AD9717 and AD9117 CML Pins

What is the purpose of the CMLI and CMLQ pins on the AD9717 and AD9117 DAC famlies and where do they need to be connected?

  • The CMLI and CMLQ pins are used to adjust the common level. When interfacing the AD9117,6,5,4 and AD9717,6,5,4 DAC famlies to some quadrature modulators or some amplifiers you may not be able to obtain the right common mode voltage level with just addition of load resistors on the DAC outputs. In this case the on chip common mode resitors can be used to raise the common mode voltage level.The CMLI and CMLQ pins should be left unconnected when the on chip common mode resistors are used by setting bit 7 to "1"  in registers 0x05 and 0x07. The common mode level can be shifted up to 1.2V but this is when operating with lower output currents and requires using 3.3v for AVDD.

          If the on chip common mode resistors are not needed then set  bit 7 to  "0" in  registers 0x05 and 0x07. The Functional block diagram does not show all the internal circuit connections to the CML pins. When the on chip common mode resistors are not use the CML pins must connect  to AVSS through low value resistors (typcially 0 ohms)  or the the part will not operate properly.