Hi Jeff
Had a closer look at the non inverting summing amp.
All very interesting but it also has the statement
“Note also that if the amplifier of the summing circuit is connected as a unity follower with RA equal to zero and RB equal to infinity, then with no voltage gain the output voltage VOUT will be exactly equal the average value of all the input voltages. That is VOUT = (V1 + V2)/2.”
Which is the way I have usually found this scenario. This holds true if the source impedance of the inputs is low and the 2 series input resistors can act as a voltage divider. If however ,as is often the case, there is a diode in the way and a high value resistor to ground as in the case of a half wave rectifier charging a cap with a high value R to discharge the cap this resistor is usually too high to allow the input Rs to act as a voltage divider and the output will follow the highest input voltage. Like sharing power supplies via diodes.
Assuming a low Z input and you want to have this as a summing amp you have got to provide some gain. That is if you study the text it says the gain has to be equal to the number of inputs so if you have 3 inputs the gain has to be 3. In effect you are outputting the average multiplied by the number of inputs which will indeed be the sum of the inputs.
Of course all this goes out the window if the inputs have series diodes as mentioned above.
As I see it the disadvantage here could be you have to know the number of inputs to ascertain the feedback network values before building.
On the other hand the inverting system has the capability of using and adding any number of inputs on the nun. The same thing is occurring but additional series resistors are effectively in parallel with existing ones so the overall value if the source Z is reduced so increasing the gain sort of automatically. Disadvantage here of course is the split supply requirement.
I presented a circuit recently to overcome this diode problem when the output from 2 sources need to be averaged. I used 2 Op Amps as unity gain buffers providing a low Z source impedance to drive a third OP Amp via 10k Resistors connected as a unity gain amp. This provides an overall output of the true average of the 2 inputs.
Don’t know the overall outcome yet but on test the OpAmp circuit worked wel.
The circuit is in this thread.
Op amp problem
That is the original post. Last entry recently.
Cheers Bob
