Q
A customer wanted to use a AD8075 to drive eight AD8075 for video applicationand he did not want to use a crosspoint switch because of cost. Can one AD8075
drive eight AD8075 and what should he pay attention to for such application.
A
Two main things the customer should pay attention to are (1) Input Capacitanceof the eight AD8075 and (2) Input bias of currents of the eight AD8075.
As shown attachment in the typical performance curve, driving capacitive loads
directly, introduces peaking into the frequency response of the AD8075 which
may lead to a stability issue. To avoid this make sure to add a resistor in
series with the output of the driving AD8075. Please also see page 9 of the
datasheet for some additional information regarding this.
With a series resistor in place now the input bias current of the eight AD8075
comes into play. As that current and the series resistor creates an offset
error voltage directly in the video signal path.(It is very small and should
not a problem, but should be noted)
It has been mentioned in passing that the primary cause of overshoot for the
AD8074 and AD8075 is the presence of large reactive loads at the output. If the
system exhibits excessive ringing while settling, a 10ohms to 50ohms series
resistor may be used at the output to isolate the emitter-follower output
buffer from the reactive load. If the output exhibits an overdamped response,
the system designer may add a few pF shunt capacitance at the output to tune
for a faster edge transition. A system with a small degree of overshoot will
settle faster than an overdamped system.
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