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LMH6881 OCM vs. Vcm

Other Parts Discussed in Thread: LMH6881

Dear Sir,

May I ask what is Vcm (V) when OCM=1V? And what is Vcm when OCM=1.5V?

LMH6881 Datasheet has only example of OCM=1.25V (Vcm=2.5V). 

<Background of question>

The DS says;

" There is a DC gain of 2 between the OCM pin and the output pins so that the OCM voltage should be from 1 V
to 1.5 V. This will set the output common mode voltage from 2 V to 3 V. Output common mode voltages outside
the recommended range will exhibit poor voltage swing and distortion performance. The amplifier will give
optimum performance when the output common mode is set to half of the supply voltage (2.5 V or 1.25 V at the
OCM pin)."

It is not clear from which GND or Vcc the output common mode (Vcm) is defined?

(a) If Vcm is defined from Vdd(5V),

Vcm=2V when OCM=1.5V    ..(5-2)/2

Vcm=2.5V when OCM=1.25V  .. (5-2.5)/2

Vcm=3V when OCM=1V(5V .. (5-3)/2

(b) If Vcm is defined from GND(0V)

Vcm=2V when OCM=1V   .. (2-0)/2

Vcm=2.5V when OCM=1.25V  .. (2.5-0)/2

Vcm=3V when OCM=1.5V   .. (3-0)/2

Which (a) or (b) is correct?

Best regards,

Mssa