Stability, Gain, Impedance and Trace to Trace Coupling

[YourGuitarist]

I have been turning my attention in layout to fighting instability.

I've read the bulk of routine talking points about tight NFB routing, impedance scaling, separating GND planes (divisive in modern day), star grounding etc etc.

The question I have is have you found empirically a general rule of thumb for how close nets can get?

I know that there are multiple factors here:

• Net impedances
• Inter-net gain
• PCB stackup
• Inter-net phase

In my personal work, I have found that somewhere in the 40-60dB gain range, I start getting ringing. I *think* my victim nets are on average 3-5mm distance from the offenders. Obviously, I'm focusing on increasing that.

Really just want to open a discussion.

 

[vigilante398]

I have been turning my attention in layout to fighting instability.

I've read the bulk of routine talking points about tight NFB routing, impedance scaling, separating GND planes (divisive in modern day), star grounding etc etc.

The question I have is have you found empirically a general rule of thumb for how close nets can get?

I know that there are multiple factors here:

• Net impedances
• Inter-net gain
• PCB stackup
• Inter-net phase

In my personal work, I have found that somewhere in the 40-60dB gain range, I start getting ringing. I *think* my victim nets are on average 3-5mm distance from the offenders. Obviously, I'm focusing on increasing that.

Really just want to open a discussion.

I don't do a lot of high-gain work so it's not as critical, but I have found that even in medium-gain work it is still something you need to keep in mind. I read some guidelines years ago that someone on DIYSB (I think it may have been RG Keen?) wrote regarding the ratio of trace spacing and gain levels, but typically "in the industry" we try to follow the 3W Rule, which is that trace spacing should be 3x the width of the traces, i.e. 10mil traces should get 30mil spacing. This is EXTREMELY conservative for things like pedals where we're in the audio frequency spectrum, but if you're trying to be on the safe side I generally recommend spacing be at least double the trace width.

For low gain things that don't have any particularly sensitive signals on them I generally just space things "as far as I reasonably can", and that tends to go pretty well.

One thing that I will also mention though, it helps to keep in mind which signals are AC and which are DC. AC signals need to have a DC reference, which could be ground, VCC, Vref, or whatever else you have around, and they don't (except in special circumstances) need to be spaced super far from DC signals. So I wouldn't feel the need to keep my audio signal traces spaced 30 mils from my Vref trace.

 

[YourGuitarist]

I was gonna say just that about DC, I know there is concern about supply noise on DC traces however I'm fairly convinced in most circumstances that is well enough filtered to be a non issue.

I'm sure in your world voltage clearance is the bigger concern.

 

[vigilante398]

I was gonna say just that about DC, I know there is concern about supply noise on DC traces however I'm fairly convinced in most circumstances that is well enough filtered to be a non issue.

I'm sure in your world voltage clearance is the bigger concern.

Honestly not really. High voltage arcing is a thing, but even conservative spacing for high voltages only requires 30mil spacing. I also mitigate that by avoiding power traces, I use 4-layer board so the HV is all on an internal layer, which has a very favorable dielectric breakdown so I don't have to worry as much about it.

There are always rules you have to keep in mind, but there are often tricks you can employ to get around the rules.

 

[YourGuitarist]

Honestly not really. High voltage arcing is a thing, but even conservative spacing for high voltages only requires 30mil spacing. I also mitigate that by avoiding power traces, I use 4-layer board so the HV is all on an internal layer, which has a very favorable dielectric breakdown so I don't have to worry as much about it.

There are always rules you have to keep in mind, but there are often tricks you can employ to get around the rules.

I didn't know about using 4 layer for HV. That's a cool trick!