[sdiy] Length of a summing node (was Re: AC coupling caps on MS20clone)

[JH.]

1. Shield it. Keep impedance (summing resistors and feedback resistor) low.
2. Stray capacitance might be a bigger enemy than noise. Ad a tiny series 
resistor (100 Ohms) close to the opamp before your summing node spreads out.

JH.


----- Original Message ----- 
From: "Tom Wiltshire" <tom at electricdruid.net>
To: "David G. Dixon" <dixon at interchange.ubc.ca>
Cc: "'Synth DIY'" <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, August 08, 2010 10:32 PM
Subject: [sdiy] Length of a summing node (was Re: AC coupling caps on 
MS20clone)


> 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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