Op Amp Voltage Reference

  
     
If you've done any work with op-amps, especially using a single supply, you probably have needed a voltage reference to set your virtual ground to 1/2 Vcc. And depending on what your circuit is to do, you may need several of different values. This can take up a lot of real estate on your breadboard just for references which is why I put together a little PCB using a quad op-amp that generates 4 independent voltage references. In the image to the left is the finished design with a six pin header on the right to plug right into a breadboard. The op-amp I have here is a Microchip MCP6004, but any op-amp with a standard pinout will work, even dual supply op-amps will work nicely.
    From top to bottom the pins are:
    - Output 3
    - Output 2
    - GND/VEE
    - VCC
    - Output 4
    - Output 1



   


  

 
On the left is the schematic for three of the op-amps. At its core is 1/4 of the chip configured as a voltage follower. The non inverting input is controlled by the 10K potentiometer which in turn has a four pin header connected to the top and bottom. By doing this, I can select whether or not the top is in series with a 10K resistor or connected directly to VCC by placing a shunt across the pins I need. Either 1/3 or 2/4. The bottom is the same only it is connected to GND.
    This will allow four possible output voltage ranges:
    - Vcc to GND.
    - GND to 1/2 VCC.
    - 1/2 VCC to VCC.
    - 1/3 VCC to 2/3 VCC.






 
      



    The fourth op-amp I have set up a little differently. I have attached a two pin female header to the top and bottom of the 10K pot. This way I can select the resistors to give me the voltage range and sensitivity that I need. In the article Trim Pot Voltage Divider, I describe how to calculate the values necessary.













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