[sdiy] differential integrators?

[Magnus Danielson]

dan snazelle wrote:
> last night, for the first time in months, i found some time to just sit and read electronics books.
> 
> i came across a term i hadn't thought much about before: Differential Integrator
> 
> 
> However, one of the examples in the OPAMP cookbook used a variable capacitor which i would like to avoid if i could. the other used two opamps and a few matched resistors
> 
> so my questions are
> 
> 1. what are some good uses for differential integrators

When you want to integrate the difference between two voltages directly 
rather than first using a diff-amp and a separate integrator stage.

> 2.  where are some good examples of how to build them?

A very simple approach is to take a 4-resistor diff-amp and replacing 
the two resistors at the op-amp (i.e. the feedback resistor becomes a 
capacitor and the resistor from the + input to ground becomes a 
capacitor). Make the two input resistors and the two caps equal.

> 3. do they always have to use matched caps and resistors

They need to have matched time-constants. You may trim the input 
resistance of one of them to compensate capacitor mismatch.

Cheers,
Magnus