[sdiy] Help deducing the internal resistor values of the IR3109?

[René Schmitz]

Hi Jack and all,

Am 05.10.2013 18:22, schrieb Jack Jackson:
> I've been looking at the schematic of the IR3109 freely available on the web. It is just four OTAs, four buffers and an expo converter apparently. The main missing piece of knowledge is the values of the internal resistors between the gain cells non-inverting inputs and ground:
>
> http://flic.kr/p/goAsc3

These look to me like the typical opamp input current compensation 
resistors. Usually one would keep the impedances seen at both input 
terminals equal (most often done on bipolar opamps).

However I'd think they are really superfluous, because applications are 
probably not necessarily providing this specific impedance at the other 
terminal. And also the offset is easily removed by any DC blocking caps 
that are in the path.

> The schematic also has a bit of useful information:
>
> http://flic.kr/p/goABBB
>
> I have 2 of these chips to hand. Can anyone think of a way to deduce the value of the resistors by taking measurements externally etc?

I think so. If one makes the gain stage into a unity gain follower by 
feeding back the output by a variable resistor from CAP to IN terminal, 
one would find the minimum offset at some resistance setting.
That should be close to the internal resistance. The output (=CAP) would 
sit near GND, because the + input is tied to that.
For this to work one would also need to feed something into the CV input 
of course, to open up the gain stage.

Cheers,
  René


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