Mixed signal - How route ground (return path) for analog switch digital control lines

[azone2]

I'll try to describe my question as simply and precisely as I can.

• I have a mixed signal audio design consisting of analog audio circuits controlled by a microcontroller and analog switches.
• The analog switches control fixed routing as well as some slow speed gating and switching stuff.
• The analog switches must be on the analog board.
• The microcontroller resides on a separate digital board.
• The analog switches are grounded to analog ground on the analog board.

My direct question concerns the control lines from the microcontroller that will control the analog switches on the analog board.
If I'm running the control lines from the digital board to the analog board, shouldn't there be a ground return path from the analog board to the digital board for each or all of these control lines?
Otherwise, if I don't give the control lines some sort of return path, won't they interfere/spike analog ground as they fight their way back to 'see' digtial ground and possibly result in audible spikes in the analog circuitry?
Is the control input on a typical analog switch electrically isolated? (part spec'd in this design is DG409).

 

[crutschow]

Yes you need a return path from the digital to the analog circuit.

Mux switches aren't normally isolated. If you need isolation, you would add opto couplers to the digital lines.

Are the analog and digital grounds connected together at the power supply? Is there a separate analog and digital power supply?

Ideally the analog and digital grounds should be connected together at only one point, that is at the mux ground.

 

[azone2]

Are the analog and digital grounds connected together at the power supply? Is there a separate analog and digital power supply?

Yes, the main power supply is located at one end of the digital board. There will be a ground connection from the analog board to the digital/IO board at the power supply.

Ideally the analog and digital grounds should be connected together at only one point, that is at the mux ground.

So grounds could be connected close to where ground is to the analog switches? and then a single ground to the power supply from here? Just to note there are about (12) analog switch IC's in total, so there would be some foresight requred to lay them out in orderly fashion, possibly near each other.

As an alternative, assuming the control lines are connected from the digital board to the analog board using a ribbon cable, is it nonsense to have extra ground wire(s) for one or more control lines running alongside the control lines of the ribbon cable that connected to the ground of the analog switch(es) (and digital ground at other end)?

 

[crutschow]

Can you remove the connection between the analog and digital grounds at the power supply and use only the connection between the digital and analog grounds at the mux? That would be better. But only if there's a separate isolated analog and digital power supply. It's more difficult to keep the digital noise out of the analog circuits with only one supply.

Running the ground wires in the same ribbon cable as the control lines is actually preferred. You always want the signals and associated grounds to be in close proximity. especially for signals that are high frequency and/or have fast transition times.

The purpose of all this is to minimize any digital return currents from going through the analog ground. Will you have a ground plane for all the grounds?

 

[azone2]

Can you remove the connection between the analog and digital grounds at the power supply and use only the connection between the digital and analog grounds at the mux?

The schematic is captured but the layout is not started yet so yes options are open. The analog and digital have their own power supplies. The digital has it's own 5V supply. The analog has +/-12V as well as a 5V supply. The 5V supply is for logic level threshold for the anlalog switches, the DG409 allows you to use 0/5V control logic from a micro to switch up to +/-20V analog signals by having 5V at the enable input.

The purpose of all this is to minimize any digital return currents from going through the analog ground. Will you have a ground plane for all the grounds?

Yes there will be ground planes. Both board will have a ground plane. The digital board also contains the raw 12VDC power in so will have the main power ground as well.

A brief description of the power supply design.

• DC Input is regulated to 5V for digital (local to digital/psu board.. the microcontroller is on the same board).
• DC Input is regulated to 5V (separately) for analog on the digital/psu board, on the edge where power input comes in.
• The regulated 5V for analog goes to the analog board where it is boosted to +/-13V using DC-DC converters (TI TPS63700 & TPS5430).
• +/-13V is then regulated to +/-12V w/ LDO regulators. These are the supply rails for the analog circuitry. The 5V is for analog switch enable pins.
• Currently the ground between the digital/psu board and the analog board is carried in a connector along with the regulated 5V, so it goes from the main power supply section of the digital/psu board to the local analog power supply generation section of the analog board.

From what you're suggesting it sounds like I may want to carry all the control signals on the same ribbon cable, have the ribbon cable connector somewhere in the middle of all the analog switches, and then have the ground between the two boards pass along this same ribbon cable? In this case the other ground between the two power supply sections would be eliminated, and since there's a ground plane on the analog board anyway it shouldn't be a big deal to the analog circuit exactly where the ground comes in?

 

[crutschow]

.........................................

From what you're suggesting it sounds like I may want to carry all the control signals on the same ribbon cable, have the ribbon cable connector somewhere in the middle of all the analog switches, and then have the ground between the two boards pass along this same ribbon cable? In this case the other ground between the two power supply sections would be eliminated, and since there's a ground plane on the analog board anyway it shouldn't be a big deal to the analog circuit exactly where the ground comes in?

It may not be a big deal but it does matter where you make the analog-digital ground connection. As I said, you want to minimize the flow of digital ground current in the analog ground (even if it is low impedance ground plane) so you want the digital ground connection to be made at the mux ground connections. You may want to make a separate wide ground trace for all the mux grounds and connect that to the digital ground at one end of that trace and connect to the analog ground plane at the other end of the trace.

Think about where the ground currents for both the analog and digital sections have to go. You want to keep those paths separate and as short as practical. Note that for every signal current there is a corresponding ground current that needs to return from the signal destination to the signal source.

 

[azone2]

Thanks for the insight. You're suggestions are exactly along the lines of what I was thinking and confirm why I asked the question. At the end of the day the switching is not high speed, and a lot of it is just one-time source selection, but it is a musical instrument and there is some real-time gating being done as well in the ms range. I don't want any spiking/clicking in the audio path. I've been burned before designing switching for little iphone docking stations and stuff.

Interestingly enough if you reasearch mixed-signal ground design and isolation application notes, research papers, advice etc.. they always talk about DACs, and about splitting the ground right at the DAC. I've never found any notes specifically regarding the treatment of control signals and ground to analog switches. It's so common in every consumer electronic piece, phone, docking gadget, everything... I'm just surprised not to find more info.

 

[crutschow]

If the analog switch is only switched infrequently then any noise from that switching is not likely to cause serious interference (perhaps just a small "click"). It's when you start switching at higher rates that the noise can become more intrusive. DACs are more likely to be switched at a high speed (such as converting digital audio to an analog output) so the grounding treatment for them is of greater concern.

 

[azone2]

yes true, for most application of an analog switch it's not real-time or high speed and you can reduce the clicks good enough by damping the transients or whatever. In my case some of the control is triggering control circuitry in the analog domain in-time with the tempo of music, so audible clicking is unacceptable. Even for infrequent source selection type stuff clicks aren't acceptable during realtime play. At the end of the day I guess what we're getting at is my application is unique. Thanks for the input.