Blocking Caps + Passive Summing Question

[thermionic]

Hi,

Let's say you're passively summing the O/P from 3 op-amps via low-ish value resistors (the op-amps are happy to drive low loads, so you're using something around 1k or less). The op-amps need a cap to block their DC offset. Traditionally I would put the caps right at the O/P of the op-amp, with each feeding the summing resistor (op-amp - cap - resistor).

Let's say, for whatever reason, you were to put the summing resistor directly at the O/P of the op-amp, with the cap afterwards - therefore making your 'summing node' the output of each cap (op-amp - resistor - cap).

What would be the implications for performance if you implemented this method?

Thanks in advance.

Justin

 

[JohnRoberts]

The order of passive components connected in series generally doesn't make any difference.

JR

 

[thermionic]

Thanks. John. It just felt weird, connecting the o/p of the caps together.

J

 

[JohnRoberts]

In fact if the resistors are not individually switched (which could cause clicks) you could get away with summing the Rs together and using one cap to block the DC, but the pole frequency for that RxC would now be the parallel resistance of all the R's together. 

JR