AD7484 Ouput drive capability

Document created by analog-archivist Employee on Feb 23, 2016
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What is the output drive capability of any output on the IC.  We are using a
number of the same ICs in our circuit on a common data bus.  Unused devices are
tristated.  We need to know the drive capbility of the devices outputs so that
we can estimate rise and fall times.
We are powering the device from a 3.3Volt supply.  The data bus has nine AD7484
devices connected to it feeding a Microsemi  PRoAsic3E FPGA with its input
configured as 3v3 LVTTL which has a capacitance of 8pF.  The total capacitance
of the tracks will be 30pF or less.  Therefore the AD7484 data line will be
driving eight other tri-stated AD7484 data lines and the FPGA input and track
capacitance.

 

The NMOS device on the output driver is generally quite strong on our
converters and digital outputs are capable of sinking/sourcing a current well
above 200uA. Rise time and fall time is typically in range of few ns and gets
extended at low Vdrive and high external capacitance/load. But we did not
characterize the AD7484 driving high capacitance digital lines. As this is a
parallel part and the data rates are not very fast, we expect that the part
will hold up to driving up to 100pF. We put a digital buffer on the output of
the AD7484 on the eval board, you may consider doing that, too.

Please let me ask you the followings :

1. What value of Vdrive are you using
2. What is the load you expect the AD7484 outputs to see
3. What is the external capacitance on the board

It would be helpful to know the answers to these questions before we can come
back with a more definite answer.

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