[sdiy] Combining currents from different OTA outputs

[Neil Johnson]

Hi Aaron,

Aaron Lanterman wrote:
> Suppose I have several OTAs with current outputs, and I would like  
> to add those currents and turn that sum into a voltage.

Easy...

> A) Could I tie all the OTA outputs together, and run that output  
> down a resistor to ground, and then buffer it with an op amp?

Not a good idea.  Look in the 2164 datasheet for "output  
compliance" ... if you don't know what this means go check H&H (2nd  
ed, page 73).

You may get away with it for a single output and a low resistor, but  
then you need a high gain follower, and then you're amplifying noise  
as well as signal.  And with multiple outputs all feeding into this  
one resistor (a crude I-V converter) because each output 'sees' the  
aggregate output voltage then each individual current output will  
affect all the other current outputs.

> B) Could I tie all the OTA outputs together, run that into the  
> negative terminal of an op amp with a a resistor in a feedback  
> loop, with the + tied to ground (this would be an inverting current- 
> to-voltage conversion.

This is the correct way to do it.  Indeed, the pinout of the 2164 is  
designed just for this, with all four outputs brought together for  
easily summing into an I-V converter.
By keeping the output terminals at pretty much zero you keep the  
operating point well within the compliance range, so distortion will  
be kept to a minimum.

> Assuming A and B are valid...

In this case A is not valid.

> In theory, if the OTA outputs act as ideal current sources, the  
> above should work... but I don't think I've ever seen OTA outputs  
> ganged up like that in practice.

Check out the 2164 datasheet figure 25 on page 9 - this is exactly  
what is described.
This is also, roughly, how most audio mixing console busses work.

> I'm worried that there may be some "in practice" reason why OTAs  
> might not like having their output currents summed in such a direct  
> fashion.

Quite the opposite - this is what they are designed for.

Cheers
Neil (another 2164 fanboy)
--
http://www.njohnson.co.uk