[sdiy] current conveyor?

[ASSI]

On Monday 06 February 2012, 12:47:39, David G Dixon wrote:
> The current needs to be able to flow in both directions.

Some sleep and a coffee later... you do need an active current source rather 
than a passive level shift to have the current flowing back, of course.

An OTA with linearizing diodes will do, although the biasing is a bit tricky 
and I don't know how good the precision is at low currents and you will 
probably need an offset trim.  In essence, the output of the OTA goes to the 
-5V integrator input; both Iabc and Idiode need to be the same current (you 
need a mirror there since one will be sitting at +0.7V and the other 1.4V 
above the negative supply), slightly larger than the maximum current you're 
going to use; the negative input goes to ground (here's where you add the 
buffered offset trim, since it needs to be low-impedance); and the positive 
connects to the output of the 2164.  It may be advantageous to rig the 
supply of the OTA so that the mid-supply becomes close to -5V (requires a 
+-15V system w/ additional 5V rail due to head room requirements, probably 
not worth the trouble if you only have +-12V).

It may be easier to level shift all the inputs to the 2164 down by 5V with 
some zero-offset, zero drift opamp.  I assume you do not want to change the 
integrator to have its output symmetric about zero?


Regards,
Achim.
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