I have an application with a system that has three AD7730s; they have a serial
communication with a /CS signal. When /CS is high, is DOUT is in tri-state
The DOUT line goes into a high impedance state some time (between 10ns and
80ns) after the last inactive edge on SCLK (e.g. if POL = 1 data is clocked out
on the falling edge and therefore the rising edge is inactive). See datasheet
for precise timing. The leakage current when DOUT is in tri-state is 10uA max
(page 3, Logic Outputs section).
So in answer to your question, it is possible to connect the DOUT pins for all
three devices on condition that only one at a time is outputting serial data to
the bus. For this reason, the user should leave sufficient time between
releasing the /CS line on one AD7730 and asserting the /CS line for a second
device, in order that the first device has time to relinquish the bus before
the second device starts driving the bus.