[sdiy] OTA Iabc feeding

sleepy_dog at gmx.de sleepy_dog at gmx.de
Sat Sep 2 10:40:32 CEST 2017

Hello SDIY,

looking at common circuitry to feed the Iabc pin of e.g. the LM13700, I 
noticed that, typically, there's a PNP involved, whose collector feeds 
into the Iabc pin.
Either directly, with base to GND and a resistor between control voltage 
source and emitter, or B,E in the feedback path of an opamp.

Why is that? To make sure absolutely no negative voltage gets in there? 
(that would be one thing that I newbie see what this does, next to all 
the things I don't see)
Although I haven't seen anything in the datasheet requiring that, only 
saw "abs max 2mA".

What I wonder mainly is, why it's thought of as acceptable to lose the 
lower 0.6V or so of the control voltage range.
E.g. using 3V3 coming from a microcontroller, that's almost 20% thrown away.

Would it be bad to put nothing but a resistor between a PWM filter 
output and the Iabc pin?

And by the way, as for digital power rail noise from a MCU pin, can 
slapping a low pass filter on that pin be considered dealing with it 

- Steve

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