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Voltage-controlled current source



		

			

									
						bojankoce
					
							

			
				
									
											
			
			
				
    
			
			
			    	
			                    
                
				                on Aug 4, 2019 
				
			
			
		
    

				
		
    
		    		    
		    
		    	
			
    Hello guys,


    I want to design voltage-controlled current source for my current project. The currents should be in a range from 0 to 2.5mA. Load resistance should be up to 5Kohm. My idea was to use unity gain amplifier together with the buffer in the configuration like this:

                                                         

    Voltage-dependent generated current can be expressed as 
     I = Vsine/R8;
     where R8 = 1Kohm. So, for Vsine voltage of 1V, for example, I will have generated current of 1mA. Supply voltage is equal to 18V. My idea is to use SPI or I2C controlled ADC for generating Vsine voltage.

    Resitor R9 of 5Kohm simulates my load. Generated current is monitored with shunt resistor R10=50Ohm and current-monitor module AD8211. I also use voltage divider made of R11 and R12 to monitor the voltage on the load. Both current and voltage monitoring are implemented through AD converters on my MCU. Current and voltage monitoring is implemented in order to calculate load resistance from MCU side.

    I was using LT1006 general purpose amplifier for unity gain amplifier and buffer and everything was as expected (at least in LTSpice simulations). I wonder if there are some less expensive components I could use in order to achieve the same goal? Do you have unity gain amplifiers with 4 resistors implemented inside the chip that I could use as well as some less expensive buffer IC?

    Also, if you think there is a better, more efficient way to implement a voltage-controlled current generator as well as voltage/current monitoring, don't hesitate to suggest. 

    I tried myself to use some less expensive general purpose op-amps (like LT6011, for example) instead of LT1006 but I had some issues in generating load current. Here is what I have in LT simulations with LT1006 and LT6011:

                  

    As you can see from the waveform above, with LT6011, for the Vsine voltages in a range from 0 to ~0.3V, generated current is constant and equal to ~0.3mA. LT1006, on the other hand, properly follows Vsine voltage even for Vsine voltages below 0.3V. 

    Thank you very much for your time and effort! It is really appreciated!

    Sincerely,

    Bojan.