Readout unit and organic light emitting diode display apparatus having the same

A read-out unit comprises a calculation unit and an analog-digital conversion unit. The calculation unit is read-out from a pixel circuit of a display panel, receives in input voltage indicating a voltage of a pixel based on a driving current which drives an organic light emitting diode, receives a reference voltage from a reference voltage terminal, and outputs an output voltage indicating a difference between the input voltage and the reference voltage by comparing the input voltage and the reference voltage. The analog-digital conversion unit converts a convertor input voltage into digital form by using the convertor input voltage based on the output voltage and an amplified voltage amplified from the convertor input voltage. Therefore, changes in the pixel can be accurately detected regardless of a capacitor of the display panel.