Analog Current Output with in the pA or fA range

Document created by analog-archivist Employee on Feb 23, 2016Last modified by analog-archivist Employee on Feb 23, 2016
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For our optical oxygen sensors we use currently the ADuC7023 in combination
with AD5695 for analog output with 0-2.5V. An industrial customer requests now
an analog current output in the range 0-100nA (yes, nanoampere!). Could you
advise any solution, how to convert an analog output of 0-2.5V into an current
output of 0-100nA? We have only single-supply of 3.3VDC.
The background for this request is, that this electronics should replace old
electrochemical oxygen sensors, which used to deliver an analog current output 
in the picoamp/nanoamp-range. Our customer wants to use our new optical oxygen
sensors, but they should be integrated into their old transmitter electronics
adapted to pA/nA-input.

 

You could try adding 25Mohm resistor at the output and using a current mirror
type setup but the noise would dominate I expect. This is not the best place to
start to try to generate this type of current because the noise is quite high
on these string DACs I'm afraid..

We have some current output drivers such as the AD5749 but this is in mA rather
than nA.

I send you some suggestions attached.

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