Resistor placement in virtual ground summing

Thread Starter

Jaslau

Joined Aug 15, 2022
2
Hi,

I have a question about the placement of the resistors on the board with a classic virtual ground summing circuit.
Is it preverable to put them near the source or near the summing node of the opamp?

I think the advantage of having them near the source is less radiation from the summing bus, but is it more prone to picking up noise? Where is the prefered place to minimize noise and crosstalk?

Thanks in advance!
 

MisterBill2

Joined Jan 23, 2018
18,477
That "summing virtual ground" is only zero volts to ground IF the non-inverting input is held to ground voltage. It is a summing junction with the voltage it sums to is supposed to be the same as the non-inverting input. For convenience in any analysis the resistors tied to the inverting input should be conveniently close.
What voltage would be applied that radiation would be an issue. Consider that the summing junction is a fairly low impedance point.
 

crutschow

Joined Mar 14, 2008
34,418
having them near the source is less radiation from the summing bus
The radiation from the summing junction is normally low on none.
Noise pickup of the summing junction is more likely to be a problem, so make the summing junction connections as short a feasible.
A long run to the summing junction also increases the stray capacitance at the point, which can lead to peaking and/or instability in the circuit response.
 

MisterBill2

Joined Jan 23, 2018
18,477
Actually, having the summing junction at a virtual ground will only happen with a perfect op-amp fed from a balanced bipolar supply. In the real world it would be fairly close, but not exact.
 
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