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LTC6081 LTC6082 precision bidirectional current source

From the datasheet for LTC6082 (60812fd.pdf), there is an example of nanoamp current source.

I couldn't find any detail explaining this circuit. How would I change it to make it operate between 1uA to few microamps (bidirectional)? Would change 10Mohms to 10kohms would greatly degrade the performance? I can see the bottom two OPs are part of an improved Howland current pump that is part of a the negative feedback loop. 

https://www.analog.com/en/analog-dialogue/articles/a-large-current-source-with-high-accuracy-and-fast-settling.html

What does the top OPAMP do with the FB? It'd be awesome if someone could write a follow-up article on this 3-OPAMP like Nick's article. Thanks.



Clarifies it's still a bidirectional current.
[edited by: Besty at 6:09 AM (GMT -4) on 9 Jun 2021]
  • Hi Besty:  It's a very simple circuit actually.  The bottom 2 opamps and their resistors form a difference amplifier, measuring the voltage across the 10Meg resistor and outing that voltage at the top opamp -input.  That is what closes the loop  So basically, the input voltage is divided by 1000 (100k:100 Ohms), and the circuit will put that divided down voltage across the 10Meg.  So the current in the 10Meg will be I=(Vin/(1000))/10Meg, = Vin/10Gig. 

    Yes the current can be bidirectional, because the opamp has a split supply.

    To scale for different currents, just scale the 10Meg.  Make it smaller for high currents.  The highest current possible will be 2.5V-Vload, divided by the resistance.