preamp ASIC linearity

[Gerard Visser <gvisser@indiana.edu>]

Hi Mitch,
	I have a small question, wanting to make sure that I am interpreting 
your plots correctly. If I look at the linearity plot (page 18 in
http://www.hep.upenn.edu/HEP_INST/INST_Projects/GLUeX/Design_Status/Nov_16_TSMC_Implentation/Shaper-Line%20Driver%20Design_version1.ppt
)
it shows on the X-axis the test pulse amplitude and the Y-axis the line 
driver output peak-peak, correct? (Axis labels are crossed?)
	And more importantly, the Y-axis peak-peak amplitude is starting from 
the offset baseline of about -425mV differential like on p.17, is that 
correct.? So when you have peak-peak pulse amplitude of 1V on p.18 it 
means the differential output voltage is swinging from -425 mV up to 
+575 mV. And the output linearity curve is probably symmetrical from 0 
Vpp up to 850 mVpp, correct? (I mean, an inflection point is found in 
the middle of this curve at 425 mVpp, and there is a bending over at the 
bottom end around 0 comparable to the bending over at the top end around 
850 mVpp.) What is the nonlinearity in this portion of the curve? It 
looks by eye like it might be a few %? I think we would like to have 
more data on this if your guys have some time to run the simulations.
	If the line is double-terminated so that each leg of the output sees a 
31 (rather than 62) Ohm load to +1.25 V, the linearity will presumably 
be improved? I guess I am asking, are the output transistors the 
dominant nonlinearity here? Or something else?
	Would an NMOS-only output (with common-mode termination to vdd instead 
of 1.25V) be more linear? Obviously it would allow VDS to be higher for 
the same swing on the outputs...

	Gerard

p.s. I would think we might well want to double terminate the lines in 
the experiment. I propose to provide the common-mode termination on the 
preamp board (since the common-mode range of the output is rather 
small), and receiver the lines differentially on the ADC board with a 
high-common-mode rejection high-common-mode impedance device like 
AD8129. So, perhaps your guys can also run some simulations with 31 Ohm 
loading on each output.