[sdiy] Guard Rings

[Oren Leavitt]

On 11/4/2010 10:58 PM, David Ingebretsen wrote:
> Hi all,
>
> I'm still progressing and working on the Steiner Synthasystem.
>
> I have a question. I am laying out the Dual Sample&  Hold/Random Voltage
> module schematic here:
>
> http://www.xmission.com/~dingebre/SP_Samp_Hold_1.0.pdf
>
> A current PCB layout closeup showing the guard ring such as it is, is here:
>
> http://www.xmission.com/~dingebre/PCB_part.jpg
>
> In conversations with Nyle, he recommends putting a guard ring around the
> high impedance sections and tying it to the output of the voltage follower,
> but I'm struggling trying to figure out what these are.
>
> I've decided it should include the nodes/pads which connect to the two FETs
> including a couple of resistor pads and the one capacitor pad.
>
> The ring shows as a red line tracing close to the pads of the FETs, cap,
> resistor, etc. The guard ring is a trace on both sides of the PCB. It starts
> and ends on the emitter of the voltage follower which (I think) is Q9. I
> excluded any pads going to ground or power.
>
> First question, is this correct? I put a dashed line around the pads I'm
> including in the ring on the schematic.
>
> Second, I've read somewhat about guard rings but would love a link to other
> information so I can understand this so I make an intelligent PCB.
>
> Thanks everyone.
>
> David
>

David,

Looking at the pdf, you'll want to place the guard ring around the 
traces connecting (Q11-S, C5, R13) and (R13, Q7-G). That's your "island" 
of high impedance while in "hold" mode. The ring is connected the output 
(emitter) of the emitter-follower Q9 which will be at the same potential 
but buffered.
Ditto for Q12, C14, R6, Q8.

- Oren