Summing amplifier using single-supply opamp?

[psychogenic]

Greetings all,

I'm attempting to create a summing amplifier using a single-supply op-amp. I've created and attached a schematic to describe where I'm at, and would like some input.

In order to avoid clipping and keep the sum of the incoming sine wave signals nice, I'm floating the inputs over VCC/2, then summing everything. I used a simple voltage divider to get the VCC/2 reference on the non-inverting input.

My questions are:
* it seems to work... any reason or conditions under which it wouldn't?
* is there a better way to achieve this, perhaps with a reduced parts count?
* are there any "gotchas" to look out for, in terms of impedance matching or somesuch--e.g. should the voltage dividers have resistors with much greater values, lower... does it matter?

Thanks in advance for your help--don't hesitate to ask if you need more info

 

[Grossel]

You haven't specified any resistor values.

One tip (gotchas as you call it) is to be aware about input impecance. If you draw the signal path for AC (don't know the correct english term for this) you'll see clearly how the input resistance (impedance if you deal with cap too) will act, from the signal source point of view.

 

[crutschow]

You don't need the voltage divider resistors (R6-R11) on the inputs. The voltage divider on the (+) terminal of the op amp will keep both the output and the (-) inputs (through R12) at 1/2 Vcc.

 

[audioguru]

Your Vcc/2 voltage divider feeds half the noise and hum on the power supply directly to the + input of the opamp. The Vcc/2 should have a filter capacitor to ground.

 

[Dragon Tamer]

He may be able to get away with using only one voltage divider on the inverting input with a single resistor in series with R12 for the voltage gain.

 

[crutschow]

He may be able to get away with using only one voltage divider on the inverting input with a single resistor in series with R12 for the voltage gain.

As I stated in my post, he only needs one divider on the non-inverting input. A single divider on the inverting input won't work. That would attempt to bias the output at minus Vcc/2, not plus Vcc/2 as required.

And you need one resistor for each input for a proper summer.

 

[Dragon Tamer]

How did I miss that you said that?