AD7738: Timing specs

Document created by analog-archivist Employee on Feb 23, 2016
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Please refer to Data Sheet Rev. 0 page 4. table: Timing Spec. What is the
meaning of t9: will the DOUT be tristated 10 to 80 ns after SCLK rising edge
sampling LSB, no matter if -CS is kept low? (t9 is referred to SCLK, not -CS)
Does it apply only to LSB, or DOUT is tristated t9 after each SCLK rising edge?
NEXT PROBLEM: t6+t7=100ns. Could I use 10MHz SCLK, and as t5=0..60ns, I can not
be sure of sampling by rising edge, so I would use falling edge?


Q1: The DOUT is an active logic output only during the read operation, i.e.,
during the time the data are being shifted out. For the rest, the DOUT is
tri-stated, regardless on CS. Therefore, the t9 is correctly related to the
last SCLK corresponding to the LSB in the read operation, i.e., to the 8th bit
when reading an 8-bit register, 16th bit when reading a 16-bit register, etc.

Q2: The t5 max of 60ns is specified with 50pF load on the digital output. The
load is usually less - and the max time should be shorter for lower capacitive
load. In my setup - AD7738 DOUT driving a standard ADuC824 digital input + 15pF
oscilloscope probe, room temperature, the t5 is typically 20ns. Therefore, I
believe the AD7738 should work with 10MHz SCLK, just keep the digital signal
capacitance to ground low (short tracks on PCB, low number of digital inputs
connected to the signal.)