Vout of AD8304

the  current value  of  photodiode is 2.996 uA.,and the G is 2.5, The  theoretical calculation is 2.23827V,But the actual output voltage is 2.2606V, Is the calculation formula incorrect?

The equation is correct.  I assume you are using the ideal values of 200 mV/dec and 100pA for Vy and Iz, respectively? 

Have you accounted for the following possible errors:

• Error in the PD current
• Error in Iz: this could be up to +/- 20% (see TPC21 in the datasheet)
• Error in the Vy; this is small but it is still there
• Error in the external resistor values that set the gain of the op-amp
• Error in the voltmeter you use to measure VOUT

Even an error of +/-10% of the Iz current could account for the difference in calculated value (with ideal values) and what you are measuring

I do not use the the Adaptive Biasing，I use the circuit diagram above .When I change  light ，the VOUT has not changed, and I make the photodiode negative to ground ，the  VOUT also has not changed.If I want to the photovoltaic mode ，how can I do？

Hello hai235,

I'm not sure how to do photovoltaic mode. But for most AD8304 applications, the PD needs to be either zero biased, or reverse biased. In the schematic above, the PD is forward biased, because the voltage at INPT pin is normally at +0.5V. To make the PD be zero-biased or reverse biased, the PD cathode must connect to +0.5V or higher, with respect to ground. 

NOt using the adaptive biasing，is the photodiode negative connected to ground ？

Photodiode cathode should connect to +0.5 Volts or higher. Try connecting it to the VREF pin which should have +2 V. 

Photodiode cathode should connect to +0.5 Volts or higher. Try connecting it to the VREF pin which should have +2 V.