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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 mode?
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.