Output voltage limitation

AN4451

12/15

DocID025985 Rev 1

4

Output voltage limitation

To improve the sensitivity of the UV sensor, use it in photovoltaic mode i.e. with a zero bias

operation. In this case, the dark current offset is generated by the photodiode leakage. If

the op-amp is used in single supply from GND to Vcc (as shown in Figure1), the Vol output

saturation of the OA1MPA might be a limitation for treating low UV radiation levels.

Despite the fact that the OA1MPA is an output rail-to-rail op-amp, it has a Vol output

saturation voltage of 40 mV @ 25 °C. When it is important to know precisely the current

delivered by a sensor with a low UV intensity, add a reference to avoid Vol limitation as

shown in Figure 9.

Figure 9: How to avoid Vol limitation

In the above case, there is an offset of 100 mV at Vout. It is important to connect Vref to

the ADC to obtain a precise calculation.

+

-

Vcc

8.2M

Ω

Vout

10pF

Uv sensor

STM32 Microcontroller

ADC

Rn

Cn

Vref=100mV

OA1MPA

STM32 microcontroller