[sdiy] Coping with diode drop(s) on Iabc input with LM13700/CA3080

[Roman Sowa]

If you put current mirror at 9V and control it from microcontroller DAC 
powered from 5V, then you have only limited range, only about from 40% 
to 100% current. Can't reach 0.

But you can always put the mirror one diode drop above 5V, so it could 
be possible to reach higher range. Still that doesn't alow to reach 0.000mA
You could possibly hang the mirror at some 5.4V or something delivered 
by resistive divider or shottky diode, and then maybe it's possible to 
reach small enough current with tiny nonlinearity at the lowest part.

Roman

W dniu 2020-09-30 o 19:22, Aaron B. pisze:
> 
> If I'm understanding the problem correctly, the OTA is powered by +9
> volts, the microcontroller by +5v. With the mirror sourced from +9 as
> well, the diode drop will move 3 or more volts beyond the
> microcontroller's power rail.
> 
> 
> I may also be completely wrong about everything here. :)
> 
> 
> On Wed, 30 Sep 2020 19:00:12 +0200
> Roman <modular at go2.pl> wrote:
> 
>> Current mirror will only move the problem of coping with diode drop from gnd to plus rail
>>
>> Roman
>>
>> ---- Użytkownik Aaron B. napisał ----
>>
>>> This will invert the polarity of the CV, but if that isn't a big deal -
>>> drive the OTA through a current mirror. The PIC's output will sink
>>> current through a programming resistor at the mirror's input;
>>> the mirror's output then sources Iabc to the OTA.
>>>
>>>   
>