# Breadboarding digital mixer: deriving Vref from Vdda, and capacitor between Vdda and Vssa?

I am assembling a digital audio mixer from an old IC I have on my workbench. And I am reading the data sheet and trying to make sense of a note that describes the Vref pin (reference voltage). The note says that Vref should be .44 of Vdda (analog power) and that I should connect the Vdda to Vssa with a 1000pf capacitor.

So I have drawn up a schematic and am wondering three things:

1) How to get .44 of Vdda - A voltage divider and or suggestions?

2) Is the schematic correct in that I place 1000pf capacitor across Vdda and Vssa?

3) One of the comments for this IC / digital mixer chip is that I should keep all analog ground contacts separate from any digital ground contacts?

Does that imply that I keep the analog and digital power supplies separate as well?

Here is a url to the ics2101 datasheet ... https://stuff.mit.edu/afs/sipb/contrib/doc/specs/ic/audio/ics2101.pdf

simulate this circuit – Schematic created using CircuitLab

• So Vref is an output? And I can leave that unconnected? It is listed in the datasheet as a Power pin, along with Vdda, Vssa, Vddd, Vssd Commented Dec 1, 2014 at 23:53
• The block diagram shows an arrow pointing in from the outside, so I assume from that it's a power input pin, so you need to provide the 0.44Vdda voltage. Commented Dec 1, 2014 at 23:55
• @Majenko : you are correct, Vref is an input, my mistake. It must be set to that value to define the DC output level (and all the analog inputs must be AC coupled).
– user16324
Commented Dec 2, 2014 at 0:45

1) How to get .44 of Vdda - A voltage divider and or suggestions?

Yes, a voltage divider should do fine. It'll be to set the virtual ground point of the op-amps, and when using discrete op-amps I use a voltage divider for just that reason. It'll be 0.44 because the op-amps may not be rail-to-rail, so shift it down slightly to the mid point of the active available region.

2) Is the schematic correct in that I place 1000pf capacitor across Vdda and Vssa?

Yes, that is correct. Basic decoupling capacitor. You can add others too, maybe 100nF and 1µF. You should add some decoupling to the Vddd/Vssd as well.

3) One of the comments for this IC / digital mixer chip is that I should keep all analog ground contacts separate from any digital ground contacts?

Does that imply that I keep the analog and digital power supplies separate as well?

No. You should still connect both grounds together, but do so only at one point, and maybe with a small ferrite bead between them. Both power supplies can be the same power supply (and should both be 5V anyway), but again you can separate the Vddd and Vdda with a ferrite bead.

• Okay, I can add a decoupling capacitor across Vddd/Vssd, I am assuming the same value as for the analog source - 1000pF. Commented Dec 1, 2014 at 23:46
• One question: you recommend adding a 100nF and 1µF as well. Where are you suggesting to place those? Commented Dec 1, 2014 at 23:48
• Next to the 1nF. Smallest value closes to the chip's pins. For digital I would use just 100nF Commented Dec 1, 2014 at 23:54