[sdiy] Length of a summing node (was Re: AC coupling caps on MS20 clone)

[Tom Wiltshire]

I've also heard that a summing node at the -ve input of an op-amp should be kept fairly small to limit noise.
Is this true, and if so, to what extent?

For example, in a multi-voice synth, I could have one mixer resistor on each voice board, and then have one bus wire which takes all those resistors to the summing node. This is supposed to be "bad" because the summing node is "stretched" along the whole bus wire.
The "good" way to do it would be to run 16 wires (it's a 16 voice synth, ok?!) back to the mixer resistors which are all next to the op-amp. That way the summing node is small and won't pick up noise.

How much of this is true? How much does it matter? How many inputs do you have to stick on a mixer or how long do the wires have to get before it does matter?

Thanks,
Tom


On 8 Aug 2010, at 19:40, David G. Dixon wrote:

>> I have often read that the PCB traces at the inverting input (aka
>> summing node) shold be kept as short as possible. I think the idea
>> is, if the summing node itself is viewed as an additional voltage
>> input, it's a very high-gain one (input resistor zero ->
>> theoretically infinite gain).
>> On the other hand, as long as the opamp does its job properly, the
>> input impedance of this "input" is zero (hence it's called virtual
>> ground). In other words, it's a current input, not a voltage input.
> 
> The impedance of a summing node is easy to calculate, and never really that
> large.  On that basis alone, I don't think the length of the trace matters
> at all.  I never worry about it, and it never seems to be a problem.
> Indeed, some summing nodes (such as CV inputs on oscillators and filters)
> can have many inputs and the traces can get quite long, and even involve
> flying wires to the panel.  I don't think this ever matters.
> 
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