AN4451

How does the UV sensor work

DocID025985 Rev 1

5/15

1.5

Input bias current (Iib)

By choosing a large feedback resistor while simultaneously trying to achieve a high gain,

the input bias current of the op-amp causes another DC offset: Iib * Rf. Consequently, a

low bias current op-amp is needed to obtain the highest sensitivity. To achieve this, it is

extremely important to choose a CMOS op-amp such as the OA1MPA which offers a very

low Iib (10 pA @25 °C). The total output voltage must also be considered (see formula (5)).

If we consider the UV sensor application described in this document (see Figure 2) and if

we use the OA1MPA as the transimpedance amplifier, the total Vout is:

Vout = 26 nA * 4 * 8.2 M Ω ±200 µV ±10 pA * 8.2 MΩ. Consequently, Vout is in the range

[852.5 mV:853.1 mV]. This represents an error of 330 ppm compared to the theoretical

value.

1.6

UV sensor equivalent circuit

Figure 3 exhibits the equivalent circuit of the photodiode, where Cj and Rj represent

respectively the junction capacitor and the shunt resistor of the diode junction.

Figure 3: Equivalent circuit of UV sensor

1.7

Resistors Rs and Rj

The resistance of the output source, Rs, is generally negligible. On the contrary, the diode

shunt resistance, Rj, should be as high as possible. For example, regarding the UV sensor

GUVA-

C22SD, Rj is in the range 100 GΩ. Effectively, without any current in the

photodiode, an output of 0 V is theoretically expected. However, in reality Vout is as shown

in formula (6).

Clearly, it is extremely important to have a low Vio and an UV sensor with a high Rj to limit

the offset error on the output.

Rj

Cj

Ip

photocurrent

Rs

Serial res