The TADD-3 output circuit with 3 74AC04 output stages paralleled is
stolen from Tom Clark's design in the original TAC GPS interface board.
The 47 ohm resistors aren't intended as back terminations; if you
assume a low output impedance at the chip, the three resistors are
effectively in parallel, so more together they are more like 16 ohms
than 50.
The value was a compromise between output voltage swing (trying to make
sure that TTL levels are maintained at the far end of the cable) and
protection against one '04 stage backfeeding another if their switching
times aren't identical. There's been discussion that lower values,
maybe 27 or 33 ohms, might be OK and would deliver more swing at the far
end.
Bruce is correct about the crosstalk problem -- in a desire to provide
lots of flexibility, I designed the TADD-3 so that each of the six
output channels could be jumper-set to a different pulse rate. Each
output uses 1/2 of a 74AC04, and between trace radiation and Vcc bounce,
there is cross-mod if two channels sharing an output chip are set to
different rates. The simple answer is to keep each pair (channels 1 and
2, 3 and 4, 5 and 6) set to the same rate.
John
----
Bruce Griffiths said the following on 01/10/2012 03:57 AM:
The TADD-3 uses 3 AC drivers in parallel, each going through a 51 ohm
resistor. Changing those resistors to 150 ohms should work. Maybe a
bit lower to account for the impedance in the drivers. I'd probably
check it with a scope.
That approach doesn't do anything for the Vcc and GND bounce exhibited
by the driver chip.
GND and Vcc bounce is the cause of the high frequency ringing exhibited
by the TADD-3 outputs.
This ringing can even be observed at the outputs of inverters whose
inputs are tied low or high in the same package
Damping the crossover current induced transient in the supply leads
(bondwire and lead frame) inductance is one way to minimise this.
A small resistor in series with the Vcc pin often works well, the
resistor value being chosen for near critical damping.
Another problem with the TADD-3 is the sharing of a driver chip by
different input frequencies which leads to intermodulation between the 2
outputs.
Bruce
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