We are planing to use either AD7688 or the AD7686 A2D-Converter due to their
excellent SNR in our application. These converters are capable of a throughput
of 500kSamp/sec, we want to use this full speed. Thus the aquisition time to
get out the bits is 400ns, SCK should be 66 Mhz.
We need to isolate the analog part, I though about using iCoupler, but even the
fastest type has a propagation delay of 18 ns - more than one SCK cycle @ 15
ns. I was looking for an app note to use the AD7886/7688 with isolation, but I
couldn't find anything. Do you have any documentation/recommandation to use
these A2C-Converts with isolation?
I understand the problem: The AD7686 requires that the read operation is
completed before the conversion is finished. Therefore if you are running at
maximum speed you need to have 16(or 17 with BUSY indication) SCLK cycles
during the 400ns acquisition phase.
Assuming a 3-wire interface with no BUSY signal (16 SCLK cycles) you require an
SCLK period of 23.5ns max corresponding to an SCLK of at 42.5MHz min. So
regarding the minimum timing you are in good shape for using the ADuM1301CRW,
which takes up to a 45MHz SCLK and has two channels in one direction (SCLK,
CNV) and one channel in the other (SDO).
It is possible to run the SCLK faster e.g. 66MHz but this is not a requirement
for achieving the maximum conversion rate. It just provides an extra bit of
margin to allow the uP to minimize the data transfer time.
The 3 channel devices e.g. ADuM1301 are often used for SPI connections and
there is likely to be further applications information in the datasheet.