a common MISO line but it remains undriven at all times despite read/write instructions sent on the SPI bus. 

The microcontroller used a Teensy 4.0 running at 3.3V. Inputs to the AD4695s are at 3.3V logic levels, which according to the datasheet p6 (Rev. 0) is within the working range of the device. The MISO line is attached to a 1.8V -> 3.3V level shifter.

The pin configurations on all 3 AD4695s are as follows.

AVDD:   3.3V
VDD:    1.8V
VIO:    1.8V
REF:    2.5V
RESET:  Tied to VIO
LDO_IN: Tied to GND

SDO:    Common MISO line to 1.8V -> 3.3V shifter
SDI:    Common MOSI line from MCU (3.3V)
SCK:    Common SCK line from MCU (3.3V)

CS:     Separate digital outputs from MCU (3.3V)
CNV:    Tied to CS

AGND:   GND
IOGND:  GND
REFGND: GND

All of the above voltages have been measured at the pins of each AD4695 package and confirmed to be correct. Signals on the CS, SDI and SCK have also been probed at the package and confirmed to be present and correct. No shorting of any lines has been observed from current draw and thermal measurements.


An example of a test communication to read from the DEVICE_TYPE register is detailed below:

SPI settings: SCK = 1MHz, SPI_MODE_3 (CPHA=1, CPOL=1), MSB_FIRST

Pseudocode: 
Set CS LOW;
SPI_transfer(0x80); //read bit set, top 7 bits of address set to 0
SPI_transfer(0x03); //address of DEVICE_TYPE register
SPI_transfer(0x00); //padding
Set CS High;


This test is repeated for all 3 ADCs, only switching the CS pins as appropriate. The data pins of each ADC were probed at the IC package and verified. The CS pins were observed to transition low for the duration of the transaction, 3x8 pulses of SCK were seen, and the 3 bytes appeared correctly on the SDI pin clocked in at each rising edge of SCK. However, all SDO pins remain in a High-Z state at all times - only EMI is being picked up when probed.

At this point we are unsure what the issue could be, it would be great if you could provide some assistance on this. 

Best Regards