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I'm trying to understand the datasheet for the AD5535B DAC. I think I can understand why there would be a DVcc (digital Vcc) and AVcc (analog Vcc) to prevent digital switching noise to enter the analog circuitry and to allow maybe 3.3V digital signals. In addition, there is a high voltage input Vpp. However, on the analog side we find both an AVcc and V+. They have the exact same voltage ratings. The datasheet has only this to say,

If the number of supplies must be reduced, bring all supplies back separately and make a provision on the board via a link option to drive the AVCC and V+ pins from the same supply.

I guess this means that they recommend to still have separate power supply connections to the eventual PCB, but allow them to be tied together if needed. But that still gets me no closer to why one would want them separate at all.

What is the explanation for these two separate supplies? Why should I bother? The manual for the evaluation board just specifies to use a single power supply for V+ and AVcc.

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  • \$\begingroup\$ That's an odd chip for a special use like driving a bunch of MEMS mirrors at 50-200 volts. You might want to read the supporting materials, study the reference design, etc and see if that offers clues to what parts of the internal functionality each supply powers beyond the minimal information in the data sheet. \$\endgroup\$ Sep 22 '20 at 15:27
  • \$\begingroup\$ @ChrisStratton I happen to want to drive a large bunch of piezo-electric actuators so it actually seems ideal for the job except for one or two questions which I'm trying to figure out now (I'm a mechanical engineer so I'm a little bit out of my depth here). \$\endgroup\$
    – Sanchises
    Sep 22 '20 at 15:30
  • \$\begingroup\$ Definitely, definitely study the reference design then \$\endgroup\$ Sep 22 '20 at 15:31
  • \$\begingroup\$ @ChrisStratton They have an evaluation board which tells you to just connect everything to a single 5V supply. The application example doesn't even mention AVcc. \$\endgroup\$
    – Sanchises
    Sep 22 '20 at 15:38
  • \$\begingroup\$ The EVAL-AD5535EB has no less than seven banana jacks for the power connection, there's a reason for that and it would be worth finding the manual. Connecting everything to a 5v supply won't even work as the minimum for the Vpp input is 50 volts. \$\endgroup\$ Sep 22 '20 at 15:41
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AVcc supplies the DACs, V+ supplies the output drivers.

AVcc is supposed to be very stable, and power draw there should be minimal, while a bit of fluctuation on V+ is inconsequential because the driver stages correct for it -- but this supply needs to be able to supply more current.

If you tie them together, loading one output will slightly change the DAC value on the others, so you get a bit of coupling between outputs through there.

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  • \$\begingroup\$ Actually Vpp supplies the 50-200 volt output drivers, making it a bit less clear what V+ does. Perhaps some intermediate stage. \$\endgroup\$ Sep 22 '20 at 15:32
  • \$\begingroup\$ Ah, then I have misread the datasheet \$\endgroup\$ Sep 22 '20 at 18:19

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