AD9268 Input understanding

Document created by analog-archivist Employee on Feb 23, 2016
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I have an understanding issue with the Input stage of the AD9268.

I am fine with the differential generation of the Input Span Vpp of 2 V

Vcommon= 0.9V
Vin+ = 0,9V +-0,5V
Vin- = 0.9V -+0.5V
This generates a 2Vpp (0V – 2V) at the output of the Input amplifier –
according to the datasheet.

The part is supplied with 1.8V.

How could the Differential Amplifier generate the 2V Output while only supplied
with 1.8V
Do we havea RtR Output Stage or do we need to substrate more headroom (e.g.
-0.2 – 0.3V)
How could the ADC Core deal with 2Vpp while only supplied with 1.8V?


This is actually a common misunderstanding.  The inputs to the ADC are
differential and the 2Vpp refers to the differential level of the signal.  Each
pin would have a signal of 0.9V common mode +/– 0.5Vp for a 1Vpp level on each
input (Vin+ and Vin–) with the Vin– having the opposite polarity of Vin+.  When
you look at the differential level of the signal it is Vin+ – Vin– = 1Vpp – (–
1Vpp) = 2Vpp. 

The maximum input swing is 2Vpp.  If you attempt to drive the ADC beyond that
you will start clipping and you will push the ADC into a nonlinear region and
your harmonic distortion will increase very quickly.  You can adjust the
maximum signal swing via a SPI write (down to around 1Vpp I believe), but 2Vpp
is the maximum.