Help me create a current source circuit to connect to a I to V converter

[themaccabee]

i/v converter

Hi,
I want to create a current source circuit to connect to an already existing I to V converter.
What i want to do is to vary the input current from 0-400uA and note the corresponding output voltage.currently for 10uA i get 100mV approximately at the output.ie 10 is the gain.(ignore the resistor valuesin figure if it is wrong)

I want to change the interface that feeds the current to the input of opamp.Currently it is done by varing a pot which is conntected at ends to -5 V & GND.

I want to change the pot and to make it automated say using a dac or a digital pot can somebody help??
Im really a novice in the design..

 

[barry]

i v converter

Hi,

Your circuit as shown will give negative output for positive current (current flowing INTO the opamp)

But if that's what you want, the following circuit(among many others) will work. Replace the 12.5K resistor(R1) with a digital pot and you can vary the current. Current is approxmately equal to (5.6(the zener voltage) - .6)/R1. R2 may also have to change depending on the zener you use.


(I'm not sure the image got pasted here. Not sure what I'm doing wrong. Let me know if there's a problem.)

 

[themaccabee]

i - v converter

Actually i wanted the current to flow out of the input pin of the opamp..So that +ve voltage can be achieved at the output.Can u suggest me what the changes that i must make in the circuit you proposed??
Thank you.
Stephen.

 

[barry]

i-v converter

Here you go. Just use an NPN instead of a PNP and flip everything around. This is just a starting point. You'll have to adjust values to suit your application.



Barry