[sdiy] Newbie alert: opamp resistor values

[Tom Wiltshire]

On 8 Jan 2009, at 18:25, Ian Fritz wrote:

> At 10:34 AM 1/8/2009, David G. Dixon wrote:
>> What are the basic guidelines for selecting absolute resistor  
>> values for
>> opamp circuits?  For example, say one needs a unity-gain inverter  
>> or summer,
>> and is using a generic FET-input opamp (TL07x, say).  What determines
>> whether one uses 10k, 100k, or 1M for the two resistors, or something
>> inbetween?  Now, what if input impedance is not an issue (i.e.,  
>> the incoming
>> voltage is well buffered)?
>
> Generally not too critical.   100k is standard modular input  
> impedance, so that often determines the two values.  Otherwise, too  
> large an impedance gives noise/offset issues, too small draws more  
> power than necessary.  I say anything 33k - 330k is fine, but I'm  
> sure you'll get other opinions.
>
> Another consideration is to think about other folks building your  
> circuit.  It's convenient to have as small a number of values as is  
> practical.  So don't use 33k one place and 47k another if they  
> could be the same.
>
> Ian

I agree with Ian. Years ago, I remember actually sitting down and  
doing a load of tests making various op-amp amplifiers with different  
resistor values to see if I could find any difference. I didn't have  
any sophisticated test equipment (or any sophisticated op-amps - this  
was using a 741) but whilst the values stayed above 1K and below 1M  
there wasn't a lot in it.

Ian is right to point out that resistor values too low uses more  
current than you need to, whereas resistor values too high increases  
noise from the resistors. But between your 10K and 100K there isn't  
much to choose.

T.