In article ,
Andrew Haley wrote:
Digital Audio receiver ICs like DIR9001 and CS8414 mention the diff
voltage level at their input pins RXP and RXN as 200mv min while the
Digital Audio Transsmitters like DIT4096 and CS8406 use 5V logic
levels. How are they compatible with wide difference in levels? I am
looking at using them for SPDIF transmission and reception.
For standard SPDIF you'd normally use a voltage divider and an
isolation transformer on the output side. Look at 15.2, Isolating
Transformer Requirements, in the CS8406 datasheet.
Or, more simply, just use a moderate-value coupling capacitor between
the transmitting IC and your SPDIF-out jack. The presence of the
receiver's termination resistance will pull the average value of at
the jack down to 0 volts, and so you'll end up with a voltage at the
receiver which swings both positive and negative. This lets you
simply ground one of the two differential-input pins, and feed the
other from the signal.
The receiver will probably be able to handle a +/- 2.5 volt swing
without difficulty. If you want to drop it down to +/- 1 volt or so,
you don't need a full voltage divider at the transmitter... you can
use the 75-ohm coax characteristic impedance / termination load as
half of it, and so all you need is a series resistor.
So... quick-and-dirty approach would be to go from the transmitting
IC, through a 75-ohm-or-so buildout / padding resistor, through a .1
uF cap, and out to the jack. At the receiving end, go through another
..1 uF cap, into one differential-input pin (with a 75-ohm resistor to
ground) and ground the other differential input pin.
There are, I agree, good reasons to use a proper SPDIF isolation
transformer at each end... a very good idea if your sending and
receiving devices are any great distance apart, might be on different
power circuits with different ground potentials, or are in high-RF
areas. Many less-expensive devices don't bother (just use capacitive
isolation) and work well under less-demanding conditions.
--
Dave Platt AE6EO
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