[sdiy] Buffer feedback loop vs. PCB layout

[rburnett at richieburnett.co.uk]

Agree with everything Neil said.  Just put a low value resistor at the 
"drive" end of the "long trace" to decouple the op-amp from any unwanted 
capacitive loading from this connection.

If the trace is only a few cm's long then the capacitive loading is 
going to be minimal unless it's a massive thick trace over a 
ground-plane or you're working at MHz frequencies.  So, my bet is that 
your op-amp won't complain if it's a relatively slow audio op-amp, but 
if in doubt put the resistor in place anyway to isolate it's output from 
the capacitance of the connection, and sleep soundly in your bed knowing 
that the op-amp won't become a radio transmitter ;-)

-Richie,


On 2016-10-07 14:35, Justin Owen wrote:
> Apologies for the basics/fundamentals question - I did ask this on
> another forum but I didn't get a great answer (possibly down to my
> original explanation...) - so I thought I'd try here as well...
> 
> I have a PCB layout which comprises 2x different boards connected by a
> hardware connection.
> 
> http://www.sdiy.org/juz/feedback_loop_01.png
> 
> 1) This simplified diagram represents the physical layout. A buffer
> output on PCB1 connects to an inverting op-amp input on PCB2 via an
> inter-board connector (the blue trace).
> 
> 2) Is essentially what the circuit 'sees' - a long trace from the
> output of the buffer to the input of the inverting op-amp.
> 
> 3) This is what I'm worried about though - as far as the circuit is
> concerned - it's not just one trace from output to input - it's also -
> potentially(?) -  a long, extended feedback path/loop that travels all
> the way from PCB1 to PCB2 and back again.
> 
> That strikes me as not a very good PCB layout decision - it's
> basically a trace that is going to be way longer than it needs to be
> (OK - we're talking cm IRL - but still). Is that a correct assessment
> in regards to the layout?
> 
> So - my questions is - if this is correct and is potentially a problem
> - how do I solve it. How do I constrain the feedback path to the
> shortest possible area and ensure that only a single trace is
> travelling between boards - not a whole feedback loop.
> 
> My original idea was to put some sort of small value resistor after
> the output of the buffer - is that a contender?
> 
> Thanks,
> 
> 
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