[sdiy] Newbie questions about summing buffers

[Scott Juskiw]

For the inverting summer shown in that schematic the input impedance is R1. If you use 1K for R1 then your input impedance is 1K which might not work very well (depends on what is driving this circuit). If the output impedance of the driving circuit is also 1K then you are only getting half the voltage at your inverting summer output. Not good.

Capacitor across Rf forms a low pass filter, helps dampen any high frequency oscillations at the output.

On 2012-02-24, at 7:35 AM, ganesha wrote:

> Hi,
> 
> I have 2 questions about summing buffers
> If you take this schematic:
> 
> http://upload.wikimedia.org/wikipedia/commons/thumb/3/3e/Op-Amp_Summing_Amplifier.svg/200px-Op-Amp_Summing_Amplifier.svg.png
> 
> 
> -Most of the schematics I've seen so far use large resistor values of f.e. 100K for resistors R1- Rn and Rf. 
> Is there a specific reason to use 100K 0,1% instead of  small resistor values like for example 1K 0,1%? If they both are high precision resistors of 0,1% I would think the result would be exactly the same?
> 
> -In some schematics I've seen a small capacitor in parallel with resistor Rf. Can someone explain when it is better to add a capacitor?
> 
> Thanks.
> 
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