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enter image description here I've learning a lot about op amps and constant current sources lately. I've implemented the above Improved Howland VCCS. Found it on page 6 of TI's "A Comprehensive Study of the Howland Current Pump":

http://www.ti.com/lit/an/snoa474a/snoa474a.pdf

I see that it draws 1mA/V with loads ranging from 1k to 4k. Which makes sense to me since R13 sets the gain. Yet, for some reason the circuit wont output any current less than 1mA (unless I increase the load resistance above 5k, which isn't important to me). Essentially the relationship of 1mA/V doesn't apply anymore. When I input 0.5mV I still get 1mA out. Same all the way down to zero.

Does anyone know why this might be happening? I'm using an LM741. Do I need to have the exact op amp as seen in the pic?

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  • \$\begingroup\$ You need to ensure the Vin common mode range is satisfied. \$\endgroup\$ – Sunnyskyguy EE75 Jan 16 '18 at 5:53
  • \$\begingroup\$ Both the original and the improved Howland Current Pump is very sensitive to resistor matching. Check they really are matched! \$\endgroup\$ – Damien Jan 16 '18 at 6:34
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    \$\begingroup\$ Reasons not to use a 741: electronics.stackexchange.com/questions/304521/… \$\endgroup\$ – Andy aka Jan 16 '18 at 10:18

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