Analog Ground on a buffered R2R ladder DAC

[davemisbehave]

Hi!

I've built a simple R2R DAC using resistors, a rail-to-rail opamp and some logic. So far it's been working well on a single supply, but I want to improve upon the design by adding dedicated positive and negative power rails for the analog part of the circuit. I'm re-spinning the board and this time I also want to incorporate separate analog and digital grounds, as well as the analog split supply. I've added a simplified schematic that might help the following two questions I have:

1. Should the first resistor of the R2R ladder (R9 in my picture) be connected to digital ground or analog ground?

2. Should I connect the analog and digital grounds, and if yes, where? At the power supply (DP831A), or should the GND plane and AGND plane be connected somewhere?

Please see the attachment I included for a full understanding of the context.

Cheers,
Dave Misbehave

EDIT: R13 should be 10k (copy/paste error)! Thanks for pointing it out, nitro2k01!

[Christopher]

Resistors to digital ground, I think.

Two ground planes, joined at one spot with a 0r resistor.

Don't forget your decoupling.

I'd put a series current limit resistor on the opamp output after the feedback

[nitro2k01]

Maybe it's just an error in the schematic that is not reflected when you're building the circuit, but look at R13!

[tggzzz]

Two ground planes, joined at one spot with a 0r resistor.

Usually a mistake; see books/articles  and manufactuers' literature on low power mixed signal PCBs, e.g. for ADCs and DACs.

Much better to consider where the "return" currents flow, and to keep them separate, and to minimise the loop area for each individual contribution (analogue/digital, signal/power) to the total currents flowing.

[T3sl4co1l]

The grounds are one in the same.  Digital VCC should be filtered, or use analog.  Why?  Because it's NOT a digital supply -- you are using the gate output pins as one-bit DACs!  These are, in turn, weighted by the R2R divider.

Also, the output resistance of each pin is a factor in the weight of each, and variation in that resistance limits how stable a conversion you can produce here (over manufacturing and temperature variation).

Tim

[davemisbehave]

Thanks for the replies. So, my guesstimate would be to connect R9 to AGND and to connect AGND and GND at the power supply (not the planes). Is this correct, and if not why?

P.S.: R13 should be 10k (thx, nitro)

[T3sl4co1l]

Yes.  So it's the same ground plane everywhere.

Tim