[sdiy] Resistor math

[rburnett at richieburnett.co.uk]

The 40ohm figure you quote for the on-resistance of your analogue 
switch will have far less effect on the circuit than the tolerances on 
your much larger 25K, 50K & 100K resistors.  So, I think you can safely 
ignore the effect of the 40 ohms of on-resistance.

-Richie,


On 2013-05-22 11:08, Rutger Vlek wrote:
> Thanks, yes it's indeed the summing amplifier. However, your 2 cents
> seem to be an answer to a different question. What I meant was that
> I'm using a 8:1 mux to select inputs, but I'm not selecting single
> inputs, but selecting different weighted sums of signals. My question
> is how to take the mux's ON-resistance into account when computing
> values for the weighting network?
> 
> Rutger.
> 
> 
> On 21 mei 2013, at 14:12, Julien Delgoulet wrote:
> 
>> Hum …
>> 
>> Have you checked this pdf :
>> 
>> http://www.stanford.edu/class/ee122/Parts_Info/datasheets/op_amp_circuit%20collection_AN-31.pdf
>> Your circuit is the "Inverting Summing Amplifier", a classic one.
>> Also if you are using an analog switch to "select" inputs, may be you 
>> could use an analog switch to select the resistor in the feedback loop 
>> of the opamp …
>> 
>> My 2 cents ..
>> 
>> Julien
>> 
>> 
>> Le 21 mai 2013 à 11:50, Rutger Vlek <rutgervlek at gmail.com> a écrit :
>> 
>>> Hi all,
>>> 
>>> I need a little help calculating values for a network of precision 
>>> resistors. The resistors are used for summing several signals with 
>>> different weights (1x 2x 4x 12x, etc). Summing is done at the virtual 
>>> ground of an inverting opamp. I've used 100K in the feedbackloop of 
>>> the opamp, so to weight one of the inputs with 1x a resistor of 100K 
>>> is normally used, and 50K for 2x and 25K for 4x. However... there is 
>>> an analog switch in between the summing network and the inverting 
>>> input of the opamp, with a typical ON-resistance of 40ohms. My 
>>> question is, how does this affect the values that should be used for 
>>> the summing network? And how to compute new values for those 
>>> resistors that assure summation with the exact weights I need (and 
>>> how to implement those with limited values available in a series of 
>>> 0.1% tolerance precision resistors)?
>>> 
>>> Here's an overview of the summing network:
>>> 
>>> --1xresistor----|
>>> --2xresistor----|
>>> --4xresistor----|-----analog switch----inverting opamp---
>>> --12xresistor--|
>>> 
>>> Any help is appreciated!
>>> 
>>> Rutger
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>> 
> 
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