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Regarding ADA4077

seshupower
on Jul 10, 2015

Hi,

I am using ADA4077 opamp in my SMPS design for Feedback for current control. I given single supply of +15V to pin 8 and 4 pin connected to GND. But iam getting 1.34V offset at ping 7,it is effecting my current control.Please suggest me how to minimize this offset.Find the attached schematic. U29B. I am facting same issue in U28A alos,but it will not impact my voltage control.

schematic.pdf
  • 0
    •  Analog Employees 
    on Jul 11, 2015 3:26 AM

    Here are more locations where the input-voltage-range  requirement is not being satisified:

    • U20B: The common mode is 1.025V but the op amp requires it to be at least 1.2V above the negative supply (which in this case is ground). This violates the Input Voltage Range specification shown at the bottom of datasheet Page 4.
    • U28B: Current from the 2.05V source will flow through V_set, dividing the resulting voltage drop across the two 1k resistors, R132 and R166. So the common mode input will only be (2.05V-Vset)/2, which will too low to meet the input voltage range of the op amp (assuming V_set is lower than about 1V)
    • U2B: The current I_BATT will cause a diode voltage drop across D14. This will be below the minimum voltage to satisfy the IVR for the op amp.
    • U2A: Input is tied to ground. Must be above 1.2V.

    You can consider using an op amp with rail-to-rail inputs, like the ADA4661 or ADA4084

    You may also want to refer to the following tutorials and application notes:

    MT-035 Op Amp Inputs, Outputs, Single-Supply, and Rail-to-Rail Issues

    MT-041 Op Amp Input and Output Common-Mode and Differential Voltage Range

    AN-581 Biasing and Decoupling Op Amps in Single Supply Applications