AD8152: Connection of DC inputs

Document created by analog-archivist Employee on Feb 23, 2016
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I am running the AD8152 on VEE=0, VCC=+2.5V  and would like to drive the inputs
of the inputs, DC couple, from LVPECL with a common mode range of  +1.5V to
+2.3V. Can I connect VTTI  to ground via a (say 10nF) capacitor?   VTTI would
then end up at about +1.9V

This seems to be within the allowed range of +0.8 to VCC but I need to be sure
this arrangement will not cause any problems.

 

The situation that you describe will be OK as far as the dc levels are
concerned.

One thing that you don't mention is pull-down resistors. I don't know if this
was implied, or this is something you overlooked.

PECL outputs are open emitter. If these are connected only to the AD8152
inputs, then only the input-bias current will flow (~2 uA). This is not nearly
enough to turn on the output transistors enough.

Thus, pull-down resistors are required. There are several ways to provide these.

The termination resistors on the AD8152 will provide the proper termination for
its end of the line. At the PECL drive end, the impedance will be low, because
effectively the line ends with the emitters of the output. When these are
running with their nominal dc bias current (10mA to 15 mA), then their
impedance will be only a few ohms. This low value resistance will dominate. The
pull-down resistor will have very little effect on the termination resistance
at the PECL end.

For a reference point, if the dc voltage is about 1.9V and it is desired to
have 10 mA out of the PECL emitters, then a pull-down resistor of 190 ohms to
ground will work fine. This value is not too critical.

The exact value of the capacitor used between VTTI and ground is not too
important. 1 nF should be fine. I suggest, however, that you use one capacitor
for each VTTI pin of the AD8152.

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