The SYNC command on the AD7768 can be thought of as a reset of the digital filter paths and logic within the AD7768. One of the major events that the sync triggers is the reloading of the digital filter coefficients based on the selected configuration.
A SYNC should be provided to the AD7768 after any changes to the AD7768 configuration, before valid samples are acquired.
The sync signal can be provided direct to the AD7768 SYNC_IN pin, to the START pin, or can be triggered over SPI, under software control.
To provide the SYNC over SPI or through the START pin, the SYNC_OUT and SYNC_IN pins must be tied together.
Once the SYNC is provided, it is no longer required to provide a SYNC again, until either the part is reset, or the configuration is changed.
I have played with the AD Evaluation Board of AD7768 controlled by my own FPGA interface and had the following problem:
- after power up, the ADC is working and sending serial frames, seen on DRDY and DCLK
- over SPI I can set and read back all registers, all default values are correct after power up
- when changing from sinc5 filter (default after power up) to wideband filter in some channel or several channels, they don't send more new data, when all channels are configured with the wideband filter, DRDY disappers completely!
- writing over SPI 0x00 and then 0x80 to register 0x06 (DATA_CTRL) doesn't help. I checked, that 1) SYNC_OUT and SYNC_IN are connected; 2) BUT SYNC_OUT was permanently low; 3) over SPI I can verify, that DATA_CTRL contains 0x00 and 0x80 after the two write command.
The problem was at the START pin: when NOT using the AD interface board, the START pin floats to GND. The pull up resistor R88 is not soldered by default! The solution : solder 10k to R88, this doesn't disturb the operation with the AD interface board, BUT is essential when using some own interface without driving the START pin.
I think this story demonstates the importance of pulsing the SYNC pin after change in the filter configuration and could help other users evaluating AD7768.
On power up it is recommended to perform a RESET followed by a SYNC to guarantee correct operation of the device. Although you may be able to get samples, we cannot guarantee correct operation of the AD7768 until it receives a SYNC pulse. This SYNC pulse resets the digital paths and logic in the device.
Dear engineer of ADI,
i just used a AD7768, and i want to use pin control mode, when i power on the board,and use RESET pin to reset the AD7768. Do i need to use the SYNCIN pin to config it? Or just wait RDRY pin and ready the data. Because I did not connect SYNCIN to FPGA, just use a 0 transistor to connect the SYNCIN and SYNCOUT.