Hi Experts,
Is this table mean if 3 wire SPI is used, 70MHz clock is necessary for 1Mhz data rate?
Do we have SPI isolator recommendations for 70MHz communications with ADS8900B?
Currently, my customer wants to develop an system to acquire data with 3 wire SPI by ADS8900B, and isolation is required but we are still researching the isolators. Do you have any recommendations?
Marsh
Hello Marsh,
In order to lower the SCLK frequency to 22MHz, you would need to operate in Zone 2. Please see section 7.5.3 in the datasheet for details. In Zone 2, you have most of the entire cycle time to transfer the data.
In order to support 22MHz with 3 isolation channels, you will also need to capture data on the same SCLK edge as the launch edge, to allow for the clock delay through the isolator, and then the additional delay for the SDO to return, t-D_CKDO. This will require very precise timing to ensure their are no timing specification violations. Please take a look at this application note that describes this use case in much more detail.
https://www.ti.com/lit/an/sbaa241/sbaa241.pdf
The ISO7731 appears to be one of the better choices, and should be able to support 22MHz SCLK data with 3 channels. However, in order to provide more timing margin, I would suggest using the 4 channel ISO7742, which will allow looping the SCLK signal back through the isolator to reduce the timing error between the SCLK launch edge and the SDO data.
I am not familiar with the entire isolation device family; you may want to reach out to that team to see if they have a different recommendation for the isolator. In order to support 3-wire SPI, the key spec will be the propagation delay. As an example, the ISO77xx family has a maximum propagation delay of 16nsec.
Regards,
Keith Nicholas
Precision ADC Applications
