Hi,
I want to use the opa547 in my one application & i do not want any offset at output. for testing purpose i have assembled the opa 547 circuit on general purpose board. right now i am providing power supply of +12v w.r.t. gnd & i am getting offset of 600mV. it is clearly written on datasheet of opa547 for VO=0V use -1V on pin number 4. can i use negative voltage reference? Can you suggest me any -ve voltage reference from positive supply?
Ashitosh,
In a single supply application, OPA547 output voltage range does NOT include the ground (no linear op amp can swing all the way to its rails) - please see below the graph of the Output Voltage Swing (to negative rail) vs Output Current; under unloaded output current, Iout=omA, the min output voltage is 600mV above negative rail (GND) and thus what you see is NOT an offset voltage but rather output voltage swing from the rail. Therefore, in your application while using gain of G=2, the minimum valid input voltage is Vin>300mV resulting in Vout>600mV.
Most reference voltages have very limited current sink capabilities and you do not need the precision of the reference to implement a negative supply voltage; using a negative supply (V-) < -1V (instead of GND) or simply using +/-12V dual supply will fix the problem you see.
Ashitosh,
Your output current limit setting does not work properly because you must reference Ilim NOT to GND but rather to the negative rail (the same as pin 4) - see below.
Therefore, as you run the negative supply (V-) with -1V or -12V, you need to connect the lower side of your current limiting resistor to the same -1V or -12V, respectively, potential - see below. Of course, in order to maintain the original current limit, this may also require adjustment of current limiting resistors (800 ohm and 5k) - see below.
