ej_whyte
Well-known member
I'm trying to do some noise calculations at the minute but just need to clarify a couple of things up and need a bit of help please. So it is for an 8 channel virtual earth mixer, this is what I was working with at the moment:
Calculate noise for individual channel input circuitry (including summing resistor)
RMS addition of 8x that figure for 8 channels (Vn-in)
RMS addition of summing amp voltage noise x gain+1, summing amp current noise and feedback resistor noise (Vn-sum)
RMS addition of (Vn-in x gain) + Vn-sum)
There are three things I am not sure on
1. I assume that there is no attenuation of the channel input's noise as the virtual earth means that there is no real potential divider formed with other channel's summing resistors
2. Does the channel input noise need to be multiplied by the summing amp's gain, or is it treated as unity?
3.Not quite sure on how to treat the summing resistors A) calculate johnson noise for 1 resistor and then do RMS addition for 8 of these, or B) Treat them all as parallel and calculate johnson noise from the parallel value. I would guess A as it is actually the physically correct amount of resistors.
Thanks
Calculate noise for individual channel input circuitry (including summing resistor)
RMS addition of 8x that figure for 8 channels (Vn-in)
RMS addition of summing amp voltage noise x gain+1, summing amp current noise and feedback resistor noise (Vn-sum)
RMS addition of (Vn-in x gain) + Vn-sum)
There are three things I am not sure on
1. I assume that there is no attenuation of the channel input's noise as the virtual earth means that there is no real potential divider formed with other channel's summing resistors
2. Does the channel input noise need to be multiplied by the summing amp's gain, or is it treated as unity?
3.Not quite sure on how to treat the summing resistors A) calculate johnson noise for 1 resistor and then do RMS addition for 8 of these, or B) Treat them all as parallel and calculate johnson noise from the parallel value. I would guess A as it is actually the physically correct amount of resistors.
Thanks