HMC675LC3C VCCO and VEE

why in data sheet's characterization  the vcco is 0V for the output stage while in pin configuration Vcco is positive supply and what is at emitter side Vee or ground for Q and Q bar interface.

hmc675lc3c.pdf
  • Hi Salman,

    Responses are given below;

    why in data sheet's characterization  the vcco is 0V for the output stage while in pin configuration Vcco is positive supply

    Answer: Vcco controls the output common mode level of the CML output and it can typically be adjusted anywhere between 0 and 2V depending on the users need.  The output common mode level will be approximately Vout_cm= (Vcco – 0.15V).

    what is at emitter side Vee or ground for Q and Q bar interface.

    Answer: We are not sure of exactly what is being asked here.  If the question is about the output transistor emitter voltage then the answer is that it is neither ground or Vee.  The output transistor emitters are driven by a current sink. The output common mode level of this CML output is as described above.  The negative rail for the output stage current sink  is Vee but the output voltage is relatively independent of Vee.  The output voltage varies from Vcco (logic high) to approximately (Vcco-0.3V) (logic low).  Normal circuit operation requires Vee=-3V.  Looking back into the outputs, the equivalent circuit looks like a 50 ohm resistor connected back to Vcco with either current flowing through it (pulled by the transistors for logic low) or no current flowing through it (logic high).  For further information the customer may want to consult a  tutorial on current mode logic (CML), available at many sites on the web. 


    Thanks

    Ahmet