depend how do you terminate the, cable if the cable's impedance is Z,
use two terminating resistors each R =2Z, one is connected to thee
ground the other is connected to the supply voltage of the receiving
chip, that way although the cables input and output termination will
"eat up" half of the signal voltage [since the two resistors at the end
of the cable are for AC parallel and the two 2Z parallel is =Z ], it
will be still enough since the left over 2,5V level change will be
centered at the threshold of the input device...
73
Alex
On 9/15/2014 2:48 PM, Bob Camp wrote:
Hi
These are saturated logic signals. When you terminate both source and load you
get an interesting issue with compatible logic levels.
For instance: 5V CMOS switches at roughly 2.5V. If you series terminate and
load terminate, your destination now sees a 0 to 2.5V signal. Either it’s
running 2.5V CMOS and switching at 1.25V or you have a problem.
Bob
On Sep 15, 2014, at 9:03 AM, Dave Martindale <[email protected]> wrote:
Is there any reason (other than cost) not to both series-terminate the source
and parallel-terminate the sink?
When I was dealing with analog video, the standard distribution method was :
1. Buffer amplifier with high input impedance, very low output impedance, and a
gain of 2 (so 1 V P-P input becomes 2 V P-P out)
2. A series 75 ohm resistor from the amp output to each individual video
output. This formed a 2:1 voltage divider with the 75 ohm coax to give 1 V P-P
on the cable. It also isolates the loads from each other.
3. A single video signal could be looped through multiple high impedance loads.
4. 75 ohm parallel termination at the far end of the signal path (usually on
the last device).
This way, every device along the way saw an undistorted copy of the signal.
The buffer amplifier sees a simple resistive load. And any reflections are
absorbed at both ends of the cable.
- Dave
On 15/09/2014 02:04, Fuqua, Bill L wrote:
A lot of devices have a low output impedance so that the signal can be split
using a TEE adapter with little loss or need for a distribution amplifier.
However, the cables must be impedance matched at far end, scope input, to
prevent reflections which are the source of the ringing.
You can match the impedance at the source and you will get a reflection which
will then be absorbed by the source resistance. One way to do this
is to get a small 15 turn pot about 100 Ohms put it, in series with the input
source and adjust it until the ringing is gone or you can put it at the far end
,input of the scope, to ground and do the same. But the best solution is to
get a good feed thru 50 Ohm terminator and put it on the input of the scope.
Bill
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