# AD8138 - Vocm RTI noise

Question asked by ddurachk on Mar 21, 2016
Latest reply on Jun 28, 2017 by daverowe

Question on the AD8138 input referred noise of the Vocm input noise specification:

The datasheet lists this as 17nV/rtHz, which is more than 3 times that of what the datasheet claims in the features.  Does this noise always apply and scale with BW and gain?  I am using this device to convert a single ended signal to differential, the Vocm pin will be held at 1.2V.  Graphs shown in Figure 37 and 38 show the voltage and current noise (RTI), the broadband voltage noise isn't anywhere close to 17nV.  If the 17nV/rtHz always applies, these graphs are very misleading.

As an example, say we have a case where the gain is unity, and the signal bandwidth is 100MHz.  The noise bandwidth is ~ 50MHz.  The broadband voltage noise at the output (just considering voltage RTI noise, neglect current noise and thermal resistor noise)

RTO Vn = 5.7nV/rtHz (50MHz)^.5 = 40.3uV
RTO Vocm = 17nV/rtHz (50MHz)^.5 = 120.2uV

Total RTO = [(40.3uV)^2 + (120uV)^2]^.5 = 127.8uV