[sdiy] More OTA meas.

[Ian Fritz]

At 03:49 PM 5/25/05, ASSI wrote:
>On Mittwoch, 25. Mai 2005 00:42, Ian Fritz wrote:
> > First I looked at another 13600 chip and verified the 9% nonlinearity
> > for Iabc of .25 mA and .5 mA.
>
>When you measure the linearity, do you keep a constant Iabc and then
>vary the input or do you keep a constant input and vary the Iabc? If
>the Iabc is constant, then no error in the Iabc mirroring should show
>up in the result, so I guess you compared the output for constant input
>at the two different Iabc settings. What's the supply voltages, common
>mode voltage and differential input voltage for the inputs?

If you read my original post you would have seen the details of the 
measurement.

It is a measurement of output current via an I/V converter as a function of 
Iabc, with Vin fixed (usually at 20 mV or so).  The input impedances are 
balanced, and the offset is nulled in the usual manner.  Supply voltages 
are +/- 12 V.  For a short report, I just quote the nonlinearity at .25 mA 
and .5 mA.

> > Then I tried the idea of tying the
> > buffer input to the (-) rail to keep it from drawing bias current.
> > This had no effect whatsoever on the transconductance.
>
>You can't cut off the suspected "current steal" that way (unless the
>circuit shown in the data sheet is completely phony).

Perhaps not, but the diagram shows that this would reverse bias the buffer.

> > Finally I looked at the 3280 Iabc linearity.  Again, just considering
> > .25 mA and .5 mA the nonlinearity measures 1.5%.
>
>The allowable Iabc for the CA3280 is 5 times larger than the LM13600,

So what?  Their specs and applications are generally below 1 mA.  And there 
are plenty of good octaves down there.

>but that wouldn't explain the CA3080 result unless it's data sheet
>underspecs the part in that respect. It might be worth a try to scale
>down the Iabc when using the LM13600.

Yes, well, why don't you try doing it (or doing anything) yourself, instead 
of just bitching about other peoples work?