Bug in LTC2400 SPI interface
Dan Michaels email (remove spam text)
Morgan Olsson wrote:
>The error occours when using the polled mode, i.e activating the CS (Chip
Select) to read the EOC flag (End Of Conversion) that LTC2400 then as
response outputs in the SDO pin, then if it is ready we read out the result,
if it is not we deactivate CS and later test again.
>It then, very rarely, and not in all implemetations, happen, that the 32
bit data comes in one bit too early. Very funny, eh?
I had a couple of other thoughts - that again may have no relationship
to your problem - but may give you some ideas. You may possibly have
a layout/grounding/ringing problem.
I have been using the LTC1404, 12-bit, 600ksps A/D. Like the LTC2400,
it has only 8-pins and does not have "separate" analog and digital gnd
pins. I use it on a pcb where it is driven alternately by a PIC and
Scenix. The SX can run the chip at max SPI rate of 6.5 Mhz, while
the PIC cannot. Unfortunately, the SX tracks are about 2.5" [7 cm]
long - I cannot get the layout any better.
Well, on "some" pcbs, SX control has been a little flaky. I suspected
ringing, but every time I put the scope probe on an A/D pin, the problem
would disappear. I found several ways to deal with it, but each "fix"
would work on some pcbs only, but not on others:
a) RC filter in the sclk line at the A/D - 330 ohm, 27 pF.
b) 2.2K termination on the conv line at the A/D.
c) placing finger on the A/D SPI pins - [not field-installable].
Since I have separate analog and digital supplies and gnds on the
pcb, single-point-connected back at the power entry point, and
since the LTC1404 does not have separate analog and digital gnd
pins, I thought there may be large inductive loops related to the
SPI tracks, so I also tried:
d) locating the single-point analog-digital gnd connection directly
under the A/D, rather than at power entry.
You are probably running a much slower clock than I am, but maybe
my experiences will give you some ideas.
[BTW, I just figured out what the problem with "my" pcb is - very
embarassing too - at least this thread helped one of us].
- Dan Michaels
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