The data sheet for the AD9910 indicates that pin 6 is AVDD, but it "can be grouped together with the DVDD 1.8 V supply". So should it be connected to AVDD or DVDD? What part of the chip does it feed?
The data sheet described that there are four AVDD(1.8V) pins: 3, 6, 89, 92. Although pin 6 is an AVDD(1.8V) pin the data sheet doesn't recommend to group it together with the other AVDD(1.8V) pins namely; 3, 89, 92. It may sound weird but it follows that pin 6 should be grouped up with DVDD(1.8) not with AVDD(1.8V). Pin 3, 89, and 92 are isolated because the noise within these supplies can directly impact the performance of the device.
AVDD goes to the Analog core while DVDD feeds the Digital core. AVDD(1.8V) and DVDD(1.8V) can be grouped together since they are of the same potential. But I advise to use a separate regulators on each supply pin if you want to optimize the performance of the device because its best if you have a significant isolation between AVDD and DVDD supplies.
I understood the difference between AVDD and DVDD and the need for isolation. My confusion has to do with the data sheet statement about pin 6, which is said to be AVDD, but "can be grouped together with the DVDD 1.8 V supply". This would seem to violate the separation between AVDD and DVDD.
Focusing on the pin 6 statement, it is true that you can group them together but the data sheet also recommends that "a ferrite bead should be used for isolation, with a separate regulator being ideal". So either you group them together, resort to a ferrite bead, or isolate them with a separate regulator would depend on the specifications of your circuit. If your application can afford to have regulators on each critical supply pins, then it is best to follow it that way, however it would be more costly.
OK. I understand about feeding pin 6 separately for best performance. But if pin 6 is "grouped together" with some other pins, I still don't understand if it should be connected to +1.8AVDD or +1.8DVDD.
Putting it another way, if my design has two separate regulators for +1.8AVDD and +1.8DVDD, which one should I connect to pin 6?
The data sheet is not helpful. The pin is listed in Table 3 of the data sheet as AVDD. But on page 47 of the same data sheet, it says to connect pin 6 to DVDD! Which one is correct?
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